21MX proms (per request)

[Glen Slick]

On Sun, Sep 6, 2015 at 11:41 AM, Jay West <jwest at classiccmp.org> wrote:
> Several people asked, here's the scoop:
>
> The common proms for HP 21MX M/E/F are 1K, 4K, and 8K.
>
> 1K are used for either loader roms on the cpu board or microcode on the FAB
> 4K are used for microcode on the FAB or FEM
> 8K are used for microcode on the FEM
>

The loader PROMs are 256x4 while the firmware PROMs are either 512x8
or 1024x8. The firmware has some interesting code to unpack the 256x4
loader PROMs into 64x16 format in memory, and patch in the I/O select
code while it is doing it.

I programmed at least one loader PROM using a BP Micro programmer. I
forget exactly which PROM part I used. I probably have a very small
number of blank PROMs left. I forget what loader PROM I swapped out
and which one I swapped in. Sometime I'll have to fire up the system
and dump the loaders PROMs into memory and check which ones I
currently have installed, and make some notes so I don't have to
wonder again later.

-Glen