Reading from write-only TIA registers

[vdub_bobby]

What do you get when you read from the write-only TIA registers, like GRP0, HMP0, etc.?

 

I know you don't get the value that's actually in the registers, but what do you get?

 

I was messing around with PCAE testing some of this but I'm not sure if that emulator correctly emulates this stuff - anybody know? - so I thought I'd ask you smart folk

 

For example, in PCAE, after this:

lda #$FF

sta GRP0

lda GRP0

A is zero.

[vdub_bobby]

Trying to follow the trace file in z26...

 

And after this code:

lda #$FE

sta GRP0

lda GRP0

A holds $1B, which is the address of GRP0, when run by z26. Which seems odd.

[Thomas Jentzsch]

Have a look at the source codes of Stella or z26 (PCAE is quite outdated and not nearly as reliable as those two). There you should find some information.

 

And search the [stella] mailing list, IIRC your question was diskussed there too.

[vdub_bobby]

Thanks, I found some stuff on Stella that answered most of my questions.

[Bruce Tomlin]

What do you get when you read from the write-only TIA registers, like GRP0, HMP0, etc.?

You get the contents of the collision registers, which are mirrored every 16 bytes. And I guess you get bus float from the remaining 4 (?) unused read locations.

[vdub_bobby]

What do you get when you read from the write-only TIA registers, like GRP0, HMP0, etc.?

You get the contents of the collision registers, which are mirrored every 16 bytes. And I guess you get bus float from the remaining 4 (?) unused read locations.

 

Yeah, the stuff I found on Stella (http://www.biglist.com/lists/stella/archives/200107/msg00045.html) said basically the same thing.

 

What is the bus, though?

[Nukey Shay]

Riders Then they get off and there's just the driver until he goes home.