base:8-bit_ranged_comparison

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base:8-bit_ranged_comparison [2015-04-17 04:30] 127.0.0.1 external edit |
base:8-bit_ranged_comparison [2015-04-22 20:15] (current) ftc |
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+ | ====== Range Checking a Byte ====== | ||

+ | ===== Approach by White Flame ===== | ||

+ | |||

+ | In checking for the range [x,y), instead of performing 2 comparisons the idea is to subtract x to align the range to [0,y-x). This then allows us to perform a single unsigned comparison to check both ends of the range. | ||

+ | |||

+ | Any numbers lower than the original range will have wrapped the byte into the high values <=255, and any number higher than the original range will still be too large. | ||

+ | |||

+ | <code asm> | ||

+ | ; Check .A for the range [10,100) | ||

+ | sec | ||

+ | sbc #10 ; start of the range | ||

+ | cmp #90 ; length of the range | ||

+ | bcs fail ; result needs to be 0-89 to pass the original 10-99 check | ||

+ | ... ; .A is in range here, and Carry is clear | ||

+ | </code> | ||

+ | |||

+ | ===== Approach by Lee Davison ===== | ||

+ | |||

+ | 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. | ||

+ | |||

+ | <code asm> | ||

+ | CLC ; clear carry for add | ||

+ | ADC #$FF-m; make m = $FF | ||

+ | ADC #m-n+1; carry set if in range n to m | ||

+ | </code> | ||

+ | |||

+ | Obviously, if the state of the carry flag is always known before executing this code, you can skip the CLC as well and adjust the routine accordingly. |

base/8-bit_ranged_comparison.txt ยท Last modified: 2015-04-22 20:15 by ftc