[sebhc] 64K memory

Carroll Waddell CarrollWaddell at sc.rr.com
Sun May 16 23:07:07 CDT 2004

I heard that. I'm old with gouty fingers and not so great eyesight.
(But playing with this stuff is fun,,, even if I am old.)

Lee Hart wrote:

>Carroll Waddell wrote:
>>I have been designing a 64K memory card for the H8... If there is
>>enough interest, we could get a PCB fab house like PCBExpress to
>>make the PC boards... This is a half size (6 x 6) card.
>Carroll, rather than make it a half-size board, I would rather see it a
>full-size board, and make good use of the space.
>First, H8 boards have large spacings, so they are easy to build. Clumsy
>fingers, tired old eyes, and poor soldering skills aren't a problem. Now
>that *I* am clumsy, old, and poor, these attributes would be most
>Second, the speed of a computer is largely set by the CPU-to-memory
>link. Putting the CPU on one board and the memory on another forces all
>transfers to be limited by the motherboard. The H8 has a very simple,
>but slow motherboard.
>So, I would put the Z80 and memory on the same board. Then the Z80 can
>run *very* fast (up to 20 MHz with modern chips) when accessing on-board
>memory. When accessing OFF-board memory or I/O, slow the Z80 back down
>to its standard 2 MHz. This avoids any problems with motherboard or
>other board speeds.
>I did this on my H-1000 design (a replacement CPU board for the H89, and
>it was very successful. The Z80 could run at 4 MHz or 8 MHz for on-board
>RAM, but slowed to 2 MHz for ROMs, I/O boards, or accessory memory
>boards. Every stock accessory board we tried worked perfectly with no
>cuts or patches. No BIOS patches were needed, unlike all the 4 MHz mods
>for the H89.

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