223 lines
3.2 KiB
NASM
223 lines
3.2 KiB
NASM
CPU 8086
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ORG 0x7c00
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jmp start
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nop
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;Disk description tables
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%ifdef F1440
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;1.44 MB 3.5" floppy disk (enable by passing -d F1440 to NASM)
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oemlabel db "ETTINOS "
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sectorsize dw 0x200 ;bytes
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clustersize db 0x1 ;sectors
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bootsectors dw 0x1
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fats db 0x2
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rootentries dw 0xe0
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logicalsectors dw 0xb40
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mediadescriptor db 0xf0
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sectorsperfat dw 0x9
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sectorspertrack dw 0x12
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sides dw 0x2
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hiddensectors dd 0x0
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largesectors dd 0x0
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driveid dw 0x0
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drivesignature db 0x29
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volumeid dd 0x0
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volumelabel db "ETTINOS "
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filesystem db "FAT12 "
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%else
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;360 KiB 5.25" floppy disk (default)
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oemlabel db "ETTINOS "
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sectorsize dw 0x200 ;bytes
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clustersize db 0x2 ;sectors
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bootsectors dw 0x1
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fats db 0x2
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rootentries dw 0x70
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logicalsectors dw 0x2d0
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mediadescriptor db 0xfd
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sectorsperfat dw 0x2
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sectorspertrack dw 0x9
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sides dw 0x2
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hiddensectors dd 0x0
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largesectors dd 0x0
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driveid dw 0x0
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drivesignature db 0x29
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volumeid dd 0x0
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volumelabel db "ETTINOS "
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filesystem db "FAT12 "
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%endif
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start:
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;Set up the data segment
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mov ax, 0x0
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mov ds, ax
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;Set up the stack
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cli
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mov ax, 0x0
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mov ss, ax
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mov sp, 0x7c00
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sti
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;Store the boot device number
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mov [bootdev], dl
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;Load the root
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;Set the source
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mov ah, 0x0
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mov al, [fats]
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mul word [sectorsperfat]
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add ax, [bootsectors]
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push ax
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call calcsource
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;Set the destination
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mov si, 0x7e00
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mov bx, ds
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mov es, bx
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mov bx, si
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;Set the size
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push dx
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mov ax, [rootentries]
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mov dx, 0x20
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mul dx
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mov dx, 0x0
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div word [sectorsize]
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pop dx
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push ax
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;Load
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mov ah, 0x2
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int 0x13
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;Search the root for the system
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;Set DI to the root FAT
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mov ax, ds
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mov es, ax
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mov di, 0x7e00
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;Initialise the search loop
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mov cx, word [rootentries]
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mov ax, 0x0
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search:
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;Store CX in the stack
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push cx
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;Check for the system FAT
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mov si, sysfile
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mov cx, 0xb
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rep cmpsb
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je loadfat
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;Set DI to the next entry
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add ax, 0x20
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mov di, 0x7e00
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add di, ax
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;Load CX from the stack
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pop cx
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loop search
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;Load the system FAT
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loadfat:
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;Load CX from the stack
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pop cx
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;Store the first cluster
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mov ax, word [es:di+0xf]
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mov word [cluster], ax
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;Set the source
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mov ax, 0x1
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call calcsource
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;Set the destination
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mov di, 0x7e00
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mov bx, di
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;Set the size
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mov ax, [sectorsperfat]
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;Load
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mov ah, 0x2
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int 0x13
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;Load the system file
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;Load a cluster
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loadcluster:
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;Set the source
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pop bx
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pop ax
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push ax
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push bx
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add ax, bx
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sub ax, 0x2
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add ax, word [cluster]
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call calcsource
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;Set the destination
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mov ax, 0x0
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mov es, ax
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mov bx, word [pointer]
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;Set the size
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mov al, 0x1
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;Load
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mov ah, 0x2
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int 0x13
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;Calculate the next cluster
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mov ax, [cluster]
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mov dx, 0x0
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mov bx, 0x3
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mul bx
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mov bx, 0x2
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div bx
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mov si, 0x7e00
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add si, ax
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mov ax, word [ds:si]
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or dx, dx
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jz even
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odd:
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shr ax, 1
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shr ax, 1
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shr ax, 1
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shr ax, 1
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jmp contcalc
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even:
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and ax, 0xfff
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contcalc:
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mov word [cluster], ax
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cmp ax, 0xff8
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jge boot
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add word [pointer], 0x200
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jmp loadcluster
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boot:
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jmp 0x0:0x9e00
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sysfile db "SYSTEM BIN"
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bootdev db 0x0
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cluster dw 0x0
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pointer dw 0x9e00
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calcsource:
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push ax
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push bx
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mov bx, ax
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mov dx, 0x0
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div word [sectorspertrack]
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add dl, 0x1
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mov cl, dl
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mov ax, bx
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mov dx, 0x0
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div word [sectorspertrack]
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mov dx, 0x0
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div word [sides]
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mov dh, dl
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mov ch, al
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pop bx
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pop ax
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mov dl, byte [bootdev]
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ret
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;Padding
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times 0x1fe-($-$$) db 0x0
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;Boot signature
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dw 0xaa55
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