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- #include <asm.h>
-
- # Start the CPU: switch to 32-bit protected mode, jump into C.
- # The BIOS loads this code from the first sector of the hard disk into
- # memory at physical address 0x7c00 and starts executing in real mode
- # with %cs=0 %ip=7c00.
-
- .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
-
- # start address should be 0:7c00, in real mode, the beginning address of the running bootloader
- .globl start
- start:
- .code16 # Assemble for 16-bit mode
- cli # Disable interrupts
- cld # String operations increment
-
- # 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
-
- # Enable A20:
- # For backwards compatibility with the earliest PCs, physical
- # address line 20 is tied low, so that addresses higher than
- # 1MB wrap around to zero by default. This code undoes this.
- seta20.1:
- inb $0x64, %al # Wait for not busy(8042 input buffer empty).
- testb $0x2, %al
- jnz seta20.1
-
- movb $0xd1, %al # 0xd1 -> port 0x64
- outb %al, $0x64 # 0xd1 means: write data to 8042's P2 port
-
- seta20.2:
- inb $0x64, %al # Wait for not busy(8042 input buffer empty).
- testb $0x2, %al
- jnz seta20.2
-
- movb $0xdf, %al # 0xdf -> port 0x60
- outb %al, $0x60 # 0xdf = 11011111, means set P2's A20 bit(the 1 bit) to 1
-
- # 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_ON, %eax
- movl %eax, %cr0
-
- # 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
- 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
-
- # Set up the stack pointer and call into C. The stack region is from 0--start(0x7c00)
- movl $0x0, %ebp
- movl $start, %esp
- call bootmain
-
- # If bootmain returns (it shouldn't), loop.
- 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 for bootloader and kernel
- SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg for bootloader and kernel
-
- gdtdesc:
- .word 0x17 # sizeof(gdt) - 1
- .long gdt # address gdt
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