About cbw,cwde,cwd,cdq

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Roman

Joined: 21 Apr 2012
Posts: 1847

Roman 25 Apr 2023, 10:20

Why need cbw,cwde,cwd,cdq ?
cbw — sign-extend byte in al to word in ax. Why do this ?

Show example task where is needed cbw,cwde,cwd,cdq.

25 Apr 2023, 10:20

Roman

Joined: 21 Apr 2012
Posts: 1847

Roman 25 Apr 2023, 10:27

O !

25 Apr 2023, 10:27

Roman

Joined: 21 Apr 2012
Posts: 1847

Roman 25 Apr 2023, 10:27

I found. For negative numbers.

Code:

xor eax,eax
mov al,-5 ;-5=255-5=251
cbw       ;in ax=-5 but 16bits 65535-5

25 Apr 2023, 10:27

macomics

Joined: 26 Jan 2021
Posts: 1042
Location: Russia

macomics 25 Apr 2023, 11:17

Forgot cqo. rax => rdx:rax

25 Apr 2023, 11:17

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 25 Apr 2023, 11:21

In twos complement:
0xFB == -5
0xFFFB == -5

In unsigned:
0xFB == 251
0xFFFB == 65531

What happens if you add 65531 + 65531 with 16-bit registers?
0xFFFB + 0xFFFB == 0xFFF6 --> -5 + -5 == -10

What happens when you add 65531 + 5 with 16-bit registers?
0xFFFB + 0x0005 == 0x0000 --> -5 + 5 == 0

25 Apr 2023, 11:21

macomics

Joined: 26 Jan 2021
Posts: 1042
Location: Russia

macomics 25 Apr 2023, 11:27

In order not to ask such questions, it is enough to read about the additional format of numbers.

https://en.wikipedia.org/wiki/Two%27s_complement#Sign_extension

25 Apr 2023, 11:27

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 25 Apr 2023, 11:33

I think a lot of confusion happens if someone implicitly extends values with zeros.

The following might not be true:
0xFB == 0x00FB
We need to justify whether extending to the left with zeros is correct, or whether extending with the top-most bit is correct. And it depends upon the context.

25 Apr 2023, 11:33

Roman

Joined: 21 Apr 2012
Posts: 1847

Roman 25 Apr 2023, 11:35

macomics wrote:

Forgot cqo. rax => rdx:rax

Why out to rdx:rax ?

25 Apr 2023, 11:35

Roman

Joined: 21 Apr 2012
Posts: 1847

Roman 25 Apr 2023, 11:38

Oh !

Code:

mov     al,-5 ;al=FB
movsx ax, al  ;the same work as cbw ! I checked in IDA Pro ax=FFFB

;more handful then cbw,cwde,cwd
movsx   edx,cl
movsx   dx,cl
movsx   edx,cx

Crazy Intel Laughing

Last edited by Roman on 25 Apr 2023, 12:09; edited 7 times in total

25 Apr 2023, 11:38

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 25 Apr 2023, 11:38

Roman wrote:

macomics wrote:
Forgot cqo. rax => rdx:rax

Why out to rdx:rax ?

For div

25 Apr 2023, 11:38

macomics

Joined: 26 Jan 2021
Posts: 1042
Location: Russia

macomics 25 Apr 2023, 12:13

revolution wrote:

Roman wrote:

macomics wrote:
Forgot cqo. rax => rdx:rax

Why out to rdx:rax ?
For div

For idiv

25 Apr 2023, 12:13

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 27 Apr 2023, 14:30

Roman wrote:

Oh !

Code:

mov     al,-5 ;al=FB
movsx ax, al  ;the same work as cbw ! I checked in IDA Pro ax=FFFB

;more handful then cbw,cwde,cwd
movsx   edx,cl
movsx   dx,cl
movsx   edx,cx

Crazy Intel Laughing

It’s just important to note that movsx and movzx didn’t exist before 80386.

27 Apr 2023, 14:30

Ali.Z

Joined: 08 Jan 2018
Posts: 732

Ali.Z 16 May 2023, 18:23

you can play with the snippet here, trying removing cdq and see what happen.

https://board.flatassembler.net/topic.php?t=22616

_________________
Asm For Wise Humans

16 May 2023, 18:23

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