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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.

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

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.

	CLC	; clear carry for add
	ADC	#$FF-m; make m = $FF
	ADC	#m-n+1; carry set if in range n to m

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 · Last modified: 2015-04-22 20:15 by ftc