calc string length in asm

AGS · 27 February 2016, 11:17

Hi,

I am a bit confused. Is the following code correct to calc the length of a string in bytes or should the "not.w" rather be a "neg.w"?

Code:

                      moveq    #-1,d0
.name_length          tst.b    (a0)+
                      dbeq    d0,.name_length
                      not.w    d0

hooverphonique · 27 February 2016, 12:17

if you use neg, the count will include the terminating zero, if you use not, it won't.

AGS · 27 February 2016, 12:21

ah, ok. thanks. this was exactly what I need to kow.

Photon · 27 February 2016, 15:31

A standard technique was also to put a byte before the first char containing the length of the string from 0 to 255. All string functions that insert or delete characters must then update the length value, but that's just 1 instruction. It's much more efficient.

TheDarkCoder · 09 March 2016, 19:31

neg is equivalent to not followed by add #1

So the sequence

neg d0
add #1,d0

can be optimized as

not d0

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27 February 2016, 11:17	#1
AGS XoXo/Tasko Developer Join Date: Dec 2013 Location: Munich Age: 48 Posts: 450	calc string length in asm Hi, I am a bit confused. Is the following code correct to calc the length of a string in bytes or should the "not.w" rather be a "neg.w"? Code: moveq #-1,d0 .name_length tst.b (a0)+ dbeq d0,.name_length not.w d0

27 February 2016, 12:17	#2
hooverphonique ex. demoscener "Bigmama" Join Date: Jun 2012 Location: Fyn / Denmark Posts: 1,635	if you use neg, the count will include the terminating zero, if you use not, it won't.

27 February 2016, 12:21	#3
AGS XoXo/Tasko Developer Join Date: Dec 2013 Location: Munich Age: 48 Posts: 450	ah, ok. thanks. this was exactly what I need to kow.

27 February 2016, 15:31	#4
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,647	A standard technique was also to put a byte before the first char containing the length of the string from 0 to 255. All string functions that insert or delete characters must then update the length value, but that's just 1 instruction. It's much more efficient.

09 March 2016, 19:31	#5
TheDarkCoder Registered User Join Date: Dec 2007 Location: Dark Kingdom Posts: 213	neg is equivalent to not followed by add #1 So the sequence neg d0 add #1,d0 can be optimized as not d0

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