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authorrsc <rsc>2007-08-24 18:52:59 +0000
committerrsc <rsc>2007-08-24 18:52:59 +0000
commit5d1f4b8a3ae587c918293f79e44e87e6f185f7de (patch)
tree7a27202a6694787b272226621b0049b6ce4f5bcb
parent2036534834841641edf3a6d1e142c4798e146b7b (diff)
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Rework comments a bit.
Move initialization of %sp after protected mode jump and change to %esp.
-rw-r--r--bootasm.S54
1 files changed, 25 insertions, 29 deletions
diff --git a/bootasm.S b/bootasm.S
index 65026f0..c313cbb 100644
--- a/bootasm.S
+++ b/bootasm.S
@@ -1,27 +1,18 @@
#include "asm.h"
-.set PROT_MODE_CSEG,0x8 # code segment selector
-.set PROT_MODE_DSEG,0x10 # data segment selector
-.set CR0_PE_ON,0x1 # protected mode enable flag
+# Start the first CPU: switch to 32-bit protected mode, jump into C.
+# The BIOS loads this code from the first sector of the hard disk into
+# memory at physical address 0x7c00 and starts executing in real mode
+# with %cs=0 %ip=7c00.
-#########################################################################
-# ENTRY POINT for the bootstrap processor
-# This code should be stored in the first sector of the hard disk.
-# After the BIOS initializes the hardware on startup or system reset,
-# it loads this code at physical address 0x7c00 - 0x7d00 (512 bytes).
-# Then the BIOS jumps to the beginning of it, address 0x7c00,
-# while running in 16-bit real-mode (8086 compatibility mode).
-# The Code Segment register (CS) is initially zero on entry.
-#
-# This code switches into 32-bit protected mode so that all of
-# memory can accessed, then calls into C.
-#########################################################################
+.set PROT_MODE_CSEG, 0x8 # kernel code segment selector
+.set PROT_MODE_DSEG, 0x10 # kernel data segment selector
+.set CR0_PE_ON, 0x1 # protected mode enable flag
.globl start # Entry point
start:
.code16 # This runs in real mode
cli # Disable interrupts
- cld # String operations increment
# Set up the important data segment registers (DS, ES, SS).
xorw %ax,%ax # Segment number zero
@@ -29,26 +20,25 @@ start:
movw %ax,%es # -> Extra Segment
movw %ax,%ss # -> Stack Segment
- # Set up the stack pointer, growing downward from 0x7c00.
- movw $start,%sp # Stack Pointer
-
# Enable A20:
# For backwards compatibility with the earliest PCs, physical
# address line 20 is tied low, so that addresses higher than
# 1MB wrap around to zero by default. This code undoes this.
seta20.1:
- inb $0x64,%al # Get status
- testb $0x2,%al # Busy?
- jnz seta20.1 # Yes
- movb $0xd1,%al # Command: Write
- outb %al,$0x64 # output port
+ inb $0x64,%al # Wait for not busy
+ testb $0x2,%al
+ jnz seta20.1
+
+ movb $0xd1,%al # 0xd1 -> port 0x64
+ outb %al,$0x64
seta20.2:
- inb $0x64,%al # Get status
- testb $0x2,%al # Busy?
- jnz seta20.2 # Yes
- movb $0xdf,%al # Enable
- outb %al,$0x60 # A20
+ inb $0x64,%al # Wait for not busy
+ testb $0x2,%al
+ jnz seta20.2
+
+ movb $0xdf,%al # 0xdf -> port 0x60
+ outb %al,$0x60
# Switch from real to protected mode
# The descriptors in our GDT allow all physical memory to be accessed.
@@ -80,16 +70,22 @@ protcseg:
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
movw %ax, %ss # -> SS: Stack Segment
+
+ # Set up the stack pointer, growing downward from 0x7c00.
+ movl $start, %esp
+
call cmain # finish the boot load from C.
# cmain() should not return
spin:
jmp spin # ..but in case it does, spin
+# Bootstrap GDT
.p2align 2 # force 4 byte alignment
gdt:
SEG_NULLASM # null seg
SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
+
gdtdesc:
.word 0x17 # sizeof(gdt) - 1
.long gdt # address gdt