On Sun, Sep 6, 2015 at 11:41 AM, Jay West <jw...@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