6502 Advanced Tips
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Contents
jump table instead of comparison branch chains
When you need to choose between 4 or more destinations, it's more efficient to use a jump table instead of a sequence of CMP and BEQ. Here's a jump table example that uses RTS instead of JMP (ADDRESS) to save even more ROM and cycles...
LDX FunctionIndex LDA FunctionJumptablehi,x PHA LDA FunctionJumptablelo,x PHA RTS FunctionJumptablehi .byte >(ClearRam-1) .byte >(SetRam-1) .byte >(MemCpy-1) .byte >(MemCmp-1) FunctionJumptablelo .byte <(ClearRam-1) .byte <(SetRam-1) .byte <(MemCpy-1) .byte <(MemCmp-1)
fast range test
For all of these we assume that the byte to be tested is in A and that the start and end values, n and m, are already defined. Also that 0 < n < m < $FF.
If you don't need to preserve the byte in A then testing the byte can be done in five bytes and only six cycles. This sets the carry if A is in the range n to m.
CLC ; clear carry for add ADC #$FF-m ; make m = $FF ADC #m-n+1 ; carry set if in range n to m
Courtesy Lee Davidson. [1]
fast unsigned integer divisions
; Unsigned Integer Division Routines (rev 2) ; by Omegamatrix ; ; Rev 1 (June 14, 2014) ; Divide by 6,10,12,20,24,26, and 28 have all been replace with new and better routines. ; ; Rev 2 (June 21, 2014) ; Divide by 22 routines has been upgraded to one that saves 3 cycles, same amount of bytes as before. ; ; ; ; To use these routines begin with unsigned value to be divided (0-255) in the accumulator, ; and the routine will finish with the integer result in the accumulator. ; ; - All divisions (2-32) are covered below ; - X, Y, and BCD mode are not used by any of these routines ; - All these routines are constant cycles ; - Most routines require 1 temp register
;Divide by 2 (trival) ;1 byte, 2 cycles lsr
;Divide by 3 ;18 bytes, 30 cycles sta temp lsr adc #21 lsr adc temp ror lsr adc temp ror lsr adc temp ror lsr
;Divide by 4 (trival) ;2 bytes, 4 cycles lsr lsr
;Divide by 5 ;18 bytes, 30 cycles sta temp lsr adc #13 adc temp ror lsr lsr adc temp ror adc temp ror lsr lsr
;Divide by 6 ;17 bytes, 30 cycles lsr sta temp lsr lsr adc temp ror lsr adc temp ror lsr adc temp ror lsr
;Divide by 7 (From December '84 Apple Assembly Line) ;15 bytes, 27 cycles sta temp lsr lsr lsr adc temp ror lsr lsr adc temp ror lsr lsr
;Divide by 8 (trival) ;3 bytes, 6 cycles lsr lsr lsr
;Divide by 9 ;17 bytes, 30 cycles sta temp lsr lsr lsr adc temp ror adc temp ror adc temp ror lsr lsr lsr
;Divide by 10 ;17 bytes, 30 cycles lsr sta temp lsr adc temp ror lsr lsr adc temp ror adc temp ror lsr lsr
;Divide by 11 ;20 bytes, 35 cycles sta temp lsr lsr adc temp ror adc temp ror adc temp ror lsr adc temp ror lsr lsr lsr
;Divide by 12 ;17 bytes, 30 cycles lsr lsr sta temp lsr adc temp ror lsr adc temp ror lsr adc temp ror lsr
; Divide by 13 ; 21 bytes, 37 cycles sta temp lsr adc temp ror adc temp ror adc temp ror lsr lsr clc adc temp ror lsr lsr lsr
;Divide by 14 ;1/14 = 1/7 * 1/2 ;16 bytes, 29 cycles sta temp lsr lsr lsr adc temp ror lsr lsr adc temp ror lsr lsr lsr
;Divide by 15 ;14 bytes, 24 cycles sta temp lsr adc #4 lsr lsr lsr adc temp ror lsr lsr lsr
;Divide by 16 (trival) ;4 bytes, 8 cycles lsr lsr lsr lsr
;Divide by 17 ;18 bytes, 30 cycles sta temp lsr adc temp ror adc temp ror adc temp ror adc #0 lsr lsr lsr lsr
;Divide by 18 = 1/9 * 1/2 ;18 bytes, 32 cycles sta temp lsr lsr lsr adc temp ror adc temp ror adc temp ror lsr lsr lsr lsr
;Divide by 19 ;17 bytes, 30 cycles sta temp lsr adc temp ror lsr adc temp ror adc temp ror lsr lsr lsr lsr
;Divide by 20 ;18 bytes, 32 cycles lsr lsr sta temp lsr adc temp ror lsr lsr adc temp ror adc temp ror lsr lsr
;Divide by 21 ;20 bytes, 36 cycles sta temp lsr adc temp ror lsr lsr lsr lsr adc temp ror adc temp ror lsr lsr lsr lsr
;Divide by 22 ;21 bytes, 34 cycles lsr cmp #33 adc #0 sta temp lsr adc temp ror adc temp ror lsr adc temp ror lsr lsr lsr
;Divide by 23 ;19 bytes, 34 cycles sta temp lsr lsr lsr adc temp ror adc temp ror lsr adc temp ror lsr lsr lsr lsr
;Divide by 24 ;15 bytes, 27 cycles lsr lsr lsr sta temp lsr lsr adc temp ror lsr adc temp ror lsr
;Divide by 25 ;16 bytes, 29 cycles sta temp lsr lsr lsr adc temp ror lsr adc temp ror lsr lsr lsr lsr
;Divide by 26 ;21 bytes, 37 cycles lsr sta temp lsr adc temp ror adc temp ror adc temp ror lsr lsr adc temp ror lsr lsr lsr
;Divide by 27 ;15 bytes, 27 cycles sta temp lsr adc temp ror lsr lsr adc temp ror lsr lsr lsr lsr
;Divide by 28 ;14 bytes, 24 cycles lsr lsr sta temp lsr adc #2 lsr lsr adc temp ror lsr lsr
;Divide by 29 ;20 bytes, 36 cycles sta temp lsr lsr adc temp ror adc temp ror lsr lsr lsr adc temp ror lsr lsr lsr lsr
;Divide by 30 ;14 bytes, 26 cycles sta temp lsr lsr lsr lsr sec adc temp ror lsr lsr lsr lsr
;Divide by 31 ;14 bytes, 26 cycles sta temp lsr lsr lsr lsr lsr adc temp ror lsr lsr lsr lsr
;Divide by 32 (trival) ;5 bytes, 10 cycles lsr lsr lsr lsr lsr
Courtesy Omegamatrix. [2]
