not di does not seem to perform bitwise negation (lemmy.world)

submitted 9 months ago* (last edited 9 months ago) by qaz@lemmy.world to c/programming@programming.dev

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I've been trying to debug a program I'm working on and have managed to narrow it down to a single not statement. I fail to understand how this could fail. My current suspicion is the fact that I'm using a 16 bit (aka short) register. Should I even be using this?


Before `not di`	`1101111110000000`
Expected after `not di`	`0010000001111111`
Actually after `not di`	`10000001111111`

Code

.certain:
not di ; ← this line is the problem
bsf ax, di

GDB output

(gdb) p/t (short) $di $4 = 1101111110000000
(gdb) nexti
(gdb) p/t (short) $di $5 = 10000001111111

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[-] mrkite@programming.dev 20 points 9 months ago

Your result is correct, is just not displaying the leading zeros.

[-] qaz@lemmy.world 2 points 9 months ago

Thanks, I somehow completely missed that.

[-] qaz@lemmy.world 2 points 9 months ago

Do you perhaps know how to show the leading zeros?

[-] mrkite@programming.dev 3 points 9 months ago

I'm not great with gdb but I think using the x cmd shows them.

[-] qaz@lemmy.world 1 points 9 months ago

Yes, but that doesn't seem to work with registers

this post was submitted on 01 Feb 2024

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