Is it possible to upgrade a Lisa to run at 8MHz or more by replacing the CPU & clock

[TorZidan]

I know about the unobtainable proprietary Lisa XLerator card and the relevant discussion at https://lisalist2.com/index.php?topic=594.0

But I wonder if anyone was able to speed up their Lisa by replacing just the CPU and the clock generator  with faster one?

I am sure this has been discussed in the past and also at 68kmla.org, but I can't find a good answer.

People mention the custom video refresh timings and the slow RAM, but somehow XLerator was able to overcome them. It seems that it is using the Lisa RAM for video buffer, and its own faster on-board RAM for system memory. 

Relevant links:

https://bitsavers.org/pdf/apple/lisa/hardware/1983_Whopper/Whopper_ERS_V2.0_Jul83.pdf describes a Lisa "Whopper" project  at Apple that never got completed. They wanted to run the CPU at 10 MHz: "A 10 MHz CPU clock has been selected because 10MHz  is 1/3 the 30MHZ clock used in the video circuit".

Why Lisa didn't use 8MHz 68000? https://68kmla.org/bb/threads/why-lisa-didnt-use-8mhz-68000.39609/

A 68010 in a Lisa? https://lisalist2.com/index.php?topic=43.0

[ried]

You know, I was recently wondering the same thing. Some time ago I picked up a Macintosh Plus with a 16MHz "Brainstorm Accelerator" installed and wondered what it might take to get that part working in a Lisa. It looks to be quite simple.

I'm sure there are quite a few Lisa-specific complications to consider, and sigma7's XLerator is a much more robust and capable solution with additional RAM, SCSI, FPU support and so on.

Still, I wonder. 

[patrick]

Typical 68k accelerators use local memory. This means you have a computer inside the computer that runs at high speed. For external access, such as input/output (I/O) or video, the speed is either slowed down (synchronous access) or the accelerator waits for a DTACK signal from the memory or peripheral (asynchronous access).

There was usually a GAL that defined the address space, i.e., where the CPU had to slow down. This made it easy to adapt the board to Amiga, Atari, and Macintosh computers.

I assume the XLerator works in a similar way.

[Eschaton]

If you just replace a Lisa's CPU itself, you'll see no difference since the 5ish MHz clock is still driving it; the CPU speed is a rating, the actual clocking of the CPU is external. (It wouldn't actually surprise me if "newer" Lisa actually used 8MHz part simply because the slower-rated models weren't on the market for very long.)

The other thing is that Lisa's 5MHz clock results from a bunch of different things share one clock domain, as was the style at the time. Something like XLerator has to implement separate clock domains for (say) video timing/RAM versus everything else. This is just a matter of engineering, and is made easier by the 68000's asynchronous bus, but it's not always trivial.