137 lines
2.6 KiB
NASM
137 lines
2.6 KiB
NASM
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; This boots *from a FAT partition*
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; So it starts at 0x7C00+90, the offset of
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; the boot code in a FAT VBR
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secreadcnt equ 0x38
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kernelreads equ 0x10000/(512*secreadcnt) ; 64K / Sector Size FIXME This underestimates when kernel size is not divisible by bytes per read
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[ORG 0x7c00+90]
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[BITS 16]
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xor ax, ax
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mov ds, ax
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mov es, ax
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mov ax, 0x8000
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mov ss, ax
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mov sp, 0xFF00
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; FIXME Assumes INT 13 Extension support
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mov ah, 0x42
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mov si, addr_packet
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int 0x13
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jc err
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xor cx, cx
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mov si, 0x7E00+0x1BE
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.find_active:
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lodsb
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bt ax, 7
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jc read
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add si, 15
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inc cl
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cmp cl, 4
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jl .find_active
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jmp err
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read:
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mov dh, cl ; Store active partition
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add si, 7
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mov di, addr_packet_start_block
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lodsw
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; Offset of kernel in partition
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add ax, 4
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stosw
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lodsw
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adc ax, 0
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stosw
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mov cx, kernelreads
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read_loop:
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xor ax, ax
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mov ah, 0x42
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mov si, addr_packet
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int 0x13
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jc err
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add word [addr_packet_transfer_buff_seg], (secreadcnt*512)/16
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add word [addr_packet_start_block], secreadcnt
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loop read_loop
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entry:
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cli ; no interrupts
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xor ax,ax
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mov ds, ax
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lgdt [gdt_desc] ; load gdt register
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mov eax, cr0 ; set pmode bit
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or al, 1
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mov cr0, eax
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jmp 08h:Pmode
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err:
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mov si, string
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mov cx, string_end - string
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print:
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push 0xb800
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pop es
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xor di, di
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mov ah, 0x7
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err_print:
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lodsb
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stosw
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loop err_print
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hlt_loop:
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hlt
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jmp hlt_loop
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[BITS 32]
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Pmode:
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mov eax, 0x10
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mov ds, ax
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mov es, ax
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mov fs, ax
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mov gs, ax
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mov ss, ax
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; check A20
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.a20:
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mov edi, 0x107C00
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mov esi, 0x007C00
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mov [esi], esi
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mov [edi], edi
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cmpsd
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jne .kernel
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; FIXME It's annoying that fast A20
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; will crash old systems, but it's
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; just so small...
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in al, 0x92 ; fast A20
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test al, 2
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jnz .fa20_end
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or al, 2
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and al, 0xFE
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out 0x92, al
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.fa20_end:
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jmp .a20
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.kernel:
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mov esi, 0x7E00
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mov edi, 0x100000
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mov ecx, 0x10000
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rep movsb
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xchg bx,bx
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jmp 08h:0x100000
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gdt_desc:
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dw gdt_end - gdt
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dd gdt
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gdt:
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gdt_null: dq 0
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gdt_code: dw 0xFFFF, 0 ; bits 0-15 limit (4GB), bits 0-15 base address
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db 0 ; bits 16-23 base address
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db 10011010b ; access byte
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db 11001111b ; bits 16-19 limit (4GB), 4 bits flags
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db 0 ; bits 24-31 base address
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gdt_data: dw 0xFFFF, 0 ; bits 0-15 limit (4GB), bits 0-15 base address
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db 0 ; bits 16-23 base address
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db 10010010b ; access byte
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db 11001111b ; bits 16-19 limit (4GB), 4 bits flags
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db 0 ; bits 24-31 base address
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gdt_end:
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string: db 'DISK ERROR'
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string_end:
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addr_packet:
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db 0x10, 0x00 ; size, reserved
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dw secreadcnt ; blocks
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addr_packet_transfer_buff_off: dw 0x7E00 ; transfer buffer offset
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addr_packet_transfer_buff_seg: dw 0x0000 ; transfer buffer segment
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addr_packet_start_block: dq 0 ; start block
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