Re: C64 2 Mhz Hack
Date: 2001-02-27 12:08:54

Hallo Per,

> If you have a way of intercepting write cycles to memory it shouldn't
> be too hard to implement mirroring. This way the CPU will read at X
> MHz and write a 1 MHz, running at full speed most of the time.

Why didn't I think of that myself? That is a very good solution indeed !!!
A small addition: include the I/O area as well.

> gb> This triggered another question: the 65816 can run on 20 MHz. The
> gb> SIMMs SCPU uses ar mostly of the 70 ns type. 20 Mhz means 25 ns
> gb> cycles. How does that fit?
> Wait states. I don't have the numbers with me, but access to SuperRAM
> can take several cycles.

I thought of waitstates as well. A Z80 or 8088 needs several clockcycles to 
perform a one-byte instruction. With these CPUs it makes sense to stretch 
the cycle which actually accesses the memory or I/O. But the 6502 accesses 
the memory every cycle of an instruction. 

With 70 ns the 65816 has to insert 1 waitstate for every instruction. The 
effect is 10 MHz. But if you run it at 10 MHz, you have a 50 ns. cycle and 
you would still need a waitstate. With 6.666 Mhz you don't need waitstates 
but this is slower then the 20 MHz "10 Mhz".

The above occured to me at the moment I was going to answer that it is 
nonsense to run it at 20 MHz. The above proves that it is sometimes very 
usefull to discuss things. I certainly learned something of it.

Groetjes, Ruud

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