Support for multiple banked ROMs in BBC Micro?

[reeve]

I'm just exploring adding some of these to my machines, and found the thread with the pre-built BBC config with various versions of DFS etc. That comment mentioned the 28 pin variant of onerom didn't have X pins, but it looks like it does now?

Does that mean we can get 4-into-1 automatic bank switching for the BBC? I'm no expert but it looks like you would be able to wire X1/X2 to IC76 pin 11/12 and that would give you the extra 2 bits of the ROM select lines (known on the BBC as QC/QD, they're currently not connected to anything). That way each of the 4 physical sockets could provide 4 images at once (the BBC supports 16x16k ROMs but only has 4 physical sockets).

Here's some install instructions for a passive adapter which does this with a 27C512 : https://www.boobip.com/hardware/64kb-eprom/64kb-eprom-installation
And a BBC schematic I found and used to confirm the wiring: https://8bs.com/boards/BBCBschematicJTribbeck.png (the ROM ports and IC76 are at the top, right of center).

[reeve]

I mean not wishing to reply to my own thread but on further reading it looks like this would probably work out of the box if I wrote the JSON right...I'll give it a try when my devices arrive :)

[piersfinlayson]

I'm just exploring adding some of these to my machines, and found the thread with the pre-built BBC config with various versions of DFS etc. That comment mentioned the 28 pin variant of onerom didn't have X pins, but it looks like it does now?

Yes, fire-28-c (flat USB-C) and fire-28-d (vertical USB-C) both have X pins. I call these RGB variants as they also include an RGB LED.

Does that mean we can get 4-into-1 automatic bank switching for the BBC? I'm no expert but it looks like you would be able to wire X1/X2 to IC76 pin 11/12 and that would give you the extra 2 bits of the ROM select lines (known on the BBC as QC/QD, they're currently not connected to anything). That way each of the 4 physical sockets could provide 4 images at once (the BBC supports 16x16k ROMs but only has 4 physical sockets).

Yes. Configure as a banked set. Both X pins "open" to get ROM 0, close X1 to get ROM 1, X2 to get ROM 2, and X1/X2 to get ROM 3.

I'm not familiar enough with the BBC to comment on QC/QD, but bear in mind that closing the X pins on these boards pulls the X pin low.

[piersfinlayson]

You need a config like this:
https://github.com/piersfinlayson/one-rom/blob/main/onerom-config/test/28-bank-27xxx.json

[reeve]

re: pulling low - (and please bear in mind my experience is almost all from watching Adrian's Digital Basement videos!) - the QC/QD are just outputs from a 74LS163, so they should be consistently held either TTL high or low by it - I should be able to connect them directly to X1/X2 - correct? I don't need to add any passives or anything? I can figure out the resultant ordering by experimentation as I don't know whether they're the least or most significant bits (it doesn't really matter in most cases).

[piersfinlayson]

The X pins are 5V tolerant so they should be safe to directly connect. You can always find out which pins on your One ROM are which, and which are 5V tolerant with

onerom inspect header

[phils71]

I had a minor segway into considering whether a OneROM could be tweaked to replace the OS ROM on a BBC model A/B to implement the complete ROM expansion board behaviour like the ATPL Sidewise ROM expansion board, iirc the OS socket provides all 16 address lines to what is otherwise a 27128 hence my thought was is it practical to feed the write line into one of the X pins and have the OneROM spot writes to ROMSEL (0xFE30) and perform bank switching accordingly. I'm not in a position to test it currently, my model B needs recapping and I don't have the desk space or time for another project just yet, hence not burned too many cycles on the OneROM firmware aspects, One interesting bit would be how to serve the same OS image at 0xC000-0xFFFF whilst serving one of up to sixteen images from 0x8000-0xBFFF, I assume it might be more performant for the OneROM image serving code to treat it as 16x 32K banks and have the installation software duplicate the top 16K. It would also need a second X pin line for the chip select for the 0x8000-0xBFFF range I suspect. Not even begun to consider whether this would be electrically safe for the BBC micro even with the 5V tolerance on the OneROM.

As ever with these things it sounds like "your mileage may vary", https://stardot.org.uk/forums/viewtopic.php?t=21340

I wish I had the time to fiddle with the idea.

[phils71]

On the plus side removing / not fitting a 16 socket expansion board and a full complement of ROMs reduces the power draw on the PSU.

[piersfinlayson]

I expect this is feasible - monitoring 0xFE30 accesses with a plugin and switching the ROM. I'm unclear whether this switch has to be near instantaneous, or whether One ROM has some time to make the switch. If instantaneous, yes, have a single 512KB RAM image loaded with all 16 x 32 KB block and switch by forcing the appropriate top address GPIOs to new hardcoded values. If you have some time you can copy the appropriate new 16KB section out of flash into the existing RAM slot being served.