Re: Switchless ROMs

From: Michał Pleban <>
Date: Thu, 22 Dec 2016 11:16:16 +0100
Message-ID: <>

Jim Brain wrote:

> It seems like those are the only two options.  I looked at the
> switchless ROM circuits (basically, a '74 hooked up to RESET as CLK and
> RESTORE as D, and I thought a small uC or GAL might also work. Still, it
> requires 2 wires.  But, then I wondered if one could have the 64 or 128
> act as the uC, and make the ROM switch.

I designed something similar few years ago, though it was more
complicated beacuse it was meant to be reprogrammable from the C64 side.
But some ideas can be reused in a simple project like this.

Basically what I did was to remove the SID (as I recall, it is always
socketed in a C64), and put it back on a small PCB that goes to the SID
socket, but also steals the SID "chip enable", PHI0, R/W and RESET
signals and routes them to the second PCB that is inserted into the
KERNAL socket.

Using these signals, you can create a simple latch which resides
somewhere in the SID address space and is used to select the appropriate
KERNAL. Upon reset, the latch would be reset to 0 to select the "KERNAL
selection KERNAL", which would write the desired KERNAL number to the
latch as well as a special bit to disable the latch until next RESET.

This is exactly the same thing that our MultiMAX cartridge does - the
latch is set to bank 0 upon reset, then the software in bank 0 writes
the bank number to the latch, setting also bit 7 which means "disable
the latch". You can do that with a 74LS273 plus some simple glue logic
- you should have MultiMAX the schematic somewhere as you drew it
yourself ;-)


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Received on 2016-12-22 11:00:02

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