89 lines
3.1 KiB
ArmAsm
89 lines
3.1 KiB
ArmAsm
#include "asm.h"
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# Each non-boot CPU ("AP") is started up in response to a STARTUP
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# IPI from the boot CPU. Section B.4.2 of the Multi-Processor
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# Specification says that the AP will start in real mode with CS:IP
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# set to XY00:0000, where XY is an 8-bit value sent with the
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# STARTUP. Thus this code must start at a 4096-byte boundary.
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#
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# Because this code sets DS to zero, it must sit
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# at an address in the low 2^16 bytes.
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#
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# Bootothers (in main.c) sends the STARTUPs, one at a time.
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# It puts this code (start) at 0x7000.
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# It puts the correct %esp in start-4,
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# and the place to jump to in start-8.
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#
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# This code is identical to bootasm.S except:
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# - 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
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#define SEG_KCODE 1 // kernel code
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#define SEG_KDATA 2 // kernel data+stack
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#define CR0_PE 1 // protected mode enable bit
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.code16 # Assemble for 16-bit mode
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.globl start
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start:
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cli # Disable interrupts
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# Set up the important data segment registers (DS, ES, SS).
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xorw %ax,%ax # Segment number zero
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movw %ax,%ds # -> Data Segment
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movw %ax,%es # -> Extra Segment
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movw %ax,%ss # -> Stack Segment
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//PAGEBREAK!
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# Switch from real to protected mode, using a bootstrap GDT
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# and segment translation that makes virtual addresses
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# identical to physical addresses, so that the
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# effective memory map does not change during the switch.
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lgdt gdtdesc
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movl %cr0, %eax
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orl $CR0_PE, %eax
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movl %eax, %cr0
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# This ljmp is how you load the CS (Code Segment) register.
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# SEG_ASM produces segment descriptors with the 32-bit mode
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# flag set (the D flag), so addresses and word operands will
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# default to 32 bits after this jump.
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ljmp $(SEG_KCODE<<3), $start32
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.code32 # Assemble for 32-bit mode
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start32:
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# Set up the protected-mode data segment registers
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movw $(SEG_KDATA<<3), %ax # Our data segment selector
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movw %ax, %ds # -> DS: Data Segment
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movw %ax, %es # -> ES: Extra Segment
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movw %ax, %ss # -> SS: Stack Segment
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movw $0, %ax # Zero segments not ready for use
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movw %ax, %fs # -> FS
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movw %ax, %gs # -> GS
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# Set up the stack pointer and call into C.
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movl start-4, %esp
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call *(start-8)
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# If the call returns (it shouldn't), trigger a Bochs
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# breakpoint if running under Bochs, then loop.
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movw $0x8a00, %ax # 0x8a00 -> port 0x8a00
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movw %ax, %dx
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outw %ax, %dx
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movw $0x8ae0, %ax # 0x8ae0 -> port 0x8a00
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outw %ax, %dx
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spin:
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jmp spin
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# Bootstrap GDT
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.p2align 2 # force 4 byte alignment
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gdt:
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SEG_NULLASM # null seg
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SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg
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SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg
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gdtdesc:
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.word (gdtdesc - gdt - 1) # sizeof(gdt) - 1
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.long gdt # address gdt
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