Assembly - deSilva examples

Brian_B

I know this question has probaly been asked ,maybe even by me. In this example code does does the value of pattern stop and reset at 255 or does it fill the whole 32 bits. If it does keep counting how would you stop it at 255 and restart at 0.




Brian


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"Imagination is more important than knowledge..." Albert Einstein

Graham Stabler

Pattern would go all the way up to FFFF_FFFF and then loop around again but you would only see the first 8 bits on OUTA because of the way DIRA was set up. To make it only go up to 255 you could replace the jmp with a djnz command using another variable initialized with 255 as a counter. Or you could do a cmp with pattern and make the jmp conditional based on the flags set.

        cmp  pattern,#255  wc
if_c   jmp   #loop                         ' if pattern < 255
        mov  pattern,#0                ' Else reset pattern
        jmp   #loop

Something like that

kuroneko

Alternatively you could perform the equivalent of outa := $FF & count++ (in an endless loop).

Post Edited (kuroneko) : 3/26/2010 2:20:32 PM GMT

Graham Stabler

I guess that was a subtle point of the example, although the counter might go to 32 bits the 8 bit pattern would still repeat as it did so. A simpler example: if we imagine the prop was only 4 bit then the pattern would do this:

0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
and repeat

But notice that if only the first two bits were output you would still get 0 1 2 3 0 1 2 3 0 1 2 3 .... output on the pins

If you wanted to use other pins at the same time then you could not use desilva or kurenko's methods (which are equivalent I think because the dira register acts like the & $FF).

You could do outa ^= ((outa & $FF) ^ ($FF & pattern++))

The xor in the brackets acts to indicate which bits in the first byte of outa need to be toggled, the ^= does the toggling.

Graham

Post Edited (Graham Stabler) : 3/26/2010 10:53:54 PM GMT

Brian_B

ok,
So this is the part that always gets me, by moving pattern to outa (without the start over after 255) you are making all of outa's part of the 32 bit counting program (even thought anything above pin7 has not been set to a output in the dira register).

If you have another part of the program that uses pin 31-24 to count down how do you make sure that at some point that pattern is not interfering with pattern1.

I know that you would never write a program this way ,but I'm try to wrap my brain around how to not get one part of the program interfering with another part.

Brian

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"Imagination is more important than knowledge..." Albert Einstein

Graham Stabler

Brian_B said...

If you have another part of the program that uses pin 31-24 to count down how do you make sure that at some point that pattern is not interfering with pattern1.

That is what this does:

outa ^= ((outa & $FF) ^ ($FF & pattern++))

(outa & $FF) gives you the current state of the lowest 8 bits of outa
($FF & pattern) gives you the lowest 8 bits of pattern

XORing them (^) gives you ones in the bits where they are different (but only in the low 8 bits)

Then when you do outa ^= you toggle the bits of outa that you found to be different from the lowest 8 bits of pattern while not changing anything else. Write out your xor truth table if it is confusing.

Graham

Brian_B

Cool,
That one has been bugging me for awhile now.

Thanks everyone

Brian

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"Imagination is more important than knowledge..." Albert Einstein