P/M with 34 in 559

[Probabilitydragon]

Hi!

 

I've been recently poking around with my Atari, exploring colour character sets and sprites. Doing animated dragons at GR13 and working on a P/M dragonslayer. My skills at hobbyist level at most.

 

With the default value 34 in the address 559, the on-screen players become a busy data flicker of 8x8 dimension pixels.

 

Now,

 

1) what the heck is going on there, where's the data that is displayed in the player from?

 

2) I like that effect, any games/programs out there utilizing that?

Edited October 5, 2022 by Probabilitydragon
typo

[Rybags]

You can have PMs without enabling the Antic DMA or setting a bit in GTIA's GRACTL.

 

You can just store direct to the GRAFPn register.  But this has to occur every scanline or at least when you want the pixels to change... so potentially CPU intensive.

If you enable the bits in GRACTL, GTIA will automatically grab whatever is on the data bus at appropriate times early each scanline and put them into the GRAFP/GRAFM registers (you can have missile without player but player will enable both) - if Antic DMA isn't fetching the data for you then you get "bus noise".

In most normal situations you set DMACTL to do the PMG DMA either in 1-line or 2-line mode.

Note that regardless, Antic will do DMA each scanline, in 2-line mode it just fetches the same data 2 lines in a row.

 

Back to the bus noise thing - generally it'll be a CPU instruction, operand or sometimes a Display List element.  It can change depending on what the system is doing.

e.g. press Ctrl-1 to pause the screen output then press a key, it will look less random than before.

 

Advanced usage - you can do object reuse on a scanline by repositioning it once the first copy starts displaying.

The GRAF data will be reused but you could also store new GRAF data to have an object twice with differrent display pixels.

This sort of thing isn't done a lot - repositioning and storing new graphics and/or colour takes a lot of CPU time and it can be fiddly to do if the 2 copies are moving around.

Edited October 5, 2022 by Rybags

[Probabilitydragon]

lots of stuff to learn there - I like the look of that bus noise  

[OxC0FFEE]

It's always reminded me of Yar's Revenge (this time, an accident rather than by design)