section .bss

		; default drive if field in FCB = 0
defdrv:		resb 1

section .text
		; Functions here generally expect the FCB to be at ES:BX

		; Helper function: read the drive number from FCB,
		; apply default if necessary,
		; and make it current I/O drive
setdrv:		mov dl, [es:bx] ; A=1
		test dl, dl
		jnz .nofix
		mov dl, [defdrv] ; A=0
		inc dl
		mov [es:bx], dl ; A=1
		; now set as current i/o drive
.nofix:		dec dl ; A=0 for dsksel
		jmp logdrv