;; lab 7
	;; step motor controller
	;; anders pearson and jason bessette

	;; initialization
	org 1800h
	ld a,90h		; clear ports
	out (0c3h), a		;
	ld (1950h),88h		; use 1950h as delay
	in a,(0fc0h)		; read input bit into a
	ld (1951h),a            ; this corresponds to "too slow"
	;; do this for each revolution
rev:	ld f,00h		; clear our flag
	;; do this for each sequence	
seq:	ld b,32h		; 50 sequences of 4 half-steps 
	call doseq		; output our bits
	dec b			; decrement our counter
	jp nz seq		; repeat sequence
	cmp f,01h		; check if the flag is set
	jp nz, nodec		; 
	dec (1950h)		; decrease delay
	jp nz, rev		; infinite loop
	jp error		; delay's made it down to 0
nodec:	inc (1950h)		; increase delay
	jp nz, rev		; infinite loop
	jp error		; went past FFh
	
	;; subrouting that does a sequence of four half steps
doseq:	push bc			;
	push d			; 
	ld b,11h		; 00010001
	ld c,33h		; 00110011
	ld d,04h		; do this four times
hs:	ld a,b			; b -> a
	and a,0fh		; mask out the high four
	out (0fc1h), a		; send it to the motor
	ld a,c			; c -> a
	and a,0fh		; mask out high four bits
	out (0fc1h), a		; send it to the motor
	in a, (0fc0h)		; read in speed bit
	cmp a,(1951h)		; compare to our slow bit
	jp nz pass		; if != skip
	ld f,01h		; set our flag
pass:	rl b			; rotate b left
	rl c			; rotate c left
	ld a,(1950h)		; delay
	call 0ffdh		; call delay function
	dec d			; decrement counter
	jp nz hs		; loop again
	pop d			; pop d off stack
	pop bc			; pop bc off stack
	ret			; return
		
error:	halt			; halt the system