Writing to a AT28 from Collapse OS

Gathering parts

* A RC2014 Classic
* An extra AT28C64B
* 1x 40106 inverter gates
* Proto board, RC2014 header pins, wires, IC sockets, etc.

Building the EEPROM holder

The AT28 is SRAM compatible so you could use a RAM module for
it. However, there is only one RAM module with the Classic
version of the RC2014 and we need it to run Collapse OS.

You could probably use the 64K RAM module for this purpose, but
I don't have one and I haven't tried it. For this recipe, I
built my own module which is the same as the regular ROM module
but with WR wired and geared for address range $2000-$3fff.

If you're tempted by the idea of hacking your existing RC2014
ROM module by wiring WR and write directly to the range
$0000-$1fff while running it, be aware that it's not that
easy. I was also tempted by this idea, tried it, but on bootup,
it seems that some random WR triggers happen and it corrupts the
EEPROM contents. Theoretically, we could go around that by
putting the AT28 in write protection mode, but I preferred
building my own module.

I don't think you need a schematic. It's really simple.

Writing contents to the AT28

If you wait 10ms between each byte you write, you can write dir-
ectly to the AT28 with regular memory access words. If you don't
wait, the AT28 writing program will fail. Because it's not very
pratical to insert waiting time between each byte writes, you
need another solution.

B321 contains an override routine called AT28$. When you call
this, It defines new "C!" and "!" words and those words ensure
that data is properly written to EEPROM before returning.

Note that because it's new definitions for "C!" and "!", these
are only going to work for direct execution or for words
defined after you've called "AT28$".

When you're done writing to the AT28, you can unset the over-
ride with "FORGET C!".

When polling, AT28 routines also verifies that the final byte in
memory is the same as the byte written. If it's not, it will
place a non-zero value in the IOERR 1b variable. Therefore, if
you want to see, after a big write operation to your AT28,
whether any write failed, do "IOERR C@ .". Re-initialize to zero
before your next write operation.