rdos/src/sys.asm

fcb1	equ 0x5C

	org 0x100
	jmp main

%include "scan.inc"
%include "print.inc"

drvnum	db 0
track	dw 39

main	mov di, 0x81
	mov al, 0x20
	mov ch, 0
	mov cl, [0x80]
	repe scasb
	repne scasb
	test cx, cx
	jnz rddrv
	call putsh
	db "BIOS drive number not specified", 0x0A, 0x0D, 0
	int 0x20
rddrv	mov si, di
	call lodnum
	mov dx, ax
	test dx, ~0x83
	jnz drverr
	mov byte [drvnum], dl
	test dl, 0x80
	jz floppy
	call drverr

drverr	call putsh
	db "Failed to access drive", 0x0A, 0x0D, 0
	int 0x20

	; read bios parameter block
floppy	mov ax, 0x0201
	mov cx, 0x0001
	mov dh, 0
	mov bx, prog
	int 0x13
	jc drverr

	mov ax, [prog+0x016]
	cmp ax, 12
	jbe .gok
	call putsh
	db "Unsupported FAT size", 0x0A, 0x0D, 0
	int 0x20
.gok	nop

	; calculate start of cluster area
	mov ax, [prog+0x011] ; number of dir entries
	mov cl, 5
	shl ax, cl
	mov bx, [prog+0x0B] ; bytes per sector
	div bx
	mov cx, ax
	mov ax, [prog+0x016] ; logical sectors per fat
	mov bl, [prog+0x010] ; number of fats
	mov bh, 0
	mul bx
	add ax, cx
	add ax, [prog+0x00E] ; reserved sectors
	mov si, ax

	; calculate sector offset of target track
	mov ax, [prog+0x018] ; sectors per track
	mul word [prog+0x01A] ; number of heads
	mul word [track]

	; calculate cluster offset of target track -> SI
	sub ax, si
	mov bl, [prog+0x00D] ; sectors per cluster
	mov bh, 0
	xor dx, dx
	div bx
	mov si, ax

	; calculate cluster length of target track -> DI
	mov ax, [prog+0x018] ; sectors per track
	add ax, dx ; remainder
	; add sectors per cluster minus one
	add ax, bx
	dec ax
	; divide by sectors per cluster and floor
	xor dx, dx
	div bx
	mov di, ax

	; load fat table
	call debug

	ret

; TODO:
; calculate offset into fat table
; check that all relevant clusters are 0 or 0xFFF7
; set all relevant clusters to 0xFFF7
; track is now available for direct writing

	; set up boot code
	mov di, prog
	mov ax, 0x7FEB
	stosw
	mov byte [di], 0x80
	add di, 0x7F - b_ld + b_rst
	mov si, b_rst
	mov cx, b_end - b_rst
	rep movsb

	; write back bootsector
	mov ax, 0x0301
	mov cx, 0x0001
	mov dh, 0
	mov bx, prog
	int 0x13
	jc drverr

	mov dx, fcb1
	mov ah, 0xF
	int 0x21
	test al, al
	jz .fok
.ferr	call putsh
	db "Failed to read input file", 0x0A, 0x0D, 0
	ret
.fok	mov si, prog
	; apply our target address
.rloop	mov dx, si
	mov ah, 0x1A
	int 0x21
	; increment address for next iteration
	add si, 0x80
	; do a read
	mov dx, fcb1
	mov ah, 0x14
	int 0x21
	; success: continue
	cmp al, 0
	jz .rloop
	; eof: break
	cmp al, 1
	jz .cok
	; segment wrap: fail
	cmp al, 2
	jz .ferr
	; we assume al=3: eof
.cok	nop
	mov ax, si
	sub ax, args
	add ax, 0x1FF
	mov cl, 9
	shr ax, cl
	call debug

	ret

	; code to copy into boot sector
b_rst	mov ah, 0
	int 0x13
b_ld	mov ax, 0x60
	mov ds, ax
	mov es, ax
	mov ss, ax
	xor sp, sp
	mov ax, 0x0208 ; read, 8 sectors
b_len	equ $ - 2
	mov cx, 0x2701 ; track 39, sector 1
b_trck	equ $ - 1
b_strt	equ $ - 2
	mov dh, 0 ; head 0
	xor bx, bx ; es:bx=0x60:0
	int 0x13
	jc b_rst
	mov byte [4], dl
	jmp 0x60:0x100
b_end	equ $

	align 2
psp	equ $
args	equ psp + 0x80
prog	equ psp + 0x100