95 lines
3.1 KiB
ArmAsm
95 lines
3.1 KiB
ArmAsm
#include "asm.h"
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# Start the first CPU: switch to 32-bit protected mode, jump into C.
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# The BIOS loads this code from the first sector of the hard disk into
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# memory at physical address 0x7c00 and starts executing in real mode
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# with %cs=0 %ip=7c00.
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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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# Enable A20:
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# For backwards compatibility with the earliest PCs, physical
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# address line 20 is tied low, so that addresses higher than
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# 1MB wrap around to zero by default. This code undoes this.
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seta20.1:
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inb $0x64,%al # Wait for not busy
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testb $0x2,%al
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jnz seta20.1
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movb $0xd1,%al # 0xd1 -> port 0x64
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outb %al,$0x64
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seta20.2:
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inb $0x64,%al # Wait for not busy
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testb $0x2,%al
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jnz seta20.2
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movb $0xdf,%al # 0xdf -> port 0x60
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outb %al,$0x60
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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, %esp
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call bootmain
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# If bootmain 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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