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89 lines
3.9 KiB
89 lines
3.9 KiB
#include <asm.h>
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# Start the 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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.set PROT_MODE_CSEG, 0x8 # kernel code segment selector
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.set PROT_MODE_DSEG, 0x10 # kernel data segment selector
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.set CR0_PE_ON, 0x1 # protected mode enable flag
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# start address should be 0:7c00, in real mode, the beginning address of the running bootloader
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.globl start
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start:
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.code16 # Assemble for 16-bit mode
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cli # Disable interrupts
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cld # String operations increment
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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(8042 input buffer empty).
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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 # 0xd1 means: write data to 8042's P2 port
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seta20.2:
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inb $0x64, %al # Wait for not busy(8042 input buffer empty).
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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 # 0xdf = 11011111, means set P2's A20 bit(the 1 bit) to 1
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movb $0xdf, %al # 0xdf -> port 0x60, do it again!
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outb %al, $0x60 # 0xdf = 11011111, means set P2's A20 bit(the 1 bit) to 1
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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_ON, %eax
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movl %eax, %cr0
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# Jump to next instruction, but in 32-bit code segment.
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# Switches processor into 32-bit mode.
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ljmp $PROT_MODE_CSEG, $protcseg
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.code32 # Assemble for 32-bit mode
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protcseg:
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# Set up the protected-mode data segment registers
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movw $PROT_MODE_DSEG, %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, %fs # -> FS
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movw %ax, %gs # -> GS
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movw %ax, %ss # -> SS: Stack Segment
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# Set up the stack pointer and call into C. The stack region is from 0--start(0x7c00)
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movl $0x0, %ebp
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movl $start, %esp
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call bootmain
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# If bootmain returns (it shouldn't), loop.
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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 for bootloader and kernel
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SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg for bootloader and kernel
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gdtdesc:
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.word 0x17 # sizeof(gdt) - 1
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.long gdt # address gdt
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