x86 ROL on ARM?

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
x86 ROL does this:

bit 0 to bit 1... bit 30 to bit 31
bit 31 to both bit 0 and carry flag.

Since I'm new to arm asm, is there a quick way of doing this?
Or should I just change the algo to use ARM's ROR?



Re: x86 ROL on ARM?
Quoted text here. Click to load it


You can't do exactly the same in one instruction you can do either

"ROR provides the value of the contents of a register rotated by a
value. The bits that are rotated off the right end are inserted into
the vacated bit positions on the left."

"RRX provides the value of the contents of a register shifted right one
bit. The old carry flag is shifted into bit[31]. If the S suffix is
present, the old bit[0] is placed in the carry flag."

-p
--
Paul Gotch
--------------------------------------------------------------------

Re: x86 ROL on ARM?

Quoted text here. Click to load it

ROR doesn't update the carry flag? (with bit 0)



Re: x86 ROL on ARM?
dot.net says...
Quoted text here. Click to load it


Hmmm,  except for the carry, could you you use  ROR  #31, R0 to get
the equivalent of a 1-bit ROL?

Could you generalize that for ROL with more than one bit as something
like  ROR  #(32-shift), RO?

I can see this one is going to cost me a sheet of paper---either that
or I give up and just go have a drink while someone else thinks about
it!   ;-)

Quoted text here. Click to load it
Mark Borgerson



Re: x86 ROL on ARM?
says...
Quoted text here. Click to load it
Are you shifting just one bit at a time?  

If so, could you use something like  

     ADD  RO,RO     ;  shifts left one bit, zero in bit zero
     ADDC #0, R0    ;  put the carry bit into bit zero


If you're shifing multiple bits, I'm not sure what significance
the carry bit would have after you're done.

Being unfamiliar with the arm assembly language,  I don't know if
the above is either valid or proper syntax.

Mark Borgerson

Re: x86 ROL on ARM?
Quoted text here. Click to load it

Yes, one bit at a time!

My father told me the same thing yesterday,
but, comming from the Z80 world, we both concluded that
ADC will destroy the final carry flag.
But not in ARM, since avery inctruction MAY have S suffix!

So, this should work:
adds r2, r2, r2 ; shift & update carry
adc  r2, r2, #0 ; complete the shift, do not change carry

Thank you



Site Timeline