CODEWITHSUNDEEP
assemblyx86tasm
chicken
chicken

Reputation: 1640

x86 question about bit comparisons

Im having a problem with a final part of my assignment. We get in a stream of bits, etc etc, in the stream is an integer with the number of 1's in the text portion. I get that integer and its 24 which is correct, now i loop through the text data i get and i try to count all the 1's in there. But my proc is always returning zero.

I was able to make sure it was looping properly and it is.

The text = Hello which is 16 1's, here is my proc for looping through that text to count the number of ones in it.

sub     AX,AX
sub     SI,SI
mov     bx,[bp+6]       ;get message offset    

@@mainLoop:
    mov     cx,8
    mov     dh,80h
    cmp     byte ptr [bx + si],0
    je      @@endChecker
@@innerLoop:
    test    byte ptr [bx + si],dh
    jz      @@zeroFound
    inc     AX
    @@zeroFound:
        shr     bh,1
        loop    @@innerLoop     
    @@continue:
        inc     si
        jmp     @@mainLoop

the rest of the proc is just push/pops. What im wanting this to actually do is use TEST to compare 100000000 to a byte, if its a 1 inc AX else shift right the mask by 1 and loop a whole byte, than inc to next byte and do again.

Upvotes: 2

Views: 334

Answers (2)

Nils Pipenbrinck
Nils Pipenbrinck

Reputation: 86433

Just to give you an alternative and shorter implementation of your inner-loop:

        mov    cx, 8
        mov    dh, byte ptr [bx+si] 
@@innerLoop:
        add    dh, dh    
        adc    ax, 0
        loop   @@innerLoop

Here we don't test the bits at all. Instead the add dh, dh shifts the topmost bit into the carry and also does the same as shl dh,1 in one instruction.

The addc ax,0 just adds the carry to AX.

Upvotes: 1

Menkboy
Menkboy

Reputation: 1635

'shr bh,1' should probably be 'shr dh,1', no?

Upvotes: 2

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