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Rework comments a bit.

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Move initialization of %sp after
protected mode jump and change to %esp.
This commit is contained in:
rsc 2007-08-24 18:52:59 +00:00
parent 2036534834
commit 5d1f4b8a3a
1 changed files with 25 additions and 29 deletions
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54
bootasm.S
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@ -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