A matter of contention

Once interrupts were fixed, I turned my attention to emulating the Spectrum's floating bus. Basically, I keep a copy of the last byte read by the video circuit. If the CPU reads an I/O address where the LSB is FF, I feed it a copy of that  last byte when the screen is being drawn, otherwise I send back 0xFF. At some point in the future, I'll alter this so that this is the response to ANY unused I/O address but it suffices for now.

The next thing to tackle was memory and IO contention. Whilst ZX Prism currently timeslices the bus between the video circuit and the CPU to avoid bus collisions, quite a lot of software uses precise timings (for screen efects etc).  After much messing around with alternatives, I bit the bullet and implemented IO and memory contention based on the 6C0001 ULA's circuit as described in detail in Chris Smith's ULA book. It didn't work. At all. In fact the screen erupted in a multicoloured mess.

Puzzled, I poked around and tried various things to the point of calculating propegation delays around the circuit, but to no avail. It was then I realised that I hadn't inverted the incoming clock signal. Hey presto. Prism initialised.

I didn't have much time for further testing at this point, but the couple of tests I did perform were encouraging although there's definitely a couple of things still need fixing:

• Aquaplane - Split border effect is almost correct (it occurs just under 1 line early)
• Fusetest - runs without crashing, but shows a contended frame length which is shorter than expected and most other tests show as failed
• ULATEST3, Floatspy - both still crash
• IR_contention - A very interesting effect here. This test shows four coloured boxes in the top and bottom border which should be parellel to each other. They are... however they are twice as long on ZXprism as they are on a real spectrum (or on the Spin emulator in 48k mode) - in fact the whole line wraps round on-screen. Looks like I need to take another look at the video and interrupt generation.

You will notice that I've implemented a 48K Spectrum contention solution on a machine which is compatible with the 128K Spectrum. I will be changing the contention so that it contends the appropriate memory pages in the future.