bits shift

[rufus16]

This is not related with masm but couldnt find any mnemonic for this so im asking here.

Is there any mnemonics for bits shift that  can do that:

before  0000000000000000000000000001 -32 bits

after    00000000000000000000000000111

thanks in advanced

[qWord]

maybe this does it:

Code Select Expand

mov eax,1
shl eax,4
sub eax,1
more general:

Code Select Expand

mov eax,1
mov ecx,nBits
lea ecx,[ecx+1]
shl eax,cl
sbb eax,1

[dedndave]

not exactly a "bit shift"
but.....

Code Select Expand

       mov     eax,1    ;or 0
       imul    eax,7

[raymond]

How about:

Code Select Expand

mov  eax,1
mov  edx,-1
shld eax,edx,2  ;you can use any count you need
;with a count of 2, EAX becomes 00000000000000000000000000000111
;and EDX remains -1

You could actually start with EAX = 0 followed by
mov  edx,-1
shld eax,edx,3

to get the same result in EAX.

[dedndave]

my assumption was that if the input value was 0, the output value would also be 0
otherwise, we'd just use

Code Select Expand

        mov     eax,7
:P

or, perhaps...

Code Select Expand

        or      eax,7

[dedndave]

that being the case, this may be the fastest way   :bg

Code Select Expand

        mov     eax,1   ;or 0
        neg     eax
        and     eax,7

but IMUL EAX,7 is probably smaller

[raymond]

I was only trying to answer his specific question:

Is there any mnemonics for bits shift that  can do that:

[dedndave]

you could also use CDQ

Code Select Expand

        ror     eax,1
        cdq
        and     edx,7
but, it uses an additional register - and an additional instruction

another way...
combine CDQ with Ray's SHLD idea

i am guessing that this is fastest, though

Code Select Expand

        neg     eax
        and     eax,7
and it uses only the one register

[donkey]

you could also use CDQ

Code Select Expand

        ror     eax,1
        cdq
        and     edx,7
but, it uses an additional register - and an additional instruction

another way...
combine CDQ with Ray's SHLD idea

i am guessing that this is fastest, though

Code Select Expand

        neg     eax
        and     eax,7
and it uses only the one register

mov eax,7  :bg

I'm guessing that although the example was fixed at 7 he was looking to shift because he needed to vary the size of the shift otherwise there really is no point in using anythng but MOV. In which case Ray's works best.

[dedndave]

 :lol

Code Select Expand

        or     eax,7
2 bytes shorter   :bg

[rufus16]

Hello once again. I should have given a little more information about my problem but QWords ans Raymonds answers might work.

The numbers of bits starting  from the lest significiant  that i want to set to 1  is generated dynamicially . number 7 was only an example.

[donkey]

Hi rufus16,

If you only want to set a specific number of low order bits to 1 then you can use this:

Code Select Expand

mov eax,1
shl eax, nBits
dec eax

Where nBits is the number of bits to set (0-31) - 32 is a special case.

[sinsi]

Code Select Expand


sub eax,eax
or edx,-1
shld eax,edx,number_of_bits

donkey, huh?

edit: donkey, oops (must remember to read and comprehend...) :red

[rufus16]

Thanks. Looks like donkeys sulution works.