How to do... If X = 1 AND Y = 1 Then ... in ASM

Chris_D

Hi guys,

I sort of understand the CMP(s) function now and sort of understand the IF_C etc. functions and need to expand on them.· As the title shows...

·If X = 1 AND Y = 1 Then

is easy enough to understand in BASIC and perhaps even SPIN.· However, how would I go about this in ASM?

Chris
·

hippy

Something like (untested) ...

      cmp X,#1 WZ
IF_Z  cmp Y,#1 WZ

IF_Z  jmp #okay    ' X=1  and Y=1
IF_NZ jmp #fail    ' X<>1 or  Y<>1

There are probably more optimal ways as well.

virtuPIC

You simply have to convert the AND operator to two nested IFs:

IF X = 1 THEN
IF Y = 1 THEN

In assembler this gives

       cmp X, #1 WZ
IF_Z cmp Y, #1 WZ

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Airspace V - international hangar flying!
www.airspace-v.com/ggadgets for tools & toys

Phil Pilgrim (PhiPi)

For this particular example (testing both for 1):

        djnz    X,#fail nr
        djnz    Y,#fail nr
succeed ...

-Phil

MagIO2

Hi Phil,

again a nice solution from you. But for a newbie you should have mentioned, that your solution will change the values of X and/or Y.

hippy

@ MagIO2 : Not with NR as a qualifier

lonesock

I would probably still mention that with each DJNZ, if the jump is not taken it would require 4 extra clocks. So to get to the 'succeed' label, it would take 16 clocks. Not that this is a problem, especially if you are going for code density!

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lonesock
Piranha are people too.

MagIO2

Argh ... sure ... did I already tell you that I like this propeller? You can throw away all your experience.

Phil Pilgrim (PhiPi)

Good point, lonesock. However, if you compare the djnz code with

        cmp     X,#1 wz
   if_z cmp     Y,#1 wz
  if_nz jmp     #fail
succeed ...

you will see that the above requires 12 clocks to get to succeed or fail, while the djnz code takes 16 to get to succeed and 4 or 8 to get to fail. So the optimum choice will depend on the relative probabilities of the two cases.

-Phil

Fixed formatting.

Post Edited (Phil Pilgrim (PhiPi)) : 3/28/2009 9:06:30 PM GMT

Beau Schwabe

Phil Pilgrim (PhiPi),

I like your use of the djnz, however if you wanted something that followed more closely to ... If N=0 then ... or ... if N=1 then ... you could use the TJNZ or TJZ command.

        tjnz    X,#fail     'jump to address if not zero
        tjnz    Y,#fail     'jump to address if not zero 
succeed ...

·or..

        tjz     X,#fail     'jump to address if zero
        tjz     Y,#fail     'jump to address if zero 
succeed ...

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

Beau,

Yup! But I think you meant, "If N<>0", rather than, "If N==1", right?

Actually, "If X==0 AND Y==0" is quicker with

        or      X,Y nr,wz
  if_nz jmp     #fail
succeed ...

-Phil

Beau Schwabe

lol @ Phil Pilgrim (PhiPi)

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

This is one reason I love the Propeller instruction set so much. With selective result and flag writes, along with conditional execution, a lot can be accomplished in just a few instructions. It's almost like writing microcode.

-Phil

Chris_D

Wow,

That's a lot of suggestions!· Thanks very much for all of them!

I will have to do some experimenting with the various examples so I can understand them but this should be enough to help me work out the structure for more complex decision making branching and such.



Chris