xv6-cs450/bootother.S
Robert Morris 1afc9d3fca add some comments
find out the hard way why user and kernel must have separate segment descriptors
2010-08-05 21:16:55 -04:00

88 lines
3.1 KiB
ArmAsm

#include "asm.h"
# Each non-boot CPU ("AP") is started up in response to a STARTUP
# IPI from the boot CPU. Section B.4.2 of the Multi-Processor
# Specification says that the AP will start in real mode with CS:IP
# set to XY00:0000, where XY is an 8-bit value sent with the
# STARTUP. Thus this code must start at a 4096-byte boundary.
#
# Because this code sets DS to zero, it must sit
# at an address in the low 2^16 bytes.
#
# Bootothers (in main.c) sends the STARTUPs, one at a time.
# It puts this code (start) at 0x7000.
# It puts the correct %esp in start-4,
# and the place to jump to in start-8.
#
# This code is identical to bootasm.S except:
# - it does not need to enable A20
# - it uses the address at start-4 for the %esp
# - it jumps to the address at start-8 instead of calling bootmain
#define SEG_KCODE 1 // kernel code
#define SEG_KDATA 2 // kernel data+stack
#define CR0_PE 1 // protected mode enable bit
.code16 # Assemble for 16-bit mode
.globl start
start:
cli # Disable interrupts
# Set up the important data segment registers (DS, ES, SS).
xorw %ax,%ax # Segment number zero
movw %ax,%ds # -> Data Segment
movw %ax,%es # -> Extra Segment
movw %ax,%ss # -> Stack Segment
//PAGEBREAK!
# Switch from real to protected mode, using a bootstrap GDT
# and segment translation that makes virtual addresses
# identical to physical addresses, so that the
# effective memory map does not change during the switch.
lgdt gdtdesc
movl %cr0, %eax
orl $CR0_PE, %eax
movl %eax, %cr0
# This ljmp is how you load the CS (Code Segment) register.
# SEG_ASM produces segment descriptors with the 32-bit mode
# flag set (the D flag), so addresses and word operands will
# default to 32 bits after this jump.
ljmp $(SEG_KCODE<<3), $start32
.code32 # Assemble for 32-bit mode
start32:
# Set up the protected-mode data segment registers
movw $(SEG_KDATA<<3), %ax # Our data segment selector
movw %ax, %ds # -> DS: Data Segment
movw %ax, %es # -> ES: Extra Segment
movw %ax, %ss # -> SS: Stack Segment
movw $0, %ax # Zero segments not ready for use
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
# Set up the stack pointer and call into C.
movl start-4, %esp
call *(start-8)
# If the call returns (it shouldn't), trigger a Bochs
# breakpoint if running under Bochs, then loop.
movw $0x8a00, %ax # 0x8a00 -> port 0x8a00
movw %ax, %dx
outw %ax, %dx
movw $0x8ae0, %ax # 0x8ae0 -> port 0x8a00
outw %ax, %dx
spin:
jmp 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 (gdtdesc - gdt - 1) # sizeof(gdt) - 1
.long gdt # address gdt