Question about the use of 0 in prop assembly

Chris Kraft

I am reading through the code in TV.spin and there are a couple of lines that I am not sure what they do. They all seem to use 0 as a location to copy things into.

Specifically (I don't have the line numbers handy):

: par          rdlong 0,t1

(there should not be a space between the : and par but the forums code keeps interpreting it as emoticons even inside the code block)

If I understand rdlong correctly it is taking main memory and, in this case, copying it into 0.

Further on I see the following two samples:

:wr           mov 0,t21

:ltab         mov 0,01

Does anyone know what this does? Why use 0 as a destination?

The only thing I can think of is that maybe 0 ends up representing the bottom of cog memory but that doesn't explain the use of mov 0,0 then.

If anyone has an explanation for what this is doing I would appreciate the help.

Thanks

--Chris

Post Edited (Chris Kraft) : 11/27/2006 2:28:49 PM GMT

Mike Green

All of these instructions are modified at run-time. The destination field is set to some address earlier in the code. Look for the labels (:wr, :ltab, etc.) in the destination field of other instructions and you will see how this is used. The reason for this is that the instruction set doesn't provide for indexing, so self-modifying code has to be used to access arrays/tables.

Chris Kraft

Ok. I think I got it. I saw the modification being done but it didn't register what was happening.

So if I understand correctly:

: par rdlong 0,t1

establishes : par as being those commands, then a movd modify's the destination address, for example:

movd : par, #_enable

This causes the line to modify into the following:

: par rdlong #_enable,t1

Although its not really #_enable but the address that _enable is pointing too.

--Chris

Cliff L. Biffle

Chris,

You've got it. Zero is being used here as a "don't-care" value. Some other assemblers provide an explicit symbol for distinguishing "I want this to be zero" from "I don't care what this is," often * or _, and that might be something for Parallax to consider.

Phil Pilgrim (PhiPi)

I've always used 0-0 in these situations. It evaluates to 0, of course, but it's enough of a visual cue to flag it as a value that gets modified.

-Phil