rdos/fat.asm

	; Bootsector for FAT12 filesystems
	; Memory layout:
	; 0000-03FF  IVT
	; 0400-0500  Bios data area
	; 0501-0600  Stack for us
	; 0600-0800  This code
	org 0x0600
	; 0800-0A00  Buffer for disk
buf:	equ 0x0800

	jmp near init

	; current cluster number
cclus:	dw 0
	; sector number in cluster
csec:	dw 0

	times (0x0B - ($-$$)) db 0
fdc:	; sector size
.ss:	dw 0x200
	; sectors per cluster
.sc:	db 2
	; reserved sector count
.rsc:	dw 1
.fn:	; number of file allocation tables
	db 2
.rde:	; number of root directory entries
	dw 0x70
.ts:	; total number of sectors
	dw 720
.mid:	db 0xFD
.sf:	dw 2
.spt:	dw 9
.nos:	dw 2
.po:	dd 0
.lrgts:	; large sector count - relevant if .ts is 0
	dd 0

	; mformat keeps writing until here
	; if we place init earlier, code gets overwritten
	times (62 - ($-$$)) nop

init:	xor ax, ax
	mov ds, ax
	mov es, ax
	mov ss, ax
	mov sp, 0x0600

	; Relocate and jump
	mov di, sp
	mov si, 0x7C00
	mov cx, 0x0100
	rep movsw
	jmp 0:main

	; Set adress for sector read/write
	; Number is relative to current partition
	; IN	dx:ax	sector number
seek:	push ax
	push dx
	push ds
	push cs
	pop ds
	; add partition offset (required for HDD)
	add ax, [fdc.po]
	adc dx, [fdc.po+2]
	; ax:temp     = (lba / spt)
	; dx:sector   = (lba % spt) + 1
	div word [fdc.spt]
	inc dx
	; Sector number (bits 0-5) into CL
	; This is blindly assumed to never exeed 63
	mov byte [.cl], dl
	; ax:cylinder = (tmp / heads)
	; dx:head     = (tmp % heads)
	xor dx, dx
	div word [fdc.nos]
	; Cylinder number bit 0-7 into CH
	mov byte [.ch], al
	; Head number into DL
	mov byte [.dh], dl
	; Bits 8-9 of cylinder number into bits 6-7 of CL
	and ah, 0x03
	ror ah, 1
	ror ah, 1
	or byte [.cl], ah
	pop ds
	pop dx
	pop ax
	ret
.cl:	db 0
.ch:	db 0
.dl:	db 0
.dh:	db 0

	; Read sector into ES:BX
	; Sector number has been previously set with seek
read:	push ax
	push cx
	push dx
	; ah = int13 subfunction 2
	; al = read 1 sector
	mov ax, 0x0201
	; load seek data into registers
	mov cx, word [cs:seek.cl]
	mov dx, word [cs:seek.dl]

	int 0x13

	pop dx
	pop cx
	pop ax
	ret

	; Get start sector of root dirrectory
	; OUT	ax	sector number
dirsec:	; Multiply sectors per fat with number of fats
	mov ax, [cs:fdc.sf]
	mul byte [cs:fdc.fn]
	; Add number of reserved sectors
	add ax, [cs:fdc.rsc]
	ret

	; Get root directory length in sectors
	; OUT	cx	sectors
dirlen: push ax
	mov ax, [cs:fdc.rde]
	mov cl, 4
	shr ax, cl ; 32 bytes per entry
	mov cx, ax
	pop ax
	ret

	; Get first sector of data area
	; OUT	ax	sector number
clusec:	push cx
	call dirsec
	call dirlen
	add ax, cx
	pop cx
ret:	ret

	; Get total number of sectors
	; OUT:	dx:ax	number of sectors
secnum: xor dx, dx
	mov ax, [cs:fdc.ts]
	test ax, ax
	jnz ret
	mov ax, [cs:fdc.lrgts]
	mov dx, [cs:fdc.lrgts+2]
	ret

	; Get number of clusters
	; OUT:	ax	number of clusters
	; Cant overflow for FAT16 :P
clunum:	push cx
	push dx
	call clusec
	mov cx, ax
	call secnum
	sub ax, cx
	sbb dx, 0
	mov cl, byte [cs:fdc.sc]
	xor ch, ch
	div cx
	pop dx
	pop cx
	ret

	; Scan current buffer for matching file
	; IN	ds	= cs
	;	es	= cs
	; OUT	bx	start cluster if file found
	;		trashed if no match
	; carry is set if no file was found
scadir:
	push cx
	push si
	mov si, buf
	mov cx, 0x10
.loop:	push cx
	push si
	mov bx, [si+0x1A]
	mov di, .name
	mov cx, 11
	repe cmpsb
	pop si
	pop cx
	je .found
	add si, 0x20
	loop .loop
	stc
.found:	pop si
	pop cx
	ret
.name:	db "HELLO   "
.ext:	db "COM"

open:
	push ax
	push cx
	push dx
	push bx
	push ds
	push es
	xor ax, ax
	mov ds, ax
	mov es, ax
	mov [csec], ax
	call dirsec
	call dirlen
.loop:	xor dx, dx
	mov bx, buf
	call seek
	call read
	call scadir
	jnc .found
	inc ax
	loop .loop
	stc
.found:
	mov [cclus], bx
	pop es
	pop ds
	pop bx
	pop dx
	pop cx
	pop ax
	ret

main:	mov [seek.dl], dl ; save drive number

	int 3
	call open
	int 3

.hlt:	hlt
	jmp .hlt

	; Padding and signature
	times (0x1FE - ($-$$)) db 0
	dw 0xAA55