xv6-cs450/bootother.S

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ArmAsm
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#include "asm.h"
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# 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.
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#
# 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.
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# 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
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# - it uses the address at start-4 for the %esp
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# - it jumps to the address at start-8 instead of calling bootmain
.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
start:
.code16 # Assemble for 16-bit mode
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cli # Disable interrupts
cld # String operations increment
# Set up the important data segment registers (DS, ES, SS).
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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 their physical addresses, so that the
# effective memory map does not change during the switch.
lgdt gdtdesc
movl %cr0, %eax
orl $CR0_PE_ON, %eax
movl %eax, %cr0
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# Jump to next instruction, but in 32-bit code segment.
# Switches processor into 32-bit mode.
ljmp $PROT_MODE_CSEG, $protcseg
.code32 # Assemble for 32-bit mode
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protcseg:
# Set up the protected-mode data segment registers
movw $PROT_MODE_DSEG, %ax # Our data segment selector
movw %ax, %ds # -> DS: Data Segment
movw %ax, %es # -> ES: Extra Segment
movw %ax, %fs # -> FS
movw %ax, %gs # -> GS
movw %ax, %ss # -> SS: Stack Segment
movl start-4, %esp
movl start-8, %eax
call *%eax
# If bootmain 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 $0x8e00, %ax # 0x8e00 -> port 0x8a00
outw %ax, %dx
spin:
jmp spin
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# Bootstrap GDT
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.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
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gdtdesc:
.word 0x17 # sizeof(gdt) - 1
.long gdt # address gdt