From 01a6c054d548d9fff8bbdfac4d3f3de4ae8677a1 Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Wed, 7 Sep 2011 11:49:14 -0400 Subject: Remove web directory; all cruft or moved to 6.828 repo --- web/l-interrupt.html | 174 --------------------------------------------------- 1 file changed, 174 deletions(-) delete mode 100644 web/l-interrupt.html (limited to 'web/l-interrupt.html') diff --git a/web/l-interrupt.html b/web/l-interrupt.html deleted file mode 100644 index 363af5e..0000000 --- a/web/l-interrupt.html +++ /dev/null @@ -1,174 +0,0 @@ - -Lecture 6: Interrupts & Exceptions - - -

Interrupts & Exceptions

- -

-Required reading: xv6 trapasm.S, trap.c, syscall.c, usys.S. -
-You will need to consult -IA32 System -Programming Guide chapter 5 (skip 5.7.1, 5.8.2, 5.12.2). - -

Overview

- -

-Big picture: kernel is trusted third-party that runs the machine. -Only the kernel can execute privileged instructions (e.g., -changing MMU state). -The processor enforces this protection through the ring bits -in the code segment. -If a user application needs to carry out a privileged operation -or other kernel-only service, -it must ask the kernel nicely. -How can a user program change to the kernel address space? -How can the kernel transfer to a user address space? -What happens when a device attached to the computer -needs attention? -These are the topics for today's lecture. - -

-There are three kinds of events that must be handled -by the kernel, not user programs: -(1) a system call invoked by a user program, -(2) an illegal instruction or other kind of bad processor state (memory fault, etc.). -and -(3) an interrupt from a hardware device. - -

-Although these three events are different, they all use the same -mechanism to transfer control to the kernel. -This mechanism consists of three steps that execute as one atomic unit. -(a) change the processor to kernel mode; -(b) save the old processor somewhere (usually the kernel stack); -and (c) change the processor state to the values set up as -the “official kernel entry values.” -The exact implementation of this mechanism differs -from processor to processor, but the idea is the same. - -

-We'll work through examples of these today in lecture. -You'll see all three in great detail in the labs as well. - -

-A note on terminology: sometimes we'll -use interrupt (or trap) to mean both interrupts and exceptions. - -

-Setting up traps on the x86 -

- -

-See handout Table 5-1, Figure 5-1, Figure 5-2. - -

-xv6 Sheet 07: struct gatedesc and SETGATE. - -

-xv6 Sheet 28: tvinit and idtinit. -Note setting of gate for T_SYSCALL - -

-xv6 Sheet 29: vectors.pl (also see generated vectors.S). - -

-System calls -

- -

-xv6 Sheet 16: init.c calls open("console"). -How is that implemented? - -

-xv6 usys.S (not in book). -(No saving of registers. Why?) - -

-Breakpoint 0x1b:"open", -step past int instruction into kernel. - -

-See handout Figure 9-4 [sic]. - -

-xv6 Sheet 28: in vectors.S briefly, then in alltraps. -Step through to call trap, examine registers and stack. -How will the kernel find the argument to open? - -

-xv6 Sheet 29: trap, on to syscall. - -

-xv6 Sheet 31: syscall looks at eax, -calls sys_open. - -

-(Briefly) -xv6 Sheet 52: sys_open uses argstr and argint -to get its arguments. How do they work? - -

-xv6 Sheet 30: fetchint, fetcharg, argint, -argptr, argstr. - -

-What happens if a user program divides by zero -or accesses unmapped memory? -Exception. Same path as system call until trap. - -

-What happens if kernel divides by zero or accesses unmapped memory? - -

-Interrupts -

- -

-Like system calls, except: -devices generate them at any time, -there are no arguments in CPU registers, -nothing to return to, -usually can't ignore them. - -

-How do they get generated? -Device essentially phones up the -interrupt controller and asks to talk to the CPU. -Interrupt controller then buzzes the CPU and -tells it, “keyboard on line 1.” -Interrupt controller is essentially the CPU's -~~secretary~~ administrative assistant, -managing the phone lines on the CPU's behalf. - -

-Have to set up interrupt controller. - -

-(Briefly) xv6 Sheet 63: pic_init sets up the interrupt controller, -irq_enable tells the interrupt controller to let the given -interrupt through. - -

-(Briefly) xv6 Sheet 68: pit8253_init sets up the clock chip, -telling it to interrupt on IRQ_TIMER 100 times/second. -console_init sets up the keyboard, enabling IRQ_KBD. - -

-In Bochs, set breakpoint at 0x8:"vector0" -and continue, loading kernel. -Step through clock interrupt, look at -stack, registers. - -

-Was the processor executing in kernel or user mode -at the time of the clock interrupt? -Why? (Have any user-space instructions executed at all?) - -

-Can the kernel get an interrupt at any time? -Why or why not? cli and sti, -irq_enable. - - - -- cgit v1.2.3