Delay code

Delay code

Functions that cause a parametrised number of cycles of delay.

25..280 cycles of delay

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A clocks + overhead
; Preserved: X, Y
; Time: A+25 clocks (including JSR)
;;;;;;;;;;;;;;;;;;;;;;;;
:       sbc #7          ; carry set by CMP
delay_a_25_clocks:
	cmp #7
	bcs :-          ; do multiples of 7
	lsr a           ; bit 0
	bcs :+
                       ; A=clocks/2, either 0,1,2,3
	beq @zero       ; 0: 5
	lsr a
	beq :+          ; 1: 7
	bcc :+          ; 2: 9
@zero:  bne :+          ; 3: 11
:       rts             ; (thanks to dclxvi for the algorithm)

33..65568 cycles of delay

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A:X clocks+overhead
; Time: 256*A+X+33 clocks (including JSR)
; Clobbers A. Preserves X,Y. Has relocations.
;;;;;;;;;;;;;;;;;;;;;;;;
:	; do 256 cycles.	; 5 cycles done so far. Loop is 2+1+ 2+3+ 1 = 9 bytes.
	sbc #1			; 2 cycles - Carry was set from cmp
	pha			; 3 cycles
	 lda #(256-25-10-2-4)   ; +2
	 jsr delay_a_25_clocks
	pla                     ; 4 cycles
delay_256a_x_33_clocks:
	cmp #1			; +2; 2 cycles overhead
	bcs :-			; +2; 4 cycles overhead
	; 0-255 cycles remain, overhead = 4
	txa 			; +2; 6; +27 = 33
        ; 15 + JSR + RTS overhead for the code below. JSR=6, RTS=6. 15+12=27
        ;          ;    Cycles        Accumulator     Carry flag
        ;          ; 0  1  2  3  4       (hex)        0 1 2 3 4
        sec        ; 0  0  0  0  0   00 01 02 03 04   1 1 1 1 1
:       sbc #5     ; 2  2  2  2  2   FB FC FD FE FF   0 0 0 0 0
        Jcs :-     ; 4  4  4  4  4   FB FC FD FE FF   0 0 0 0 0
        lsr a      ; 6  6  6  6  6   7D 7E 7E 7F 7F   1 0 1 0 1
        Jcc :+     ; 8  8  8  8  8   7D 7E 7E 7F 7F   1 0 1 0 1
:       sbc #$7E   ;10 11 10 11 10   FF FF 00 00 01   0 0 1 1 1
        Jcc :+     ;12 13 12 13 12   FF FF 00 00 01   0 0 1 1 1
        Jeq :+     ;      14 15 14         00 00 01       1 1 1
        Jne :+     ;            16               01           1
:       rts        ;15 16 17 18 19   (thanks to dclxvi for the algorithm)

30..65565 cycles of delay

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays X:A clocks+overhead
; Time: 256*X+A+30 clocks (including JSR)
; Clobbers A,X. Preserves Y. Has relocations.
;;;;;;;;;;;;;;;;;;;;;;;;
delay_256x_a_30_clocks:
	cpx #0			; +2
	Jeq delay_a_25_clocks	; +3  (25+5 = 30 cycles overhead)
	; do 256 cycles.        ;  4 cycles so far. Loop is 1+1+ 2+3+ 1+3 = 11 bytes.
	dex                     ;  2 cycles
	pha                     ;  3 cycles
	 lda #(256-25-9-2-7)    ; +2
	 jsr delay_a_25_clocks
	pla                        ; 4
	jmp delay_256x_a_30_clocks ; 3.

16..271 cycles of delay

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A*256 clocks + overhead
; Preserved: X, Y
; Time: A*256+16 clocks (including JSR)
;;;;;;;;;;;;;;;;;;;;;;;;
delay_256a_16_clocks:
	cmp #0
	bne :+
	rts
delay_256a_11_clocks_:
:       pha
	 lda #256-19-22
	 jsr delay_a_25_clocks
	pla
	clc
	adc #-1&$FF
	bne :-
	rts

31..65566 cycles of delay

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A:X clocks+overhead
; Time: 256*A+X+31 clocks (including JSR)
; Clobbers A. Preserves X,Y. Has relocations.
;;;;;;;;;;;;;;;;;;;;;;;;
:	; do 256 cycles.	; 5 cycles done so far. Loop is 2+1+ 2+3+ 1 = 9 bytes.
	sbc #1			; 2 cycles - Carry was set from cmp
	pha			; 3 cycles
	 lda #(256-25-10-2-4)   ; +2
	 jsr delay_a_25_clocks
	pla                     ; 4 cycles
delay_256a_x_31_clocks:
	cmp #1			; +2; 2 cycles overhead
	bcs :-			; +2; 4 cycles overhead
	; 0-255 cycles remain, overhead = 4
	txa 			; +2; 6; +25 = 31
	;passthru
<<Place the function delay_a_25_clocks immediately following here>>

See also

• Fixed cycle delay