Merge branch 'cow' into net

Conflicts:
	.gitignore
	Makefile
	conf/lab.mk
	kernel/defs.h
	kernel/syscall.c
	kernel/vm.c
	user/pingpong.c
	user/user.h
	user/usys.pl

41 files changed, 1882 insertions, 77 deletions

diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000..065dcdf
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,225 @@
+---
+Language:        Cpp
+# BasedOnStyle:  Mozilla
+AccessModifierOffset: -2
+AlignAfterOpenBracket: Align
+AlignArrayOfStructures: None
+AlignConsecutiveAssignments:
+  Enabled:         false
+  AcrossEmptyLines: false
+  AcrossComments:  false
+  AlignCompound:   false
+  PadOperators:    true
+AlignConsecutiveBitFields:
+  Enabled:         false
+  AcrossEmptyLines: false
+  AcrossComments:  false
+  AlignCompound:   false
+  PadOperators:    false
+AlignConsecutiveDeclarations:
+  Enabled:         false
+  AcrossEmptyLines: false
+  AcrossComments:  false
+  AlignCompound:   false
+  PadOperators:    false
+AlignConsecutiveMacros:
+  Enabled:         false
+  AcrossEmptyLines: false
+  AcrossComments:  false
+  AlignCompound:   false
+  PadOperators:    false
+AlignEscapedNewlines: Right
+AlignOperands:   Align
+AlignTrailingComments:
+  Kind:            Always
+  OverEmptyLines:  0
+AllowAllArgumentsOnNextLine: true
+AllowAllParametersOfDeclarationOnNextLine: false
+AllowShortBlocksOnASingleLine: Never
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortEnumsOnASingleLine: true
+AllowShortFunctionsOnASingleLine: Inline
+AllowShortIfStatementsOnASingleLine: Never
+AllowShortLambdasOnASingleLine: All
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakAfterDefinitionReturnType: TopLevel
+AlwaysBreakAfterReturnType: TopLevel
+AlwaysBreakBeforeMultilineStrings: false
+AlwaysBreakTemplateDeclarations: Yes
+AttributeMacros:
+  - __capability
+BinPackArguments: false
+BinPackParameters: false
+BitFieldColonSpacing: Both
+BraceWrapping:
+  AfterCaseLabel:  false
+  AfterClass:      true
+  AfterControlStatement: Never
+  AfterEnum:       true
+  AfterExternBlock: true
+  AfterFunction:   true
+  AfterNamespace:  false
+  AfterObjCDeclaration: false
+  AfterStruct:     true
+  AfterUnion:      true
+  BeforeCatch:     false
+  BeforeElse:      false
+  BeforeLambdaBody: false
+  BeforeWhile:     false
+  IndentBraces:    false
+  SplitEmptyFunction: true
+  SplitEmptyRecord: false
+  SplitEmptyNamespace: true
+BreakAfterAttributes: Never
+BreakAfterJavaFieldAnnotations: false
+BreakArrays:     true
+BreakBeforeBinaryOperators: None
+BreakBeforeConceptDeclarations: Always
+BreakBeforeBraces: Mozilla
+BreakBeforeInlineASMColon: OnlyMultiline
+BreakBeforeTernaryOperators: true
+BreakConstructorInitializers: BeforeComma
+BreakInheritanceList: BeforeComma
+BreakStringLiterals: true
+ColumnLimit:     80
+CommentPragmas:  '^ IWYU pragma:'
+CompactNamespaces: false
+ConstructorInitializerIndentWidth: 2
+ContinuationIndentWidth: 2
+Cpp11BracedListStyle: false
+DerivePointerAlignment: false
+DisableFormat:   false
+EmptyLineAfterAccessModifier: Never
+EmptyLineBeforeAccessModifier: LogicalBlock
+ExperimentalAutoDetectBinPacking: false
+FixNamespaceComments: false
+ForEachMacros:
+  - foreach
+  - Q_FOREACH
+  - BOOST_FOREACH
+IfMacros:
+  - KJ_IF_MAYBE
+IncludeBlocks:   Preserve
+IncludeCategories:
+  - Regex:           '^"(llvm|llvm-c|clang|clang-c)/'
+    Priority:        2
+    SortPriority:    0
+    CaseSensitive:   false
+  - Regex:           '^(<|"(gtest|gmock|isl|json)/)'
+    Priority:        3
+    SortPriority:    0
+    CaseSensitive:   false
+  - Regex:           '.*'
+    Priority:        1
+    SortPriority:    0
+    CaseSensitive:   false
+IncludeIsMainRegex: '(Test)?$'
+IncludeIsMainSourceRegex: ''
+IndentAccessModifiers: false
+IndentCaseBlocks: false
+IndentCaseLabels: true
+IndentExternBlock: AfterExternBlock
+IndentGotoLabels: true
+IndentPPDirectives: None
+IndentRequiresClause: true
+IndentWidth:     2
+IndentWrappedFunctionNames: false
+InsertBraces:    false
+InsertNewlineAtEOF: false
+InsertTrailingCommas: None
+IntegerLiteralSeparator:
+  Binary:          0
+  BinaryMinDigits: 0
+  Decimal:         0
+  DecimalMinDigits: 0
+  Hex:             0
+  HexMinDigits:    0
+JavaScriptQuotes: Leave
+JavaScriptWrapImports: true
+KeepEmptyLinesAtTheStartOfBlocks: true
+LambdaBodyIndentation: Signature
+LineEnding:      DeriveLF
+MacroBlockBegin: ''
+MacroBlockEnd:   ''
+MaxEmptyLinesToKeep: 1
+NamespaceIndentation: None
+ObjCBinPackProtocolList: Auto
+ObjCBlockIndentWidth: 2
+ObjCBreakBeforeNestedBlockParam: true
+ObjCSpaceAfterProperty: true
+ObjCSpaceBeforeProtocolList: false
+PackConstructorInitializers: BinPack
+PenaltyBreakAssignment: 2
+PenaltyBreakBeforeFirstCallParameter: 19
+PenaltyBreakComment: 300
+PenaltyBreakFirstLessLess: 120
+PenaltyBreakOpenParenthesis: 0
+PenaltyBreakString: 1000
+PenaltyBreakTemplateDeclaration: 10
+PenaltyExcessCharacter: 1000000
+PenaltyIndentedWhitespace: 0
+PenaltyReturnTypeOnItsOwnLine: 200
+PointerAlignment: Left
+PPIndentWidth:   -1
+QualifierAlignment: Leave
+ReferenceAlignment: Pointer
+ReflowComments:  true
+RemoveBracesLLVM: false
+RemoveSemicolon: false
+RequiresClausePosition: OwnLine
+RequiresExpressionIndentation: OuterScope
+SeparateDefinitionBlocks: Leave
+ShortNamespaceLines: 1
+SortIncludes:    CaseSensitive
+SortJavaStaticImport: Before
+SortUsingDeclarations: LexicographicNumeric
+SpaceAfterCStyleCast: false
+SpaceAfterLogicalNot: false
+SpaceAfterTemplateKeyword: false
+SpaceAroundPointerQualifiers: Default
+SpaceBeforeAssignmentOperators: true
+SpaceBeforeCaseColon: false
+SpaceBeforeCpp11BracedList: false
+SpaceBeforeCtorInitializerColon: true
+SpaceBeforeInheritanceColon: true
+SpaceBeforeParens: ControlStatements
+SpaceBeforeParensOptions:
+  AfterControlStatements: true
+  AfterForeachMacros: true
+  AfterFunctionDefinitionName: false
+  AfterFunctionDeclarationName: false
+  AfterIfMacros:   true
+  AfterOverloadedOperator: false
+  AfterRequiresInClause: false
+  AfterRequiresInExpression: false
+  BeforeNonEmptyParentheses: false
+SpaceBeforeRangeBasedForLoopColon: true
+SpaceBeforeSquareBrackets: false
+SpaceInEmptyBlock: false
+SpaceInEmptyParentheses: false
+SpacesBeforeTrailingComments: 1
+SpacesInAngles:  Never
+SpacesInConditionalStatement: false
+SpacesInContainerLiterals: true
+SpacesInCStyleCastParentheses: false
+SpacesInLineCommentPrefix:
+  Minimum:         1
+  Maximum:         -1
+SpacesInParentheses: false
+SpacesInSquareBrackets: false
+Standard:        Latest
+StatementAttributeLikeMacros:
+  - Q_EMIT
+StatementMacros:
+  - Q_UNUSED
+  - QT_REQUIRE_VERSION
+TabWidth:        8
+UseTab:          Never
+WhitespaceSensitiveMacros:
+  - BOOST_PP_STRINGIZE
+  - CF_SWIFT_NAME
+  - NS_SWIFT_NAME
+  - PP_STRINGIZE
+  - STRINGIZE
+...
+
diff --git a/.gitignore b/.gitignore
index d036a8b..1f45623 100644
--- a/.gitignore
+++ b/.gitignore
@@ -25,3 +25,5 @@ barrier
 .DS_Store
 *.dSYM
 *.pcap
+riscv/
+compile_flags.txt
diff --git a/Makefile b/Makefile
index da43ce2..561311a 100644
--- a/Makefile
+++ b/Makefile
@@ -13,12 +13,14 @@ OBJS = \
   $K/kalloc.o \
   $K/string.o \
   $K/main.o \
+  $K/cow.o \
   $K/vm.o \
   $K/proc.o \
   $K/swtch.o \
   $K/trampoline.o \
   $K/trap.o \
   $K/syscall.o \
+  $K/sysinfo.o \
   $K/sysproc.o \
   $K/bio.o \
   $K/fs.o \
@@ -90,7 +92,7 @@ CFLAGS = -Wall -Werror -O -fno-omit-frame-pointer -ggdb -gdwarf-2
 
 ifdef LAB
 LABUPPER = $(shell echo $(LAB) | tr a-z A-Z)
-XCFLAGS += -DSOL_$(LABUPPER) -DLAB_$(LABUPPER)
+XCFLAGS += -DSOL_$(LABUPPER) -DLAB_$(LABUPPER) -DLAB_PGTBL
 endif
 
 CFLAGS += $(XCFLAGS)
@@ -188,6 +190,13 @@ UPROGS=\
 	$U/_grind\
 	$U/_wc\
 	$U/_zombie\
+	$U/_sleep\
+	$U/_pingpong\
+	$U/_primes\
+	$U/_find\
+	$U/_xargs\
+	$U/_trace\
+	$U/_sysinfotest\
 
 
 
@@ -197,21 +206,18 @@ UPROGS += \
 	$U/_stats
 endif
 
-ifeq ($(LAB),traps)
 UPROGS += \
 	$U/_call\
-	$U/_bttest
-endif
+	$U/_bttest\
+	$U/_alarmtest
 
 ifeq ($(LAB),lazy)
 UPROGS += \
 	$U/_lazytests
 endif
 
-ifeq ($(LAB),cow)
 UPROGS += \
 	$U/_cowtest
-endif
 
 ifeq ($(LAB),thread)
 UPROGS += \
@@ -231,10 +237,8 @@ barrier: notxv6/barrier.c
 	gcc -o barrier -g -O2 $(XCFLAGS) notxv6/barrier.c -pthread
 endif
 
-ifeq ($(LAB),pgtbl)
 UPROGS += \
 	$U/_pgtbltest
-endif
 
 ifeq ($(LAB),lock)
 UPROGS += \
@@ -255,9 +259,7 @@ UPROGS += \
 endif
 
 UEXTRA=
-ifeq ($(LAB),util)
-	UEXTRA += user/xargstest.sh
-endif
+UEXTRA += user/xargstest.sh
 
 
 fs.img: mkfs/mkfs README $(UEXTRA) $(UPROGS)
@@ -266,11 +268,13 @@ fs.img: mkfs/mkfs README $(UEXTRA) $(UPROGS)
 -include kernel/*.d user/*.d
 
 clean:
-	rm -rf *.tex *.dvi *.idx *.aux *.log *.ind *.ilg *.dSYM *.zip *.pcap \
+	rm -f *.tex *.dvi *.idx *.aux *.log *.ind *.ilg *.dSYM *.zip \
 	*/*.o */*.d */*.asm */*.sym \
-	$U/initcode $U/initcode.out $U/usys.S $U/_* \
-	$K/kernel \
-	mkfs/mkfs fs.img .gdbinit __pycache__ xv6.out* \
+	$U/initcode $U/initcode.out $K/kernel fs.img \
+	mkfs/mkfs .gdbinit \
+        $U/usys.S \
+	$(UPROGS) \
+	*.zip \
 	ph barrier
 
 # try to generate a unique GDB port
diff --git a/answers-pgtbl.txt b/answers-pgtbl.txt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/answers-pgtbl.txt
diff --git a/answers-syscall.txt b/answers-syscall.txt
new file mode 100644
index 0000000..ca0ce77
--- /dev/null
+++ b/answers-syscall.txt
@@ -0,0 +1,9 @@
+usertrap()
+7
+SYS_exec
+user mode
+lw	a3,0(zero)
+a3
+no
+yes, `scause` 0xd -> interrupt = 0, exception code = 13 -> instruction page fault (riscv-priviledged p71)
+'initcode', 1
diff --git a/answers-traps.txt b/answers-traps.txt
new file mode 100644
index 0000000..dbb40f1
--- /dev/null
+++ b/answers-traps.txt
@@ -0,0 +1,6 @@
+a0~a7, a2
+0x26 (optimized during compile time), 0x14 (inline, unfolded)
+0x666
+0x38 (next instruction)
+He110 World
+the value in register a2
diff --git a/grade-lab-cow b/grade-lab-cow
new file mode 100755
index 0000000..08b6990
--- /dev/null
+++ b/grade-lab-cow
@@ -0,0 +1,55 @@
+#!/usr/bin/env python3
+
+import re
+from gradelib import *
+
+r = Runner(save("xv6.out"))
+
+@test(0, "running cowtest")
+def test_cowtest():
+    r.run_qemu(shell_script([
+        'cowtest'
+    ]))
+
+@test(30, "simple", parent=test_cowtest)
+def test_simple():
+    matches = re.findall("^simple: ok$", r.qemu.output, re.M)
+    assert_equal(len(matches), 2, "Number of appearances of 'simple: ok'")
+
+@test(30, "three", parent=test_cowtest)
+def test_three():
+    matches = re.findall("^three: ok$", r.qemu.output, re.M)
+    assert_equal(len(matches), 3, "Number of appearances of 'three: ok'")
+
+@test(20, "file", parent=test_cowtest)
+def test_file():
+    r.match('^file: ok$')
+
+@test(0, "usertests")
+def test_usertests():
+    r.run_qemu(shell_script([
+        'usertests -q'
+    ]), timeout=1000)
+    r.match('^ALL TESTS PASSED$')
+
+def usertest_check(testcase, nextcase, output):
+    if not re.search(r'\ntest {}: [\s\S]*OK\ntest {}'.format(testcase, nextcase), output):
+        raise AssertionError('Failed ' + testcase)
+
+@test(5, "usertests: copyin", parent=test_usertests)
+def test_sbrkbugs():
+    usertest_check("copyin", "copyout", r.qemu.output)
+
+@test(5, "usertests: copyout", parent=test_usertests)
+def test_sbrkbugs():
+    usertest_check("copyout", "copyinstr1", r.qemu.output)
+
+@test(19, "usertests: all tests", parent=test_usertests)
+def test_usertests_all():
+    r.match('^ALL TESTS PASSED$')
+
+@test(1, "time")
+def test_time():
+    check_time()
+
+run_tests()
diff --git a/grade-lab-pgtbl b/grade-lab-pgtbl
new file mode 100755
index 0000000..121bb3b
--- /dev/null
+++ b/grade-lab-pgtbl
@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+
+import re
+from gradelib import *
+
+r = Runner(save("xv6.out"))
+
+PTE_PRINT = """page table 0x0000000087f6b000
+ ..0: pte 0x0000000021fd9c01 pa 0x0000000087f67000
+ .. ..0: pte 0x0000000021fd9801 pa 0x0000000087f66000
+ .. .. ..0: pte 0x0000000021fda01b pa 0x0000000087f68000
+ .. .. ..1: pte 0x0000000021fd9417 pa 0x0000000087f65000
+ .. .. ..2: pte 0x0000000021fd9007 pa 0x0000000087f64000
+ .. .. ..3: pte 0x0000000021fd8c17 pa 0x0000000087f63000
+ ..255: pte 0x0000000021fda801 pa 0x0000000087f6a000
+ .. ..511: pte 0x0000000021fda401 pa 0x0000000087f69000
+ .. .. ..509: pte 0x0000000021fdcc13 pa 0x0000000087f73000
+ .. .. ..510: pte 0x0000000021fdd007 pa 0x0000000087f74000
+ .. .. ..511: pte 0x0000000020001c0b pa 0x0000000080007000"""
+
+VAL_RE = "(0x00000000[0-9a-f]+)"
+INDENT_RE = r"\s*\.\.\s*"
+INDENT_ESC = "\\\s*\.\.\\\s*"
+
+@test(0, "pgtbltest")
+def test_pgtbltest():
+    r.run_qemu(shell_script([
+        'pgtbltest'
+    ]), timeout=300)
+
+@test(10, "pgtbltest: ugetpid", parent=test_pgtbltest)
+def test_nettest_():
+    r.match('^ugetpid_test: OK$')
+
+@test(10, "pgtbltest: pgaccess", parent=test_pgtbltest)
+def test_nettest_():
+    r.match('^pgaccess_test: OK$')
+
+@test(10, "pte printout")
+def test_pteprint():
+    first = True
+    r.run_qemu(shell_script([
+        'echo hi'
+    ]))
+    r.match('^hi')
+    p = re.compile(VAL_RE)
+    d = re.compile(INDENT_RE)
+    for l in PTE_PRINT.splitlines():
+        l = d.sub(INDENT_ESC, l)
+        l = p.sub(VAL_RE, l)
+        r.match(r'^{}$'.format(l))
+        if first:
+            first = False
+        else:
+            matches = re.findall(r'^{}$'.format(l), r.qemu.output, re.MULTILINE)
+            assert_equal(len(matches[0]), 2)
+            pa = (int(matches[0][0], 16) >> 10) << 12
+            assert_equal(int(matches[0][1], 16), pa)
+
+@test(5, "answers-pgtbl.txt")
+def test_answers():
+    # just a simple sanity check, will be graded manually
+    check_answers("answers-pgtbl.txt")
+
+@test(0, "usertests")
+def test_usertests():
+    r.run_qemu(shell_script([
+        'usertests -q'
+    ]), timeout=300)
+
+@test(10, "usertests: all tests", parent=test_usertests)
+def test_usertests():
+    r.match('^ALL TESTS PASSED$')
+
+@test(1, "time")
+def test_time():
+    check_time()
+
+run_tests()
diff --git a/grade-lab-traps b/grade-lab-traps
new file mode 100755
index 0000000..10613d3
--- /dev/null
+++ b/grade-lab-traps
@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+
+import os
+import re
+import subprocess
+from gradelib import *
+
+r = Runner(save("xv6.out"))
+
+@test(5, "answers-traps.txt")
+def test_answers():
+    # just a simple sanity check, will be graded manually
+    check_answers("answers-traps.txt")
+
+BACKTRACE_RE = r"^(0x000000008[0-9a-f]+)"
+
+def addr2line():
+    for f in ['riscv64-unknown-elf-addr2line', 'riscv64-linux-gnu-addr2line', 'addr2line', ]:
+        try:
+            devnull = open(os.devnull)
+            subprocess.Popen([f], stdout=devnull, stderr=devnull).communicate()
+            return f
+        except OSError:
+            continue
+    raise AssertionError('Cannot find the addr2line program')
+
+@test(10, "backtrace test")
+def test_backtracetest():
+    r.run_qemu(shell_script([
+        'bttest'
+    ]))
+    a2l = addr2line()
+    matches = re.findall(BACKTRACE_RE, r.qemu.output, re.MULTILINE)
+    assert_equal(len(matches), 3)
+    files = ['sysproc.c', 'syscall.c', 'trap.c']
+    for f, m in zip(files, matches):
+        result = subprocess.run([a2l, '-e', 'kernel/kernel', m], stdout=subprocess.PIPE)
+        if not f in result.stdout.decode("utf-8"):
+            raise AssertionError('Trace is incorrect; no %s' % f)
+
+@test(0, "running alarmtest")
+def test_alarmtest():
+    r.run_qemu(shell_script([
+        'alarmtest'
+    ]))
+
+@test(20, "alarmtest: test0", parent=test_alarmtest)
+def test_alarmtest_test0():
+    r.match('^test0 passed$')
+
+@test(20, "alarmtest: test1", parent=test_alarmtest)
+def test_alarmtest_test1():
+    r.match('^\\.?test1 passed$')
+
+@test(10, "alarmtest: test2", parent=test_alarmtest)
+def test_alarmtest_test2():
+    r.match('^\\.?test2 passed$')
+
+@test(10, "alarmtest: test3", parent=test_alarmtest)
+def test_alarmtest_test3():
+    r.match('^test3 passed$')
+
+@test(19, "usertests")
+def test_usertests():
+    r.run_qemu(shell_script([
+        'usertests -q'
+    ]), timeout=300)
+    r.match('^ALL TESTS PASSED$')
+
+@test(1, "time")
+def test_time():
+    check_time()
+
+run_tests()
diff --git a/kernel/cow.c b/kernel/cow.c
new file mode 100644
index 0000000..b3634fc
--- /dev/null
+++ b/kernel/cow.c
@@ -0,0 +1,30 @@
+// COW pagefault handler
+#include "types.h"
+#include "riscv.h"
+#include "defs.h"
+
+int
+cow_handler(pagetable_t pagetable, uint64 va)
+{
+  // you can't really write to rediculous pointers
+  if(va >= MAXVA || PGROUNDDOWN(va) == 0)
+    return -1;
+  pte_t *pte = walk(pagetable, va, 0);
+  if(pte == 0 || (*pte & PTE_V) == 0 || (*pte & PTE_U) == 0)
+    return -1;
+  if(*pte & PTE_C){
+    uint64 pa_orig = PTE2PA(*pte);
+    uint64 pa_new = (uint64)kalloc();
+    if(pa_new == 0){
+      printf("cow pagefault: kalloc failed\n");
+      return -1;
+    }
+    // copy the page and add write permission
+    memmove((void*)pa_new, (void*)pa_orig, PGSIZE);
+    uint64 flags = (PTE_FLAGS(*pte) | PTE_W) & ~PTE_C;
+    *pte = PA2PTE(pa_new) | flags;
+    kfree((void*)pa_orig);
+  } else if ((*pte & PTE_W) == 0)
+    return -1;
+  return 0;
+}
diff --git a/kernel/defs.h b/kernel/defs.h
index c6fef8c..02226b5 100644
--- a/kernel/defs.h
+++ b/kernel/defs.h
@@ -12,6 +12,7 @@ struct superblock;
 struct mbuf;
 struct sock;
 #endif
+struct sysinfo;
 
 // bio.c
 void            binit(void);
@@ -26,6 +27,9 @@ void            consoleinit(void);
 void            consoleintr(int);
 void            consputc(int);
 
+// cow.c
+int             cow_handler(pagetable_t, uint64);
+
 // exec.c
 int             exec(char*, char**);
 
@@ -64,9 +68,12 @@ void            ramdiskintr(void);
 void            ramdiskrw(struct buf*);
 
 // kalloc.c
+int             refcnt_inc(uint64);
+int             refcnt_dec(uint64);
 void*           kalloc(void);
 void            kfree(void *);
 void            kinit(void);
+int             get_freemem(void);
 
 // log.c
 void            initlog(int, struct superblock*);
@@ -84,6 +91,7 @@ int             pipewrite(struct pipe*, uint64, int);
 void            printf(char*, ...);
 void            panic(char*) __attribute__((noreturn));
 void            printfinit(void);
+void            backtrace(void);
 
 // proc.c
 int             cpuid(void);
@@ -110,6 +118,8 @@ void            yield(void);
 int             either_copyout(int user_dst, uint64 dst, void *src, uint64 len);
 int             either_copyin(void *dst, int user_src, uint64 src, uint64 len);
 void            procdump(void);
+int             get_nproc(void);
+int             pgaccess(uint64 base, int len, uint64 mask);
 
 // swtch.S
 void            swtch(struct context*, struct context*);
@@ -149,6 +159,9 @@ int             fetchstr(uint64, char*, int);
 int             fetchaddr(uint64, uint64*);
 void            syscall();
 
+// sysinfo.c
+int             sys_info(uint64);
+
 // trap.c
 extern uint     ticks;
 void            trapinit(void);
@@ -181,6 +194,7 @@ uint64          walkaddr(pagetable_t, uint64);
 int             copyout(pagetable_t, uint64, char *, uint64);
 int             copyin(pagetable_t, char *, uint64, uint64);
 int             copyinstr(pagetable_t, char *, uint64, uint64);
+void            vmprint(pagetable_t);
 
 // plic.c
 void            plicinit(void);
diff --git a/kernel/exec.c b/kernel/exec.c
index e18bbb6..35b35f5 100644
--- a/kernel/exec.c
+++ b/kernel/exec.c
@@ -128,6 +128,10 @@ exec(char *path, char **argv)
   p->trapframe->sp = sp; // initial stack pointer
   proc_freepagetable(oldpagetable, oldsz);
 
+  if(p->pid == 1){
+    vmprint(p->pagetable);
+  }
+
   return argc; // this ends up in a0, the first argument to main(argc, argv)
 
  bad:
diff --git a/kernel/kalloc.c b/kernel/kalloc.c
index 0699e7e..581f0f6 100644
--- a/kernel/kalloc.c
+++ b/kernel/kalloc.c
@@ -23,10 +23,41 @@ struct {
   struct run *freelist;
 } kmem;
 
+int phypg_refcnt[PHYSTOP/PGSIZE];
+
+// Increase the refcnt
+int
+refcnt_inc(uint64 pa)
+{
+  acquire(&kmem.lock);
+  int *prefcnt = &phypg_refcnt[pa/PGSIZE];
+  if(pa > PHYSTOP || *prefcnt < 1)
+    panic("increase refcnt");
+  (*prefcnt)++;
+  release(&kmem.lock);
+  return *prefcnt;
+}
+
+// Decrease the refcnt
+int
+refcnt_dec(uint64 pa)
+{
+  acquire(&kmem.lock);
+  int *prefcnt = &phypg_refcnt[pa/PGSIZE];
+  if(pa > PHYSTOP || *prefcnt < 1)
+    panic("decrease refcnt");
+  (*prefcnt)--;
+  release(&kmem.lock);
+  return *prefcnt;
+}
+
 void
 kinit()
 {
   initlock(&kmem.lock, "kmem");
+  // init all refcnt to 1, which would later be freed to 0 by kfree()
+  for(uint64 p = PGROUNDUP((uint64)end); p + PGSIZE <= PHYSTOP; p += PGSIZE)
+    phypg_refcnt[p/PGSIZE] = 1;
   freerange(end, (void*)PHYSTOP);
 }
 
@@ -51,6 +82,12 @@ kfree(void *pa)
   if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
     panic("kfree");
 
+  refcnt_dec((uint64)pa);
+
+  if(phypg_refcnt[(uint64)pa/PGSIZE] > 0)
+    // We still have refs to this phy page, do not actually free it
+    return;
+
   // Fill with junk to catch dangling refs.
   memset(pa, 1, PGSIZE);
 
@@ -72,11 +109,29 @@ kalloc(void)
 
   acquire(&kmem.lock);
   r = kmem.freelist;
-  if(r)
+  if(r){
+    if(phypg_refcnt[(uint64)r/PGSIZE])
+      panic("kalloc: invalid refcnt");
+    phypg_refcnt[(uint64)r/PGSIZE] = 1;
     kmem.freelist = r->next;
+  }
   release(&kmem.lock);
 
   if(r)
     memset((char*)r, 5, PGSIZE); // fill with junk
   return (void*)r;
 }
+
+int
+get_freemem(void)
+{
+  int n;
+  struct run *r;
+
+  acquire(&kmem.lock);
+  for (n = 0, r = kmem.freelist; r; r = r->next)
+    n++;
+  release(&kmem.lock);
+
+  return n * PGSIZE;
+}
diff --git a/kernel/printf.c b/kernel/printf.c
index 1a50203..509c1c5 100644
--- a/kernel/printf.c
+++ b/kernel/printf.c
@@ -122,6 +122,8 @@ panic(char *s)
   printf("panic: ");
   printf(s);
   printf("\n");
+  backtrace();
+
   panicked = 1; // freeze uart output from other CPUs
   for(;;)
     ;
@@ -133,3 +135,18 @@ printfinit(void)
   initlock(&pr.lock, "pr");
   pr.locking = 1;
 }
+
+void
+backtrace(void)
+{
+  uint64 fp = r_fp();
+  printf("backtrace:\n");
+  uint64 stackpg = PGROUNDDOWN(fp);
+  // Whereever fp points to should always live in the stack page
+  while(PGROUNDDOWN(fp) == stackpg){
+    // print the return addr (stored in fp-8)
+    printf("%p\n", *(uint64 *)(fp-8));
+    // load previous (upper stack) fp
+    fp = *(uint64 *)(fp-16);
+  }
+}
diff --git a/kernel/proc.c b/kernel/proc.c
index 58a8a0b..9a9bae9 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -39,6 +39,7 @@ proc_mapstacks(pagetable_t kpgtbl)
     if(pa == 0)
       panic("kalloc");
     uint64 va = KSTACK((int) (p - proc));
+  p->alarm_tickspassed = 0;
     kvmmap(kpgtbl, va, (uint64)pa, PGSIZE, PTE_R | PTE_W);
   }
 }
@@ -132,6 +133,25 @@ found:
     return 0;
   }
 
+  // Allocate a usyscall page and fill pid.
+  if((p->usyscall = (struct usyscall *)kalloc()) == 0){
+    freeproc(p);
+    release(&p->lock);
+    return 0;
+  }
+  p->usyscall->pid = p->pid;
+
+  // reset sigalarm props 
+  p->alarm_interval = 0;
+  p->alarm_handler = 0;
+  p->alarm_tickspassed = 0;
+  p->alarm_caninvoke = 1;
+  if((p->atpfm = (struct trapframe *)kalloc()) == 0){
+    freeproc(p);
+    release(&p->lock);
+    return 0;
+  }
+
   // An empty user page table.
   p->pagetable = proc_pagetable(p);
   if(p->pagetable == 0){
@@ -158,8 +178,18 @@ freeproc(struct proc *p)
   if(p->trapframe)
     kfree((void*)p->trapframe);
   p->trapframe = 0;
+  if(p->usyscall)
+    kfree((void*)p->usyscall);
+  p->usyscall = 0;
   if(p->pagetable)
     proc_freepagetable(p->pagetable, p->sz);
+  if(p->atpfm)
+    kfree((void*)p->atpfm);
+  p->atpfm = 0;
+  p->alarm_interval = 0;
+  p->alarm_handler = 0;
+  p->alarm_tickspassed = 0;
+  p->alarm_caninvoke = 1;
   p->pagetable = 0;
   p->sz = 0;
   p->pid = 0;
@@ -172,7 +202,7 @@ freeproc(struct proc *p)
 }
 
 // Create a user page table for a given process, with no user memory,
-// but with trampoline and trapframe pages.
+// but with trampoline, trapframe and usyscall pages.
 pagetable_t
 proc_pagetable(struct proc *p)
 {
@@ -202,6 +232,14 @@ proc_pagetable(struct proc *p)
     return 0;
   }
 
+  // map the usyscall page below the trapframe page, for
+  // ugetpid().
+  if(mappages(pagetable, USYSCALL, PGSIZE,
+              (uint64)(p->usyscall), PTE_R | PTE_U) < 0){
+    uvmunmap(pagetable, USYSCALL, 1, 0);
+    uvmfree(pagetable, 0);
+    return 0;
+  }
   return pagetable;
 }
 
@@ -212,6 +250,7 @@ proc_freepagetable(pagetable_t pagetable, uint64 sz)
 {
   uvmunmap(pagetable, TRAMPOLINE, 1, 0);
   uvmunmap(pagetable, TRAPFRAME, 1, 0);
+  uvmunmap(pagetable, USYSCALL, 1, 0);
   uvmfree(pagetable, sz);
 }
 
@@ -299,6 +338,9 @@ fork(void)
   // copy saved user registers.
   *(np->trapframe) = *(p->trapframe);
 
+  // inherit trace_mask
+  np->trace_mask = p->trace_mask;
+
   // Cause fork to return 0 in the child.
   np->trapframe->a0 = 0;
 
@@ -686,3 +728,43 @@ procdump(void)
     printf("\n");
   }
 }
+
+int
+get_nproc(void)
+{
+  int n = 0;
+  struct proc *p;
+
+  for(int i = 0; i < NPROC; i++) {
+    p = &proc[i];
+    acquire(&p->lock);
+    if(p->state != UNUSED)
+      n++;
+    release(&p->lock);
+  }
+
+  return n;
+}
+
+// lab pagetable: report which pages have been accessed (r/w)
+// according to PTE_A and store it in a bit mask (3rd param)
+int
+pgaccess(uint64 base, int len, uint64 mask_addr)
+{
+  struct proc *p = myproc();
+  pagetable_t pgtbl = p->pagetable;
+  pte_t *pte;
+  int mask = 0;
+  
+  // iterater thru pages
+  for(int i = 0; i < len; i++) {
+    pte = walk(pgtbl, base + i * PGSIZE, 0);
+    if(*pte & PTE_A) {
+      *pte &= (~PTE_A); // clear PTE_A to avoid setting it forever
+      mask |= (1L << i);
+    }
+  }
+
+  // now copyout the mask to user memory
+  return copyout(pgtbl, mask_addr, (char *)&mask, sizeof(mask));
+}
diff --git a/kernel/proc.h b/kernel/proc.h
index d021857..a195b02 100644
--- a/kernel/proc.h
+++ b/kernel/proc.h
@@ -91,6 +91,7 @@ struct proc {
   int killed;                  // If non-zero, have been killed
   int xstate;                  // Exit status to be returned to parent's wait
   int pid;                     // Process ID
+  int trace_mask;              // SYS_trace mask (1 << SYS_xxx)
 
   // wait_lock must be held when using this:
   struct proc *parent;         // Parent process
@@ -100,8 +101,14 @@ struct proc {
   uint64 sz;                   // Size of process memory (bytes)
   pagetable_t pagetable;       // User page table
   struct trapframe *trapframe; // data page for trampoline.S
+  struct usyscall *usyscall;   // data page for usyscall
   struct context context;      // swtch() here to run process
   struct file *ofile[NOFILE];  // Open files
   struct inode *cwd;           // Current directory
   char name[16];               // Process name (debugging)
+  int alarm_interval;          // sigalarm syscall interval
+  uint64 alarm_handler;        // sigalarm syscall handler
+  int alarm_tickspassed;       // record how many ticks passed since last sigalarm handler call
+  int alarm_caninvoke;         // prevent re-entrant calls to handler
+  struct trapframe *atpfm;     // trapframe to resume after handling, must hold p->lock
 };
diff --git a/kernel/riscv.h b/kernel/riscv.h
index 20a01db..7d6eb6e 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -327,6 +327,15 @@ sfence_vma()
   asm volatile("sfence.vma zero, zero");
 }
 
+// read the frame pointer of currently executing func
+static inline uint64
+r_fp()
+{
+  uint64 x;
+  asm volatile("mv %0, s0" : "=r" (x) );
+  return x;
+}
+
 typedef uint64 pte_t;
 typedef uint64 *pagetable_t; // 512 PTEs
 
@@ -343,6 +352,8 @@ typedef uint64 *pagetable_t; // 512 PTEs
 #define PTE_W (1L << 2)
 #define PTE_X (1L << 3)
 #define PTE_U (1L << 4) // user can access
+#define PTE_A (1L << 6) // riscv access bit                      
+#define PTE_C (1L << 8) // RSW low bit, use it to mark whether a page is COW
 
 // shift a physical address to the right place for a PTE.
 #define PA2PTE(pa) ((((uint64)pa) >> 12) << 10)
diff --git a/kernel/syscall.c b/kernel/syscall.c
index beea0ef..172c5ea 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -101,6 +101,17 @@ extern uint64 sys_unlink(void);
 extern uint64 sys_link(void);
 extern uint64 sys_mkdir(void);
 extern uint64 sys_close(void);
+extern uint64 sys_trace(void);
+extern uint64 sys_sysinfo(void);
+
+#ifdef LAB_NET
+extern uint64 sys_connect(void);
+#endif
+#ifdef LAB_PGTBL
+extern uint64 sys_pgaccess(void);
+#endif
+extern uint64 sys_sigalarm(void);
+extern uint64 sys_sigreturn(void);
 
 #ifdef LAB_NET
 extern uint64 sys_connect(void);
@@ -139,8 +150,46 @@ static uint64 (*syscalls[])(void) = {
 #ifdef LAB_PGTBL
 [SYS_pgaccess] sys_pgaccess,
 #endif
+[SYS_trace]   sys_trace,
+[SYS_sysinfo] sys_sysinfo,
+[SYS_sigalarm] sys_sigalarm,
+[SYS_sigreturn] sys_sigreturn,
 };
 
+// syscall name maps for SYS_trace:
+static char *syscall_names[] = {
+[SYS_fork]    "fork",
+[SYS_exit]    "exit",
+[SYS_wait]    "wait",
+[SYS_pipe]    "pipe",
+[SYS_read]    "read",
+[SYS_kill]    "kill",
+[SYS_exec]    "exec",
+[SYS_fstat]   "fstat",
+[SYS_chdir]   "chdir",
+[SYS_dup]     "dup",
+[SYS_getpid]  "getpid",
+[SYS_sbrk]    "sbrk",
+[SYS_sleep]   "sleep",
+[SYS_uptime]  "uptime",
+[SYS_open]    "open",
+[SYS_write]   "write",
+[SYS_mknod]   "mknod",
+[SYS_unlink]  "unlink",
+[SYS_link]    "link",
+[SYS_mkdir]   "mkdir",
+[SYS_close]   "close",
+#ifdef LAB_NET
+[SYS_connect] "connect",
+#endif
+#ifdef LAB_PGTBL
+[SYS_pgaccess] "pgaccess",
+#endif
+[SYS_trace]   "trace",
+[SYS_sysinfo] "sysinfo",
+[SYS_sigalarm]  "sigalarm",
+[SYS_sigreturn] "sigreturn",
+};
 
 
 void
@@ -154,9 +203,17 @@ syscall(void)
     // Use num to lookup the system call function for num, call it,
     // and store its return value in p->trapframe->a0
     p->trapframe->a0 = syscalls[num]();
+    
+    // SYS_trace: match all the syscalls which number < mask asked
+    // p->trace_mask == 1 << SYS_xxx
+    if(p->trace_mask >> num) {
+      printf("%d: syscall %s -> %d\n", p->pid, syscall_names[num], p->trapframe->a0);
+    }
+
   } else {
     printf("%d %s: unknown sys call %d\n",
             p->pid, p->name, num);
     p->trapframe->a0 = -1;
   }
 }
+
diff --git a/kernel/sysinfo.c b/kernel/sysinfo.c
new file mode 100644
index 0000000..c66324d
--- /dev/null
+++ b/kernel/sysinfo.c
@@ -0,0 +1,24 @@
+#include "types.h"
+#include "riscv.h"
+#include "param.h"
+#include "spinlock.h"
+#include "defs.h"
+#include "sysinfo.h"
+#include "proc.h"
+
+// Get current system info
+// addr is a user virtual address, pointing to a struct sysinfo.
+int
+sys_info(uint64 addr) {
+  struct proc *p = myproc();
+  struct sysinfo info;
+
+  // Fill nums into the sysinfo struct
+  info.freemem = get_freemem();
+  info.nproc = get_nproc();
+
+  if(copyout(p->pagetable, addr, (char *)&info, sizeof(info)) < 0)
+    return -1;
+  return 0;
+}
+
diff --git a/kernel/sysinfo.h b/kernel/sysinfo.h
new file mode 100644
index 0000000..fb878e6
--- /dev/null
+++ b/kernel/sysinfo.h
@@ -0,0 +1,4 @@
+struct sysinfo {
+  uint64 freemem;   // amount of free memory (bytes)
+  uint64 nproc;     // number of process
+};
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index 3b4d5bd..715a511 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -1,7 +1,7 @@
 #include "types.h"
 #include "riscv.h"
-#include "defs.h"
 #include "param.h"
+#include "defs.h"
 #include "memlayout.h"
 #include "spinlock.h"
 #include "proc.h"
@@ -54,9 +54,8 @@ sys_sleep(void)
   int n;
   uint ticks0;
 
+
   argint(0, &n);
-  if(n < 0)
-    n = 0;
   acquire(&tickslock);
   ticks0 = ticks;
   while(ticks - ticks0 < n){
@@ -66,10 +65,29 @@ sys_sleep(void)
     }
     sleep(&ticks, &tickslock);
   }
+
+  // backtrace();
+
   release(&tickslock);
   return 0;
 }
 
+
+#ifdef LAB_PGTBL
+int
+sys_pgaccess(void)
+{
+  uint64 base, mask;
+  int len;
+
+  
+  argaddr(0, &base);
+  argint(1, &len);
+  argaddr(2, &mask);
+  return pgaccess(base, len, mask);
+}
+#endif
+
 uint64
 sys_kill(void)
 {
@@ -91,3 +109,44 @@ sys_uptime(void)
   release(&tickslock);
   return xticks;
 }
+
+uint64
+sys_trace(void)
+{
+  argint(0, &myproc()->trace_mask);
+
+  return -(myproc()->trace_mask <= 1);
+}
+
+uint64
+sys_sysinfo(void)
+{
+  uint64 si; // user pointer to struct sysinfo
+
+  argaddr(0, &si);
+  return sys_info(si);
+}
+
+uint64
+sys_sigalarm(void)
+{
+  struct proc *p = myproc();
+  uint64 handler;
+
+  argint(0, &p->alarm_interval);
+  argaddr(1, &handler);
+  p->alarm_handler = handler;
+
+  return 0;
+}
+
+uint64 sys_sigreturn(void)
+{
+  struct proc *p = myproc();
+  // retore saved trapframe to resume
+  memmove(p->trapframe, p->atpfm, sizeof(struct trapframe));
+  p->alarm_tickspassed = 0;
+  p->alarm_caninvoke = 1;
+  // make sure return the original a0 in trapframe to pass test3
+  return p->trapframe->a0;
+}
diff --git a/kernel/trap.c b/kernel/trap.c
index 5332dda..7cc69b1 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -6,6 +6,12 @@
 #include "proc.h"
 #include "defs.h"
 
+/*
+ * Always remember that RISC-V disables interrupts when it starts to take a trap,
+ * so there's no need to call intr_off() at the beginning of trap handling.
+ * Reference: xv6-riscv-book 4.5
+ */
+
 struct spinlock tickslock;
 uint ticks;
 
@@ -67,11 +73,18 @@ usertrap(void)
     syscall();
   } else if((which_dev = devintr()) != 0){
     // ok
+  } else if(r_scause() == 13 || r_scause() == 15){
+    // deal with page fault
+    uint64 va = r_stval();
+    if(cow_handler(p->pagetable, va) < 0)
+      goto err;
   } else {
 
     
     printf("usertrap(): unexpected scause %p pid=%d\n", r_scause(), p->pid);
     printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
+  err:
+    printf("killing the process...\n");
     setkilled(p);
   }
 
@@ -79,9 +92,22 @@ usertrap(void)
     exit(-1);
   
 
-  // give up the CPU if this is a timer interrupt.
-  if(which_dev == 2)
+  if(which_dev == 2){
+    // timer interrupt
+    if(p->alarm_interval > 0 && p->alarm_caninvoke){
+      // record sigalarm
+      p->alarm_tickspassed++;
+      if(p->alarm_tickspassed == p->alarm_interval){
+        // store original trapframe in p->atpfm
+        memmove(p->atpfm, p->trapframe, sizeof(struct trapframe));
+        p->alarm_tickspassed = 0;
+        p->alarm_caninvoke = 0;
+        p->trapframe->epc = p->alarm_handler;
+      }
+    }
+    // give up the CPU.
     yield();
+  }
 
   usertrapret();
 }
diff --git a/kernel/vm.c b/kernel/vm.c
index 7119242..be7d042 100644
--- a/kernel/vm.c
+++ b/kernel/vm.c
@@ -320,20 +320,26 @@ uvmcopy(pagetable_t old, pagetable_t new, uint64 sz)
   pte_t *pte;
   uint64 pa, i;
   uint flags;
-  char *mem;
+  // char *mem;
 
   for(i = 0; i < sz; i += PGSIZE){
     if((pte = walk(old, i, 0)) == 0)
       panic("uvmcopy: pte should exist");
     if((*pte & PTE_V) == 0)
       panic("uvmcopy: page not present");
-    pa = PTE2PA(*pte);
-    flags = PTE_FLAGS(*pte);
+    // do not do the actual copy, just increase the refcnt and mark pages readonly COW
+    /*
     if((mem = kalloc()) == 0)
       goto err;
     memmove(mem, (char*)pa, PGSIZE);
-    if(mappages(new, i, PGSIZE, (uint64)mem, flags) != 0){
-      kfree(mem);
+    */
+    *pte &= ~PTE_W;
+    *pte |= PTE_C;
+    pa = PTE2PA(*pte);
+    refcnt_inc(pa);
+    flags = PTE_FLAGS(*pte);
+    if(mappages(new, i, PGSIZE, (uint64)pa, flags) != 0){
+      // kfree(mem);
       goto err;
     }
   }
@@ -364,22 +370,21 @@ int
 copyout(pagetable_t pagetable, uint64 dstva, char *src, uint64 len)
 {
   uint64 n, va0, pa0;
-  pte_t *pte;
+  // pte_t *pte;
 
   while(len > 0){
     va0 = PGROUNDDOWN(dstva);
-    if (va0 >= MAXVA)
-      return -1;
-    if((pte = walk(pagetable, va0, 0)) == 0) {
-      // printf("copyout: pte should exist 0x%x %d\n", dstva, len);
-      return -1;
-    }
-
 
-    // forbid copyout over read-only user text pages.
-    if((*pte & PTE_W) == 0)
+    if(cow_handler(pagetable, va0) < 0)
       return -1;
     
+    /*
+    pte = walk(pagetable, va0, 0);
+    if(pte == 0 || (*pte & PTE_V) == 0 || (*pte & PTE_U) == 0 ||
+       (*pte & PTE_W) == 0)
+      return -1;
+    pa0 = PTE2PA(*pte);
+    */
     pa0 = walkaddr(pagetable, va0);
     if(pa0 == 0)
       return -1;
@@ -463,5 +468,29 @@ copyinstr(pagetable_t pagetable, char *dst, uint64 srcva, uint64 max)
   }
 }
 
+static void
+walkprint(pagetable_t pgtbl, int level)
+{
+  for(int i = 0; i < 512; i++){
+    pte_t pte = pgtbl[i];
+    if(pte & PTE_V){
+      for(int j = 0; j < level; j++){
+        printf(" ..");
+      }
+      printf("%d: pte %p pa %p\n", i, pte, PTE2PA(pte));
+      if((pte & (PTE_R|PTE_W|PTE_X)) == 0){
+        // this PTE points to a lower-level page table.
+        walkprint((pagetable_t)PTE2PA(pte), level+1);
+      }
+    }
+  }
+}
 
+// Print the contents of a page table
+void
+vmprint(pagetable_t pgtbl)
+{
+  printf("page table %p\n", pgtbl);
 
+  walkprint(pgtbl, 1);
+}
diff --git a/time.txt b/time.txt
new file mode 100644
index 0000000..00750ed
--- /dev/null
+++ b/time.txt
@@ -0,0 +1 @@
+3
diff --git a/user/alarmtest.c b/user/alarmtest.c
new file mode 100644
index 0000000..b8d85f7
--- /dev/null
+++ b/user/alarmtest.c
@@ -0,0 +1,193 @@
+//
+// test program for the alarm lab.
+// you can modify this file for testing,
+// but please make sure your kernel
+// modifications pass the original
+// versions of these tests.
+//
+
+#include "kernel/param.h"
+#include "kernel/types.h"
+#include "kernel/stat.h"
+#include "kernel/riscv.h"
+#include "user/user.h"
+
+void test0();
+void test1();
+void test2();
+void test3();
+void periodic();
+void slow_handler();
+void dummy_handler();
+
+int
+main(int argc, char *argv[])
+{
+  test0();
+  test1();
+  test2();
+  test3();
+  exit(0);
+}
+
+volatile static int count;
+
+void
+periodic()
+{
+  count = count + 1;
+  printf("alarm!\n");
+  sigreturn();
+}
+
+// tests whether the kernel calls
+// the alarm handler even a single time.
+void
+test0()
+{
+  int i;
+  printf("test0 start\n");
+  count = 0;
+  sigalarm(2, periodic);
+  for(i = 0; i < 1000*500000; i++){
+    if((i % 1000000) == 0)
+      write(2, ".", 1);
+    if(count > 0)
+      break;
+  }
+  sigalarm(0, 0);
+  if(count > 0){
+    printf("test0 passed\n");
+  } else {
+    printf("\ntest0 failed: the kernel never called the alarm handler\n");
+  }
+}
+
+void __attribute__ ((noinline)) foo(int i, int *j) {
+  if((i % 2500000) == 0) {
+    write(2, ".", 1);
+  }
+  *j += 1;
+}
+
+//
+// tests that the kernel calls the handler multiple times.
+//
+// tests that, when the handler returns, it returns to
+// the point in the program where the timer interrupt
+// occurred, with all registers holding the same values they
+// held when the interrupt occurred.
+//
+void
+test1()
+{
+  int i;
+  int j;
+
+  printf("test1 start\n");
+  count = 0;
+  j = 0;
+  sigalarm(2, periodic);
+  for(i = 0; i < 500000000; i++){
+    if(count >= 10)
+      break;
+    foo(i, &j);
+  }
+  if(count < 10){
+    printf("\ntest1 failed: too few calls to the handler\n");
+  } else if(i != j){
+    // the loop should have called foo() i times, and foo() should
+    // have incremented j once per call, so j should equal i.
+    // once possible source of errors is that the handler may
+    // return somewhere other than where the timer interrupt
+    // occurred; another is that that registers may not be
+    // restored correctly, causing i or j or the address ofj
+    // to get an incorrect value.
+    printf("\ntest1 failed: foo() executed fewer times than it was called\n");
+  } else {
+    printf("test1 passed\n");
+  }
+}
+
+//
+// tests that kernel does not allow reentrant alarm calls.
+void
+test2()
+{
+  int i;
+  int pid;
+  int status;
+
+  printf("test2 start\n");
+  if ((pid = fork()) < 0) {
+    printf("test2: fork failed\n");
+  }
+  if (pid == 0) {
+    count = 0;
+    sigalarm(2, slow_handler);
+    for(i = 0; i < 1000*500000; i++){
+      if((i % 1000000) == 0)
+        write(2, ".", 1);
+      if(count > 0)
+        break;
+    }
+    if (count == 0) {
+      printf("\ntest2 failed: alarm not called\n");
+      exit(1);
+    }
+    exit(0);
+  }
+  wait(&status);
+  if (status == 0) {
+    printf("test2 passed\n");
+  }
+}
+
+void
+slow_handler()
+{
+  count++;
+  printf("alarm!\n");
+  if (count > 1) {
+    printf("test2 failed: alarm handler called more than once\n");
+    exit(1);
+  }
+  for (int i = 0; i < 1000*500000; i++) {
+    asm volatile("nop"); // avoid compiler optimizing away loop
+  }
+  sigalarm(0, 0);
+  sigreturn();
+}
+
+//
+// dummy alarm handler; after running immediately uninstall
+// itself and finish signal handling
+void
+dummy_handler()
+{
+  sigalarm(0, 0);
+  sigreturn();
+}
+
+//
+// tests that the return from sys_sigreturn() does not
+// modify the a0 register
+void
+test3()
+{
+  uint64 a0;
+
+  sigalarm(1, dummy_handler);
+  printf("test3 start\n");
+
+  asm volatile("lui a5, 0");
+  asm volatile("addi a0, a5, 0xac" : : : "a0");
+  for(int i = 0; i < 500000000; i++)
+    ;
+  asm volatile("mv %0, a0" : "=r" (a0) );
+
+  if(a0 != 0xac)
+    printf("test3 failed: register a0 changed\n");
+  else
+    printf("test3 passed\n");
+}
diff --git a/user/bttest.c b/user/bttest.c
new file mode 100644
index 0000000..05405f9
--- /dev/null
+++ b/user/bttest.c
@@ -0,0 +1,10 @@
+#include "kernel/types.h"
+#include "kernel/stat.h"
+#include "user/user.h"
+
+int
+main(int argc, char *argv[])
+{
+  sleep(1);
+  exit(0);
+}
diff --git a/user/call.c b/user/call.c
new file mode 100644
index 0000000..f725dcb
--- /dev/null
+++ b/user/call.c
@@ -0,0 +1,17 @@
+#include "kernel/param.h"
+#include "kernel/types.h"
+#include "kernel/stat.h"
+#include "user/user.h"
+
+int g(int x) {
+  return x+3;
+}
+
+int f(int x) {
+  return g(x);
+}
+
+void main(void) {
+  printf("%d %d\n", f(8)+1, 13);
+  exit(0);
+}
diff --git a/user/cowtest.c b/user/cowtest.c
new file mode 100644
index 0000000..29b918f
--- /dev/null
+++ b/user/cowtest.c
@@ -0,0 +1,197 @@
+//
+// tests for copy-on-write fork() assignment.
+//
+
+#include "kernel/types.h"
+#include "kernel/memlayout.h"
+#include "user/user.h"
+
+// allocate more than half of physical memory,
+// then fork. this will fail in the default
+// kernel, which does not support copy-on-write.
+void
+simpletest()
+{
+  uint64 phys_size = PHYSTOP - KERNBASE;
+  int sz = (phys_size / 3) * 2;
+
+  printf("simple: ");
+  
+  char *p = sbrk(sz);
+  if(p == (char*)0xffffffffffffffffL){
+    printf("sbrk(%d) failed\n", sz);
+    exit(-1);
+  }
+
+  for(char *q = p; q < p + sz; q += 4096){
+    *(int*)q = getpid();
+  }
+
+  int pid = fork();
+  if(pid < 0){
+    printf("fork() failed\n");
+    exit(-1);
+  }
+
+  if(pid == 0)
+    exit(0);
+
+  wait(0);
+
+  if(sbrk(-sz) == (char*)0xffffffffffffffffL){
+    printf("sbrk(-%d) failed\n", sz);
+    exit(-1);
+  }
+
+  printf("ok\n");
+}
+
+// three processes all write COW memory.
+// this causes more than half of physical memory
+// to be allocated, so it also checks whether
+// copied pages are freed.
+void
+threetest()
+{
+  uint64 phys_size = PHYSTOP - KERNBASE;
+  int sz = phys_size / 4;
+  int pid1, pid2;
+
+  printf("three: ");
+  
+  char *p = sbrk(sz);
+  if(p == (char*)0xffffffffffffffffL){
+    printf("sbrk(%d) failed\n", sz);
+    exit(-1);
+  }
+
+  pid1 = fork();
+  if(pid1 < 0){
+    printf("fork failed\n");
+    exit(-1);
+  }
+  if(pid1 == 0){
+    pid2 = fork();
+    if(pid2 < 0){
+      printf("fork failed");
+      exit(-1);
+    }
+    if(pid2 == 0){
+      for(char *q = p; q < p + (sz/5)*4; q += 4096){
+        *(int*)q = getpid();
+      }
+      for(char *q = p; q < p + (sz/5)*4; q += 4096){
+        if(*(int*)q != getpid()){
+          printf("wrong content\n");
+          exit(-1);
+        }
+      }
+      exit(-1);
+    }
+    for(char *q = p; q < p + (sz/2); q += 4096){
+      *(int*)q = 9999;
+    }
+    exit(0);
+  }
+
+  for(char *q = p; q < p + sz; q += 4096){
+    *(int*)q = getpid();
+  }
+
+  wait(0);
+
+  sleep(1);
+
+  for(char *q = p; q < p + sz; q += 4096){
+    if(*(int*)q != getpid()){
+      printf("wrong content\n");
+      exit(-1);
+    }
+  }
+
+  if(sbrk(-sz) == (char*)0xffffffffffffffffL){
+    printf("sbrk(-%d) failed\n", sz);
+    exit(-1);
+  }
+
+  printf("ok\n");
+}
+
+char junk1[4096];
+int fds[2];
+char junk2[4096];
+char buf[4096];
+char junk3[4096];
+
+// test whether copyout() simulates COW faults.
+void
+filetest()
+{
+  printf("file: ");
+  
+  buf[0] = 99;
+
+  for(int i = 0; i < 4; i++){
+    if(pipe(fds) != 0){
+      printf("pipe() failed\n");
+      exit(-1);
+    }
+    int pid = fork();
+    if(pid < 0){
+      printf("fork failed\n");
+      exit(-1);
+    }
+    if(pid == 0){
+      sleep(1);
+      if(read(fds[0], buf, sizeof(i)) != sizeof(i)){
+        printf("error: read failed\n");
+        exit(1);
+      }
+      sleep(1);
+      int j = *(int*)buf;
+      if(j != i){
+        printf("error: read the wrong value\n");
+        exit(1);
+      }
+      exit(0);
+    }
+    if(write(fds[1], &i, sizeof(i)) != sizeof(i)){
+      printf("error: write failed\n");
+      exit(-1);
+    }
+  }
+
+  int xstatus = 0;
+  for(int i = 0; i < 4; i++) {
+    wait(&xstatus);
+    if(xstatus != 0) {
+      exit(1);
+    }
+  }
+
+  if(buf[0] != 99){
+    printf("error: child overwrote parent\n");
+    exit(1);
+  }
+
+  printf("ok\n");
+}
+
+int
+main(int argc, char *argv[])
+{
+  simpletest();
+
+  // check that the first simpletest() freed the physical memory.
+  simpletest();
+
+  threetest();
+  threetest();
+  threetest();
+
+  filetest();
+
+  printf("ALL COW TESTS PASSED\n");
+
+  exit(0);
+}
diff --git a/user/find.c b/user/find.c
new file mode 100644
index 0000000..e185e9d
--- /dev/null
+++ b/user/find.c
@@ -0,0 +1,84 @@
+#include "kernel/types.h"
+
+#include "kernel/fcntl.h"
+#include "kernel/fs.h"
+#include "kernel/stat.h"
+#include "user/user.h"
+
+char*
+fmtname(char* path)
+{
+  char* p;
+
+  // Find first character after last slash.
+  for (p = path + strlen(path); p >= path && *p != '/'; p--)
+    ;
+  p++;
+  return p;
+}
+
+void
+find(char* root_path, char* filename)
+{
+  static char buf[512];
+  char* p;
+  int fd;
+  struct dirent de;
+  struct stat st;
+
+  if ((fd = open(root_path, O_RDONLY)) < 0) {
+    fprintf(2, "find: cannot open %s\n", root_path);
+    return;
+  }
+  if (fstat(fd, &st) < 0) {
+    fprintf(2, "find: cannot stat %s\n", root_path);
+    close(fd);
+    return;
+  }
+
+  switch (st.type) {
+    case T_FILE:
+      if (!strcmp(fmtname(root_path), filename)) {
+        printf("%s\n", root_path);
+      }
+      break;
+    case T_DIR:
+      if (strlen(root_path) + 1 + DIRSIZ + 1 > sizeof(buf)) {
+        printf("find: path too long\n");
+        break;
+      }
+
+      strcpy(buf, root_path);
+
+      p = buf + strlen(buf);
+      *p++ = '/';
+      while (read(fd, &de, sizeof(de)) == sizeof(de)) {
+        if (de.inum == 0)
+          continue;
+        memmove(p, de.name, DIRSIZ);
+        p[DIRSIZ] = '\0';
+
+        // printf("i'm finding %s!\n", fmtname(buf));
+
+        if (!strcmp(fmtname(buf), ".") || !strcmp(fmtname(buf), "..")) {
+          continue;
+        }
+
+        find(buf, filename);
+      }
+  }
+  close(fd);
+}
+
+int
+main(int argc, char* argv[])
+{
+  if (argc < 3) {
+    fprintf(2, "usage: find [root_path] filename...\n");
+    exit(1);
+  }
+
+  for (int i = 2; i < argc; i++) {
+    find(argv[1], argv[i]);
+  }
+}
diff --git a/user/pgtbltest.c b/user/pgtbltest.c
new file mode 100644
index 0000000..bce158a
--- /dev/null
+++ b/user/pgtbltest.c
@@ -0,0 +1,70 @@
+#include "kernel/param.h"
+#include "kernel/fcntl.h"
+#include "kernel/types.h"
+#include "kernel/riscv.h"
+#include "user/user.h"
+
+void ugetpid_test();
+void pgaccess_test();
+
+int
+main(int argc, char *argv[])
+{
+  ugetpid_test();
+  pgaccess_test();
+  printf("pgtbltest: all tests succeeded\n");
+  exit(0);
+}
+
+char *testname = "???";
+
+void
+err(char *why)
+{
+  printf("pgtbltest: %s failed: %s, pid=%d\n", testname, why, getpid());
+  exit(1);
+}
+
+void
+ugetpid_test()
+{
+  int i;
+
+  printf("ugetpid_test starting\n");
+  testname = "ugetpid_test";
+
+  for (i = 0; i < 64; i++) {
+    int ret = fork();
+    if (ret != 0) {
+      wait(&ret);
+      if (ret != 0)
+        exit(1);
+      continue;
+    }
+    if (getpid() != ugetpid())
+      err("missmatched PID");
+    exit(0);
+  }
+  printf("ugetpid_test: OK\n");
+}
+
+void
+pgaccess_test()
+{
+  char *buf;
+  unsigned int abits;
+  printf("pgaccess_test starting\n");
+  testname = "pgaccess_test";
+  buf = malloc(32 * PGSIZE);
+  if (pgaccess(buf, 32, &abits) < 0)
+    err("pgaccess failed");
+  buf[PGSIZE * 1] += 1;
+  buf[PGSIZE * 2] += 1;
+  buf[PGSIZE * 30] += 1;
+  if (pgaccess(buf, 32, &abits) < 0)
+    err("pgaccess failed");
+  if (abits != ((1 << 1) | (1 << 2) | (1 << 30)))
+    err("incorrect access bits set");
+  free(buf);
+  printf("pgaccess_test: OK\n");
+}
diff --git a/user/pingpong.c b/user/pingpong.c
index 6ed12e7..7b03a76 100644
--- a/user/pingpong.c
+++ b/user/pingpong.c
@@ -1,52 +1,44 @@
 #include "kernel/types.h"
-#include "kernel/stat.h"
 #include "user/user.h"
 
-#define N 5
-char buf[N];
+int
+main(int argc, char* argv[])
+{
+  int p[2];
 
-void
-pong(int *parent_to_child, int *child_to_parent) {
-  if (read(parent_to_child[0], buf, N) < 0) {
-    printf("read failed\n");
-  }
-  printf("%d: received %s\n", getpid(), buf);
-  if (write(child_to_parent[1], "pong", 4) != 4) {
-    printf("write failed\n");
+  if (argc > 1) {
+    fprintf(2, "usage: pingpong\n");
+    exit(1);
   }
-}
 
-void
-ping(int *parent_to_child, int *child_to_parent) {
-  
-  if (write(parent_to_child[1], "ping", 4) != 4) {
-    printf("write failed\n");
-  }
-  if (read(child_to_parent[0], buf, N) < 0) {
-    printf("read failed\n");
-  }
-  printf("%d: received %s\n", getpid(), buf);
-}
+  pipe(p);
 
-int
-main(int argc, char *argv[])
-{
-  int parent_to_child[2];
-  int child_to_parent[2];
+  int pid = fork();
 
-  int pid;
-
-  if (pipe(parent_to_child) < 0 || pipe(child_to_parent) < 0) {
-    printf("pipe failed\n");
-  }
-  if ((pid = fork()) < 0) {
-    printf("fork failed\n");
-  }
   if (pid == 0) {
-    pong(parent_to_child, child_to_parent);
-  } else {
-    ping(parent_to_child, child_to_parent);
+    short n;
+    read(p[0], &n, sizeof(n));
+    if (n == 42) {
+      fprintf(1, "%d: received ping\n", getpid());
+    }
+    n++;
+    write(p[1], &n, sizeof(n));
+    close(p[0]);
+    close(p[1]);
+    exit(0);
+  }
+
+  short n = 42;
+
+  write(p[1], &n, sizeof(n));
+  read(p[0], &n, sizeof(n));
+  if (n == 43) {
+    fprintf(1, "%d: received pong\n", getpid());
   }
-  
+  close(p[0]);
+  close(p[1]);
+
+  wait(0);
+
   exit(0);
 }
diff --git a/user/primes.c b/user/primes.c
new file mode 100644
index 0000000..b359524
--- /dev/null
+++ b/user/primes.c
@@ -0,0 +1,74 @@
+#include "kernel/types.h"
+#include "user/user.h"
+
+#define MAX 36
+#define FIRST_PRIME 2
+
+int
+generate_natural(); // -> out_fd
+int
+prime_filter(int in_fd, int prime); // -> out_fd
+
+int
+main(int argc, char* argv[])
+{
+  int prime;
+
+  int in = generate_natural();
+  while (read(in, &prime, sizeof(int))) {
+    // printf("prime %d: in_fd: %d\n", prime, in);  // debug
+    printf("prime %d\n", prime);
+    in = prime_filter(in, prime);
+  }
+
+  close(in);
+
+  exit(0);
+}
+
+int
+generate_natural()
+{
+  int out_pipe[2];
+
+  pipe(out_pipe);
+
+  if (!fork()) {
+    for (int i = FIRST_PRIME; i < MAX; i++) {
+      write(out_pipe[1], &i, sizeof(int));
+    }
+    close(out_pipe[1]);
+
+    exit(0);
+  }
+
+  close(out_pipe[1]);
+
+  return out_pipe[0];
+}
+
+int
+prime_filter(int in_fd, int prime)
+{
+  int num;
+  int out_pipe[2];
+
+  pipe(out_pipe);
+
+  if (!fork()) {
+    while (read(in_fd, &num, sizeof(int))) {
+      if (num % prime) {
+        write(out_pipe[1], &num, sizeof(int));
+      }
+    }
+    close(in_fd);
+    close(out_pipe[1]);
+
+    exit(0);
+  }
+
+  close(in_fd);
+  close(out_pipe[1]);
+
+  return out_pipe[0];
+}
diff --git a/user/sh.c b/user/sh.c
index 836ebcb..5eceda0 100644
--- a/user/sh.c
+++ b/user/sh.c
@@ -165,6 +165,9 @@ main(void)
         fprintf(2, "cannot cd %s\n", buf+3);
       continue;
     }
+    if(buf[0] == 'e' && buf[1] == 'x' && buf[2] == 'i' && buf[3] == 't'){
+      exit(0);
+    }
     if(fork1() == 0)
       runcmd(parsecmd(buf));
     wait(0);
diff --git a/user/sleep.c b/user/sleep.c
new file mode 100644
index 0000000..961f558
--- /dev/null
+++ b/user/sleep.c
@@ -0,0 +1,22 @@
+#include "kernel/types.h"
+#include "user/user.h"
+
+int
+main(int argc, char* argv[])
+{
+  uint sec = 0;
+
+  if (argc <= 1) {
+    fprintf(2, "usage: sleep [time (ticks)]\n");
+    exit(1);
+  }
+  sec = atoi(argv[1]);
+
+  sleep(sec);
+
+  if (argc <= 2) {
+    exit(0);
+  }
+
+  exit(0);
+}
diff --git a/user/sysinfotest.c b/user/sysinfotest.c
new file mode 100644
index 0000000..8a648a6
--- /dev/null
+++ b/user/sysinfotest.c
@@ -0,0 +1,153 @@
+#include "kernel/types.h"
+#include "kernel/riscv.h"
+#include "kernel/sysinfo.h"
+#include "user/user.h"
+
+
+void
+sinfo(struct sysinfo *info) {
+  if (sysinfo(info) < 0) {
+    printf("FAIL: sysinfo failed");
+    exit(1);
+  }
+}
+
+//
+// use sbrk() to count how many free physical memory pages there are.
+//
+int
+countfree()
+{
+  uint64 sz0 = (uint64)sbrk(0);
+  struct sysinfo info;
+  int n = 0;
+
+  while(1){
+    if((uint64)sbrk(PGSIZE) == 0xffffffffffffffff){
+      break;
+    }
+    n += PGSIZE;
+  }
+  sinfo(&info);
+  if (info.freemem != 0) {
+    printf("FAIL: there is no free mem, but sysinfo.freemem=%d\n",
+      info.freemem);
+    exit(1);
+  }
+  sbrk(-((uint64)sbrk(0) - sz0));
+  return n;
+}
+
+void
+testmem() {
+  struct sysinfo info;
+  uint64 n = countfree();
+  
+  sinfo(&info);
+
+  if (info.freemem!= n) {
+    printf("FAIL: free mem %d (bytes) instead of %d\n", info.freemem, n);
+    exit(1);
+  }
+  
+  if((uint64)sbrk(PGSIZE) == 0xffffffffffffffff){
+    printf("sbrk failed");
+    exit(1);
+  }
+
+  sinfo(&info);
+    
+  if (info.freemem != n-PGSIZE) {
+    printf("FAIL: free mem %d (bytes) instead of %d\n", n-PGSIZE, info.freemem);
+    exit(1);
+  }
+  
+  if((uint64)sbrk(-PGSIZE) == 0xffffffffffffffff){
+    printf("sbrk failed");
+    exit(1);
+  }
+
+  sinfo(&info);
+    
+  if (info.freemem != n) {
+    printf("FAIL: free mem %d (bytes) instead of %d\n", n, info.freemem);
+    exit(1);
+  }
+}
+
+void
+testcall() {
+  struct sysinfo info;
+  
+  if (sysinfo(&info) < 0) {
+    printf("FAIL: sysinfo failed\n");
+    exit(1);
+  }
+
+  if (sysinfo((struct sysinfo *) 0xeaeb0b5b00002f5e) !=  0xffffffffffffffff) {
+    printf("FAIL: sysinfo succeeded with bad argument\n");
+    exit(1);
+  }
+}
+
+void testproc() {
+  struct sysinfo info;
+  uint64 nproc;
+  int status;
+  int pid;
+  
+  sinfo(&info);
+  nproc = info.nproc;
+
+  pid = fork();
+  if(pid < 0){
+    printf("sysinfotest: fork failed\n");
+    exit(1);
+  }
+  if(pid == 0){
+    sinfo(&info);
+    if(info.nproc != nproc+1) {
+      printf("sysinfotest: FAIL nproc is %d instead of %d\n", info.nproc, nproc+1);
+      exit(1);
+    }
+    exit(0);
+  }
+  wait(&status);
+  sinfo(&info);
+  if(info.nproc != nproc) {
+      printf("sysinfotest: FAIL nproc is %d instead of %d\n", info.nproc, nproc);
+      exit(1);
+  }
+}
+
+void testbad() {
+  int pid = fork();
+  int xstatus;
+  
+  if(pid < 0){
+    printf("sysinfotest: fork failed\n");
+    exit(1);
+  }
+  if(pid == 0){
+      sinfo(0x0);
+      exit(0);
+  }
+  wait(&xstatus);
+  if(xstatus == -1)  // kernel killed child?
+    exit(0);
+  else {
+    printf("sysinfotest: testbad succeeded %d\n", xstatus);
+    exit(xstatus);
+  }
+}
+
+int
+main(int argc, char *argv[])
+{
+  printf("sysinfotest: start\n");
+  testcall();
+  testmem();
+  testproc();
+  printf("sysinfotest: OK\n");
+  exit(0);
+}
diff --git a/user/trace.c b/user/trace.c
new file mode 100644
index 0000000..dd77760
--- /dev/null
+++ b/user/trace.c
@@ -0,0 +1,27 @@
+#include "kernel/param.h"
+#include "kernel/types.h"
+#include "kernel/stat.h"
+#include "user/user.h"
+
+int
+main(int argc, char *argv[])
+{
+  int i;
+  char *nargv[MAXARG];
+
+  if(argc < 3 || (argv[1][0] < '0' || argv[1][0] > '9')){
+    fprintf(2, "Usage: %s mask command\n", argv[0]);
+    exit(1);
+  }
+
+  if (trace(atoi(argv[1])) < 0) {
+    fprintf(2, "%s: trace failed\n", argv[0]);
+    exit(1);
+  }
+  
+  for(i = 2; i < argc && i < MAXARG; i++){
+    nargv[i-2] = argv[i];
+  }
+  exec(nargv[0], nargv);
+  exit(0);
+}
diff --git a/user/ulib.c b/user/ulib.c
index c7b66c4..871adc9 100644
--- a/user/ulib.c
+++ b/user/ulib.c
@@ -1,8 +1,13 @@
 #include "kernel/types.h"
 #include "kernel/stat.h"
 #include "kernel/fcntl.h"
+#ifdef LAB_PGTBL
+#include "kernel/riscv.h"
+#include "kernel/memlayout.h"
+#endif
 #include "user/user.h"
 
+
 //
 // wrapper so that it's OK if main() does not call exit().
 //
@@ -145,3 +150,12 @@ memcpy(void *dst, const void *src, uint n)
 {
   return memmove(dst, src, n);
 }
+
+#ifdef LAB_PGTBL
+int
+ugetpid(void)
+{
+  struct usyscall *u = (struct usyscall *)USYSCALL;
+  return u->pid;
+}
+#endif
diff --git a/user/user.h b/user/user.h
index 16cf173..34591fd 100644
--- a/user/user.h
+++ b/user/user.h
@@ -1,4 +1,5 @@
 struct stat;
+struct sysinfo;
 
 // system calls
 int fork(void);
@@ -30,6 +31,10 @@ int pgaccess(void *base, int len, void *mask);
 // usyscall region
 int ugetpid(void);
 #endif
+int trace(int);
+int sysinfo(struct sysinfo*);
+int sigalarm(int ticks, void (*handler)());
+int sigreturn(void);
 
 // ulib.c
 int stat(const char*, struct stat*);
diff --git a/user/usys.pl b/user/usys.pl
index 6453fe9..33af0ad 100755
--- a/user/usys.pl
+++ b/user/usys.pl
@@ -36,5 +36,9 @@ entry("getpid");
 entry("sbrk");
 entry("sleep");
 entry("uptime");
+entry("trace");
+entry("sysinfo");
 entry("connect");
 entry("pgaccess");
+entry("sigalarm");
+entry("sigreturn");
diff --git a/user/xargs.c b/user/xargs.c
new file mode 100644
index 0000000..b5b9bee
--- /dev/null
+++ b/user/xargs.c
@@ -0,0 +1,60 @@
+#include "kernel/types.h"
+
+#include "kernel/param.h"
+#include "kernel/stat.h"
+#include "user/user.h"
+
+#define is_blank(chr) (chr == ' ' || chr == '\t')
+
+int
+main(int argc, char* argv[])
+{
+  char buf[2048], ch;
+  char* p = buf;
+  char* v[MAXARG];
+  int c;
+  int blanks = 0;
+  int offset = 0;
+
+  if (argc <= 1) {
+    fprintf(2, "usage: xargs <command> [argv...]\n");
+    exit(1);
+  }
+
+  for (c = 1; c < argc; c++) {
+    v[c - 1] = argv[c];
+  }
+  --c;
+
+  while (read(0, &ch, 1) > 0) {
+    if (is_blank(ch)) {
+      blanks++;
+      continue;
+    }
+
+    if (blanks) {
+      buf[offset++] = 0;
+
+      v[c++] = p;
+      p = buf + offset;
+
+      blanks = 0;
+    }
+
+    if (ch != '\n') {
+      buf[offset++] = ch;
+    } else {
+      v[c++] = p;
+      p = buf + offset;
+
+      if (!fork()) {
+        exit(exec(v[0], v));
+      }
+      wait(0);
+
+      c = argc - 1;
+    }
+  }
+
+  exit(0);
+}
diff --git a/user/xargstest.sh b/user/xargstest.sh
new file mode 100644
index 0000000..4362589
--- /dev/null
+++ b/user/xargstest.sh
@@ -0,0 +1,6 @@
+mkdir a
+echo hello > a/b
+mkdir c
+echo hello > c/b
+echo hello > b
+find . b | xargs grep hello