Semester has ended.. and I did some PCE timing tests today

Some bad news for the TSB/TRB instructions for VDC port $0003; it’s damn slow. It’s not the instruction, but apparently there’s a delay for when the VDC switches from reading to writing vram back to back. I.e. doing a bunch of TRB $0003 to increment the vram point is going to block by 6.5 cycles more than the expected 9 cycles. This is even the case of back to back LDA $0003 STA $0003 instructions – the processor is stalled by the VDC and ends up being ~15.6 cycles instead of 12 for the single pair.  Of course, this was only tested at lowest resolution. If the overhead should be half that for high res. I’ll need to test for that. I still have more vdc read/write setups to test.


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