Saturday, April 11, 2015

Easter Update

So yet again I've managed to go a couple of months without updating the Prism blog. Mister Polo poked me again, so here we go. In my last update I listed a number of things that I'd be working on this year. I'm happy to say that a number of them have already been successfully achieved.

Firstly, the MMU (memory control functionality) has been rewritten to make it easier to modify in future. It also now handles all memory as 8K pages instead of 16K pages which means that I could add the Timex/Spectrum SE/Chloe 280SE memory paging model.  The Spectrum 128/Pentagon memory paging model still works exactly as expected. 

Secondly, I've added in support for the Timex/Spectrum SE/Chloe 280SE screen modes. The shadow screens that the Spectrum SE and Chloe has were already implemented as Prism implements it's screen selection in the same way. Likewise, the Timex "Hi-Color" mode is the same as Prism's. The Timex/SE/Chloe 512x192 mode is different to Prism's however - where Prism's is full colour and is basically screen 0 and screen 1 side-by-side, the Timex/SE/Chloe mode is monochrome and interlaces alternate sets of 8 pixels.  This screen mode is used in SE Basic's 80 column mode, which is the main reason I implemented it (Prism's 512x192 mode is better!)

Here's a video of Prism running Andrew Owen's (Chloe) MMUtest program:


To make my life easier, I wrote a small menu program which auto-loads from SD card when Prism boots. It lets me switch between different configurations - ZX Basic with 512 or 48K, SE Basic, the ZX81 emulator etc. It also shows off 256 colour mode - though as you can see the default palette has a number of blacks and whites at the moment!

I also fixed a bug in the T80 soft CPU core where incorrect flag behaviour was observed following an LD A,R or LD A,I instruction. This fix was based on one done by the Speccy2010 team. This finally fixes Midnight Resistance, Greeen Beret, Hypersports, Gutz, King's Valley and a number of other things. After that, I was on a roll with getting games working so I also updated the floating bus emulation so now Sidewize works too (albeit with flickering sprites due to the timing diferences between Prism and a real Spectrum).

Finally, after all this time I've actually gotten around to adding support for the "flash" attribute in standard attribute decoding mode! The Manic Miner loading screen works properly for the first time!

As a bonus to make up for so long without an update, the following three pictures are a teaser of something I'm working on (and definitely not an April fool - that was over a week ago). What could this be the first stirrings of I wonder?