[BL vs BX] rol instruction not cycling?

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theguy

Joined: 22 Jan 2015
Posts: 10

theguy 26 Jan 2015, 14:29

At no surprise i'm stumped again.

First here's code that iv'e tried that works as expected.

Code:

ORG   100h


 mov     bx,00000001b
 rol     bx,2
 mov     dl,bl
 add     dl,'0'    ; sets it up with the ascii to equal 4

 mov     ah,02h
 int     21h       ; display 4 in console

 mov ah,08
 int 21h           ; readkey
 int 20h

But if i try something like this

Code:

ORG   100h


 mov     bx,10000000b
 rol     bx,2
 mov     dl,bl
 add     dl,'0'    ; sets it up with the ascii to equal 2

 mov     ah,02h
 int     21h       ; display 2 in console, but i end up with 0 instead

 mov ah,08
 int 21h           ; readkey
 int 20h

I end up with 0 which is not what i expected, i thought it was going to be 2.

26 Jan 2015, 14:29

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20632
Location: In your JS exploiting you and your system

revolution 26 Jan 2015, 14:38

bx is a 16-bit register so the bit is 'rol'ed into the high byte. Perhaps you want to use "rol bl,2"?

26 Jan 2015, 14:38

theguy

Joined: 22 Jan 2015
Posts: 10

theguy 26 Jan 2015, 14:41

Darn im an idiot, can't believe i didn't see that, thanks for your help sorry about wasting your time.

26 Jan 2015, 14:41

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20632
Location: In your JS exploiting you and your system

revolution 26 Jan 2015, 14:44

Anytime we learn something is not a waste of time.

26 Jan 2015, 14:44

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