Re: How to design non-trivial cartridges for c-64?

From: Pasi Ojala (
Date: 2007-05-27 20:34:35

> Common wisdom is to use the 1mhz system clock (PHI) to clock the
> trueness, I.E. it becomes the strobe.

According to my old hand-drawn schematics I used (PHI2 and not /IO2)
as a latch enable for a 8-bit latch (74ls373) when I created a 256kB
RAM-disk (8 x 32kB SRAM's) in 1989 to be used with my own BBS.
The latch was used as a write-only bank select for 256-byte pages
that appeared in IO1.

The software part was also neat. I implemented a RAMDISK that was
hooked to the SAVE and LOAD vectors (but not the other vectors).
Even directory load was supported. The BBS used the disk to cache
message files. Only if a message was not found, it was loaded from
disk (1581), and saved to the RAMDISK for later access.

Well, this goes a bit off-topic, but the BBS also had its own character
set to support ASCII directly, and its own keymap partly for the same
reason. The keymap also replaced normal shift-C= handling to toggle the
VIC-II blank screen -bit so the BBS could run faster when I was not on
the "console".

The keymap replacement routine became invaluable later when one of the
CIA IO-bits got damaged and I lost one character row on the keyboard:
I put the missing characters on the function keys until I got a
replacement CIA. Coding the change with one row of missing keys was
a bit of a challenge though :-)

"When I marry, I want it to be for love."
"Ah, a radical?"
	-- Vir and Lyndisty in Babylon 5:"Sic Transit Vir"

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