; FCB layout

struc FCB

FCBDRV	resb 1 ; BYTE drive (0=default, 1=A:, 2=B:, ...)
FCBFN	resb 11 ; 11 BYTES filename
FCBCLUS	resw 1 ; DWORD current cluster
FCBRN	resb 1 ; BYTE record in cluster

endstruc

FCBSIZ	equ FCB_size

section .data

defdrv	db 0
dta	dd 0

section .text

	; auto-complete drive field in fcb
	; also logs in the drive
	; IN	ds:dx	far ptr FCB
fcbdrv	push ds
	push dx
	mov si, dx
	; if currently zero, ...
	mov dl, [si] ; 1-based
	test dl, dl
	jnz .log
	; replace with current drive
	mov dl, [cs:defdrv] ; 0-based
	inc dl
	mov [si], dl ; 1-based
.log	dec dl
	mov ax, cs
	mov ds, ax
	call logdrv ; 0-based
	pop dx
	pop ds
	; if carry flag set, resume stack at bp
	; this instant-returns some higher level procs
	jnc .ret
	mov sp, bp
	; duplicate carry bit into all 8 AL bits
	; similar to SALC instruction
.ret	sbb al, al
	ret

	; set disk transfer address
	; IN	DS:DX	dta ptr
setdta	mov [cs:dta], dx
	mov [cs:dta+2], ds
	ret

	; get disk transfer address
	; OUT	ES:BX	dta ptr
getdta	les bx, [cs:dta]
	ret