126 lines
1.7 KiB
NASM
126 lines
1.7 KiB
NASM
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; Bootsector for FAT32 filesystems
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org 0x0800
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jmp near init
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; WORD reserved sector count
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rsc: equ ( $$ + 0x00E )
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; BYTE number of FATs
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fn: equ ( $$ + 0x010 )
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; DWORD hidden sector count (partition offset)
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po: equ ( $$ + 0x01C )
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; DWORD sectors per FAT
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sf: equ ( $$ + 0x024 )
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; BYTE drive number (we set it from dl)
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dn: equ ( $$ + 0x040 )
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; Area for BPB
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times (0x5A - ($-$$)) db 0
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init: xor ax, ax
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mov ds, ax
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mov es, ax
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mov ss, ax
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mov sp, 0x7C00
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mov si, sp
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mov di, 0x0800
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mov cx, 0x0100
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rep movsw
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jmp 0x0:main
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msg: pop si
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mloop: lodsb
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test al, al
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jz mend
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mov ah, 0x0e
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mov bx, 7
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int 0x10
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jmp mloop
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mend: push si
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ret
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err: call msg
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db "DISKERR", 0x0A, 0x0D, 0
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hlt: hlt
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jmp hlt
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; Requires DWORD ptr and QWORD sector on the stack
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read: pop bp
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push cx
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mov ax, 0x10
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push ax
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mov ah, 0x42
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mov dl, 0x80
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mov si, sp
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int 0x13
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jc err
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pop ax
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pop cx
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push bp
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ret
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; Advances cluster number to next entry
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next: ; Upper two words for sector num
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xor ax, ax
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push ax
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push ax
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; Lower two words for sector num
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mov ax, [clus]
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mov dx, [clus+2]
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; 32-bit >> 7
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mov cl, 7
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shftl: clc
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rcr dx, 1
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rcr ax, 1
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loop shftl
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; Add reserved sector count
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add ax, word [rsc]
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adc dx, 0
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push dx
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push ax
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; target buffer
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push cs
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mov ax, buf
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push ax
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mov cx, 1
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call read
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; Stow off offset in FAT sector
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mov si, [clus]
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sal si, 1
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sal si, 1
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and si, 0x1FF
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add si, buf
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mov di, clus
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movsw
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movsw
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; 1x QWORD, 1x DWORD = 12 bytes
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add sp, 12
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ret
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main: call next
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mov ax, [clus]
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mov dx, [clus+2]
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int 3
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xor ah,ah
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int 0x16
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int 0x19
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jmp hlt
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clus: dd 2
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; Padding and signature
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times (0x1FE - ($-$$)) db 0
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dw 0xAA55
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buf:
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