Delay code

Delay code

Functions that cause a parametrised number of cycles of delay.

Note that all branch instructions are written assuming that no page wrap occurs. If you want to ensure this condition at compile time, use the bccnw/beqnw/etc. macros that are listed at Fixed cycle delay.

A + 25 cycles of delay, clobbers A, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A clocks + overhead
; Clobbers A. Preserves 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)

A + 27 cycles of delay with no zero check, clobbers A, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A clocks + overhead
; Clobbers A. Preserves X,Y.
; Time: A+27 clocks (including JSR)
; If A = 0, is interpreted as 256.
;;;;;;;;;;;;;;;;;;;;;;;;
delay_a_27_clocks:
        ;          ;    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
        bcs :-     ; 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
        bcc :+     ; 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
        bcc :+     ;12 13 12 13 12   FF FF 00 00 01   0 0 1 1 1
        beq :+     ;      14 15 14         00 00 01       1 1 1
        bne :+     ;            16               01           1
:       rts        ;15 16 17 18 19   (thanks to dclxvi for the algorithm)

256×A + X + 33 cycles of delay, clobbers A, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A:X clocks+overhead
; Time: 256*A+X+33 clocks (including JSR)
; Clobbers A. Preserves X,Y. Has relocations.
;;;;;;;;;;;;;;;;;;;;;;;;
:	; do 256-5 cycles.
	sbc #1			; 2 cycles - Carry was set from cmp
	pha
	 lda #(256-5 - 27-7-2)
	 jsr delay_a_27_clocks
	pla
delay_256a_x_33_clocks:
	cmp #1			; +2
	bcs :-			; +3 (-1)
	; 0-255 cycles remain, overhead = 4
	txa 			; +2; 6; +27 = 33
	;passthru
<<Place the function delay_a_27_clocks immediately following here>>

Can be trivially changed to swap X, Y.

256×A + X + 33 cycles of delay, relocatable, clobbers A, Y, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A:X clocks+overhead
; Time: 256*A+X+33 clocks (including JSR)
; Clobbers A,Y. Preserves X. Relocatable.
;;;;;;;;;;;;;;;;;;;;;;;;
:	; do 256-5 cycles.
	sbc #1			; 2 cycles - Carry was set from cmp
	ldy #48  ;\
        dey      ; |- Clobbers Y; 246 cycles, 253 total
        bpl *-1  ;/
        ldy $A4  ;              ; 3 cycles, 256 total
delay_256a_x_33_clocks_b:
	cmp #1			; +2
	bcs :-			; +3 (-1)
	; 0-255 cycles remain, overhead = 4
	txa 			; +2; 6; +27 = 33
	;passthru
<<Place the function delay_a_27_clocks immediately following here>>

Can be trivially changed to swap X, Y.

256×A + X + 33 cycles of delay, relocatable, clobbers A, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A:X clocks+overhead
; Time: 256*A+X+33 clocks (including JSR)
; Clobbers A. Preserves X,Y. Relocatable.
; Does not depend on delay_a_25_clocks.
;;;;;;;;;;;;;;;;;;;;;;;;
:	; do 256 cycles.	; 5 cycles done so far. Loop is 2+1+ 1+2+1+2+1 + 1+1 = 12 bytes.
	sbc #1			; 2 cycles - Carry was set from cmp
        pha       ;\
         txa      ; |
         ldx #46  ; |
         dex      ; |-          ; 247 cycles, 254 total
         bpl *-1  ; |
         tax      ; |
        pla       ;/
        nop                     ; 2 cycles; 256 cycles total
delay_256a_x_33_clocks_c:
	cmp #1			; +2; 2 cycles overhead
	bcs :-			; +2; 4 cycles overhead
	; 0-255 cycles remain, overhead = 4
	txa 			; +2; 6; +27 = 33
	;passthru
<<Place the function delay_a_27_clocks immediately following here>>

256×A + 16 cycles of delay, clobbers A, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A*256 clocks + overhead
; Clobbers A. Preserves X,Y.
; Time: A*256+16 clocks (including JSR)
; Depends on delay_a_25_clocks
;;;;;;;;;;;;;;;;;;;;;;;;
delay_256a_16_clocks:
	cmp #0
	bne :+
	rts
delay_256a_11_clocks_:
:       pha
	 lda #(256-25-7-2-2-3)
	 jsr delay_a_25_clocks
	pla
	clc
	adc #-1&$FF
	bne :-
	rts

Alternative that depends on different function:

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays A*256 clocks + overhead
; Clobbers A. Preserves X,Y.
; Time: A*256+16 clocks (including JSR)
; Depends on delay_a_27_clocks
;;;;;;;;;;;;;;;;;;;;;;;;
delay_256a_16_clocks_b:
	cmp #0
	bne :+
	rts
delay_256a_11_clocks_b_:
:       pha
	 lda #(256-27-7-2-2-3)
	 jsr delay_a_27_clocks
	pla
	clc
	adc #-1&$FF
	bne :-
	rts

256×X + 16 cycles of delay, relocatable, clobbers X, Y, Z&N

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays X*256 clocks + overhead
; Clobbers X,Y. Preserves A. Relocatable.
; Time: X*256+16 clocks (including JSR)
;;;;;;;;;;;;;;;;;;;;;;;;
delay_256x_16_clocks:
	cpx #0
	bne :+
	rts
delay_256x_11_clocks_:
	;5 cycles done. Loop is 256 cycles
:       ldy #50
	dey
	bne *-1
	dex
	bne :-
	;Loop end is -1 cycles. Total: 4+JSR+RTS = 16
	rts

Can be trivially changed to swap X, Y.

256×X + A + 30 cycles of delay, clobbers A, X, Z&N, C, V

;;;;;;;;;;;;;;;;;;;;;;;;
; Delays X*256 clocks + overhead
; Clobbers A,X. Preserves Y.
; Depends on delay_a_25_clocks within short branch distance
; Time: X*256+16 clocks (including JSR)
;;;;;;;;;;;;;;;;;;;;;;;;
delay_256x_a_30_clocks:
	cpx #0
	beq delay_a_25_clocks
	;4 cycles done. Loop is 256 cycles
:       pha
	 lda #(256-7-2-2-3)
	 jsr delay_a_25_clocks
	pla
	dex
	beq delay_a_25_clocks ; count as 2
	bne :-
	;Loop end is -1+1 = 0 cycles. Total: 4+JSR+RTS = 16

Can be trivially changed to swap X, Y.

See also

• Fixed cycle delay