rdos/boot/fat.asm

	; Bootsector for FAT32 filesystems
	; Takes defines:
	; 	FAT12	FAT has 1.5 bytes per entry
	; 	FAT16	FAT has WORD per entry
	; 	FAT32	FAT has DWORD per entry
	; 	CHS	uses int13h/AH=02h
	; 	LARGE	requires 32 bit sector numbers
	; 	LBA	uses int13h/AH=42h
	cpu 8086

	org 0x0800
	jmp short init
	nop

	%include "inc/bpb.asm"

bpb:	equ 0x080B

	; Area for BPB
	times ( 0x5A - ($-$$) ) db 0

init:	xor ax, ax
	mov ss, ax
	mov sp, 0x7C00

	; save potential partition table entry
	push ds
	push si

	mov ds, ax
	mov es, ax

	mov si, 0x7C00
	mov di, 0x0800
	mov cx, 0x0100
	rep movsw
	jmp 0x0:main

	%include "inc/booterr.asm"

	; offsets relative to FS
readd:	; add offset of cluster data area to DX:AX
	push cx
	mov di, [bp+bpb_rde]
	mov cl, 4
	shr di, cl ; 32 bytes per entry
	add ax, di
	adc dx, 0
	pop cx
readr:	; add offset to rootdir to DX:AX (FAT12/FAT16 only)
	push cx
	xor ch, ch
	mov cl, byte [bp+bpb_fn]
fatlp:	add ax, [bp+bpb7_lsf]
	adc dx, [bp+bpb7_lsf+2]
	loop fatlp
	pop cx
readf:	; add offset to FAT table to DX:AX
	add ax, word [bp+bpb_rsc]
	adc dx, 0
readp:	; read sector DX:AX from partition
	add ax, word [bp+bpb_po]
	adc dx, word [bp+bpb_po+2]
	jnc ($+5)
	call errcll

	%ifdef LBA
read_:	; read sector DX:AX from disk
	; this uses the EBIOS extensions
	; qword sector number DX:AX
	push cs
	push cs
	push dx
	push ax

	; dword target buffer: BX:0
	push bx
	push cs

	; word sector number
	push cx

	; size & passing
	mov di, 0x10
	push di

	mov si, sp
	mov ah, 0x42
	mov dl, [bp+bpb7_dn]
	stc
	int 0x13
	jnc ($+5)
	call errcll

	add sp, di

	ret
	%endif

	%ifdef CHS
read_:	; read sector DX:AX from disk
	; uses basic CHS disk IO
	; this cant reliable read over track boundaries
	; so we read each single sector in loop over CX
chslp:	push ax
	push cx
	push dx
	push bx

	; calculate CHS data
	div word [bp+bpb_spt]    ; ax:temp     = (lba / spt)
	inc dx                   ; dx:sector   = (lba % spt) + 1
	mov cl, dl ; sector number
	xor dx, dx
	div word [bp+bpb_nos]    ; ax:cylinder = (tmp / heads)
	                         ; dx:head     = (tmp % heads)
	mov ch, al ; cylinder number
	mov dh, dl ; head number
	mov dl, [bp+bpb7_dn] ; drive number

	; set up read buffer
	mov es, bx
	xor bx, bx

	; call bios
	mov ax, 0x0201
	int 0x13
	jnc ($+5)
	call errcll

	pop bx
	pop dx
	pop cx
	pop ax

	; increment sector num + write offset
	add ax, 1
	adc dx, 0
	add bx, 0x20
	; loop on CX
	loop chslp
	ret
	%endif

next:	; Advances DX:AX to next FAT entry
	push ax
	push bx
	; shift 2 left for dword-sized FAT entries
	; shift 9 right for sector size
	mov cl, 7
shftl:	clc
	rcr dx, 1
	rcr ax, 1
	loop shftl

	mov bx, 0xA0
	%ifdef FAT12
	; FAT item might span over two sectors
	mov cx, 2
	%else
	mov cx, 1
	%endif
	call readf

	pop bx

	; get lower part of cluster number
	pop si
	%ifdef FAT16
	; double
        sal si, 1
	%endif
	%ifdef FAT32
	; multiply with 4
	sal si, 1
	sal si, 1
	%endif
	; make sure its within sector range
        and si, 0x1FF
	add si, buf
	%ifdef FAT16
	; load word from FAT
	lodsw
	xor dx, dx
	%endif
	%ifdef FAT32
	; load dword from FAT
	lodsw
	mov dx, [si]
	%endif
	ret

	; reads current cluster into [dest]
readc:	; load cluster number
	push ax
	push dx

	; subtract the two dummy entries from FAT start
	sub ax, 2
	sbb dx, 0

	; convert cluster number to sector number
	; this is some cheapo multiplication with 2^n
	mov cl, [bp+bpb_sc]
l02:	shr cl, 1
	jz l02e
	clc
        rcl ax, 1
        rcl dx, 1
	jmp l02
l02e:
	; dx:ax is now sector num in data area
	mov cx, [bp+bpb_sc]
	xor ch, ch

	call readd

	xchg di, cx
	mov cl, 5
	sal di, cl
	add bx, di

	pop dx
	pop ax

	ret

	; Load root directory
loadr:	mov ax, 2
	xor dx, dx
	jmp load

	; Load file identified by fsattr
	; Directory data is expected at 0x07C00
loadf:	mov si, (0x7C00-0x20)
sloop:	add si, 0x20
	; check if entry is valid
	mov al, [si]
	; unallocated direntry
	test al, al
	jnz ($+5)
	call errcll
	; deleted files
	cmp al, 0xE2
	je sloop
	; check attr
	mov al, [si+11]
	and al, 0x5C
	cmp al, ah
	jne sloop
	mov ax, [si+0x1A]
	mov dx, [si+0x14]

	; Load cluster chain DX:AX to 0x07C00
load:	mov bx, 0x07C0
lloop:	call readc
	call next
	cmp word dx, 0x0FFF
	jne lloop
	ret

main:	; Set up BP to point to BPB
	; Relative addressing saves one instruction byte per access
	mov bp, bpb
	; Save DL contents
	mov [bp+bpb7_dn], dl

	; load root directory
	call loadr
	; search for first system directory
	mov ah, 0x14
	call loadf
	; search for first system file
	mov ah, 0x04
	call loadf

	; restore drive number
	mov dl, [bp+bpb7_dn]

	; restore potential partition table
	pop si
	pop ds

	jmp 0x7C00

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