inverse video, Check
inverse border, Check
Double Speed, Check
Quadruple Speed.... Wait what... anyway... CHECK
just the good ole POKE ..
4X speed withg ZXpand ?? probably not... I can't get it stable at all.
4x speed is possible without modifying the main board, we now have 6 wires, 5 you already know about ( extra address lines) the extra wire is connected to the CPU's wait input and is used only to add wait states to IO operation, because at 4x when running a program and rom checks for break key it would always think break was pressed but with extra waits its all good again.
20Mhz CPU, 100ns ram (only 8k here but all i had lying around) , and 70ns rom... (bigbangx2)
IF this makes it out into the wild it will be by no means guaranteed to work.. but still great results can be achieved with bigbangx2 rom and double speed which of course works on many more machines.
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One thing could be to keep it but only mention double speed in your user documents
Or mention quad speed and mention it only works in rare circumstances.
You say 70ns ROM for Big Bang, I assume ,after seeing your video that, this is a custom chip?
Will the 2764 EPROM I use for Big Bang still work at ULA double speed?
If not do you have any plans for making the chip you used available to buy?
Being a non teccy I don't possess the skills to make one.