Signed vs unsigned comparison (was 'What am i missing')

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bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 19 Aug 2010, 01:18

Code:

        mov     cx,1000h
        mov     ax,9fc0h
        sub     ax,cx

        cmp     ax,27e0h
        jl      lies
        jb      truth

EDIT by DOS386 : enhanced subject

_________________
Coding a 3D game engine with fasm is like trying to eat an elephant,
you just have to keep focused and take it one 'byte' at a time.

19 Aug 2010, 01:18

sinsi

Joined: 10 Aug 2007
Posts: 794
Location: Adelaide

sinsi 19 Aug 2010, 01:52

signed vs unsigned?

19 Aug 2010, 01:52

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 19 Aug 2010, 02:07

bitshifter,

Does this **really** has something to do with compiler internals or I can move this to Main?

19 Aug 2010, 02:07

bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 19 Aug 2010, 02:21

ooh, its because the value has sign bit set...

facepalm Embarassed

ok to move... (or destroy)

19 Aug 2010, 02:21

Tyler

Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA

Tyler 19 Aug 2010, 02:30

jl is signed jb isn't

With jl 1111111b would be less than 0, while with jb, it would be opposite.

Edit: You beat me to it.

19 Aug 2010, 02:30

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