ASM Community

I remember someone, I think it was Erine, asked a question before on a qiuck way to see if a number lay inside a paticular range. While working on someting else this solution occured to me. Below is some code for both integer and real numbers. Both cases are for signed numbers. Basically it works on the fact that if A lies between B and C the B - A & C- A will have different signs. Otherwise the signs will be the same, either both positive or negative. A quick xor or multiplication will test this. Note for the below code it is not necessary to know the smaller and the larger of the two extreems. In this way for arbitary numbers you actually avoid three conditional jumps. Integers

 
; Test if eax is inside range edx <-> ecx.
; Does not include eax = edx / ecx

	sub edx, eax
	sub ecx, eax
	xor edx, ecx
	test edx, 80000000h

	jz @F
		; <- here if eax is inside range
	@@:
		; <- here if eax is outside range

And for Real values

; Test to see if st(0) lies inside the range st(1) <-> st(2)
; For jb then st(1) & st(2) included in range. Not for jbe

	fsub st(2), st
	fsub
	fmul
	fldz
	fcompp
	fstsw ax
	sahf

	jb @F	; Set to jbe to exclude st(1) & st(2) from range.
		; <- here if st is inside range
	@@:
		; <- here if st is outside range

All I really what to explain here is the method, my translation of the maths to assembly may not be the best, plus my assumption on the order of values in the FPU may not suit your coding. I hope however that you understand the theory, then you should have no problem translating it to suit your needs Good Luck. Zadkiel

I never actually knew there was a

jns

, thats a nice optimisation.