Support for BBC Micro Sideways ROM/RAM?

[benjamink]

Are there any plans to support being Sideways ROM/RAM for a BBC Micro?

[piersfinlayson]

I don’t have any experience with BBCs, and I don’t fully understand sideways ROM/RAM support. AFAICS this is refering to the fact that ROMs were sometimes replaced with RAM chips that required an extra wire from elsewhere on the machine (to act as the R/W line). Am I close?

[benjamink]

I don't know much about them either other than I have one. I learned you can use AT28C256 EEPROMs (28-pin) but only 16kb is accessed by default so you quadruple a standard ROM to fill up the AT28C256 (in other words, you waste 3/4 of the EEPROM). There are ROM switchers that let you bank multiple ROMs on a single EEPROM/EPROM >16kb but I haven't used them. See this: https://www.retrosix.wiki/programming-roms-bbc-micro Since the BBC Micro used ROMs (& RAMs) for so much of their software that you used on a regular basis or extended the base system with, it was common to have multiple ROMs & swap them around from time to time depending on how you wanted to use your computer. For example, if you wanted to focus on BASIC & word processing you'd have an OS ROM, BASIC ROM, Disk FS ROM, & word processor software in ROM. If you wanted to do spreadsheets you might swap the word processor ROM for a spreadsheet ROM or something like that (or have an expansion board for more ROMs installed at the same time). I'm not super clear on how the RAMs worked but I think they were more or less like RAM disks that you can swap a program from disk into RAM to make it faster to use for a session. It's kind of a fascinating system from what I understand but my knowledge about it is very limited so far. Seems like something really flexible like your OneROM could be super convenient for a system like this where you'd likely have a whole library of ROMs you'd want to swap out from time to time on a regular basis. Ben

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On Fri, Dec 26, 2025 at 3:56 AM Piers Finlayson ***@***.***> wrote: I don’t have any experience with BBCs, and I don’t fully understand sideways ROM/RAM support. AFAICS this is refering to the fact that ROMs were sometimes replaced with RAM chips that required an extra wire from elsewhere on the machine (to act as the R/W line). Am I close? — Reply to this email directly, view it on GitHub <#71 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AALD4ZHHIKGYBQH2ZOOSYWL4DTZ3TAVCNFSM6AAAAACP6TD2EGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKMZUGU4DIMA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

-- - Benjamin Krein

[benjamink]

There are a whole bunch of resources here that can probably provide more details at a much lower level that you're likely to understand better than I do: http://primrosebank.net/computers/bbc/bbcmanuals.htm

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On Fri, Dec 26, 2025 at 3:56 AM Piers Finlayson ***@***.***> wrote: I don’t have any experience with BBCs, and I don’t fully understand sideways ROM/RAM support. AFAICS this is refering to the fact that ROMs were sometimes replaced with RAM chips that required an extra wire from elsewhere on the machine (to act as the R/W line). Am I close? — Reply to this email directly, view it on GitHub <#71 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AALD4ZHHIKGYBQH2ZOOSYWL4DTZ3TAVCNFSM6AAAAACP6TD2EGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKMZUGU4DIMA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

-- - Benjamin Krein

[piersfinlayson]

I think the 28 pin One ROM probably already supports operating as a sideways ROM.

I think all that's required is emulation of a 27128 ROM, which One ROM now supports.

I probably need to step up my ebay game and get hold of a BBC to test with. It's a shame I never got into the BBCs we had at school, but I was more interested in the C64 we had at home.

[piersfinlayson]

Yes, one of the key advantages of One ROM is being able to program in situ - reducing wear and tear by not having to constantly removing and replugging the ROMs.

[Deflagrates]

I can confirm that the 28 pin One ROM works as a 27128 in a BBC Master 128.

[piersfinlayson]

I can confirm that the 28 pin One ROM works as a 27128 in a BBC Master 128.

Good to hear!

A batch of One ROM 28s has arrived from JLC - just need to figure out a good way to test them all before I list them.

[benjamink]

Excellent!  I’ll have to get my hands on a few of them & give it a try too.  Thanks!Benjamin KreinSent from my iPhoneOn Dec 30, 2025, at 7:40 AM, Deflagrates ***@***.***> wrote: I can confirm that the 28 pin One ROM works as a 27128 in a BBC Master 128. —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you authored the thread.Message ID: ***@***.***>

[benjamink]

I was looking to have some of these built for me by JLCPCB but they were out of stock for the U3 part (AP2112K-3.3). Will their part AP2112K-3.3TRG1 be a suitable substitute? It certainly looks to be the same thing.

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On Tue, Dec 30, 2025 at 10:50 AM Piers Finlayson ***@***.***> wrote: I can confirm that the 28 pin One ROM works as a 27128 in a BBC Master 128. Good to hear! A batch of One ROM 28s has arrived from JLC - just need to figure out a good way to test them all before I list them. — Reply to this email directly, view it on GitHub <#71 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AALD4ZH333FSVLG4CYQGLVD4EKNMLAVCNFSM6AAAAACP6TD2EGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKMZXGU4TAMQ> . You are receiving this because you authored the thread.Message ID: ***@***.***>

-- - Benjamin Krein

[piersfinlayson]

If it's the same part number (still AP2112K-3.3TRG1), almost certainly. It'll just be a cheaper clone, but it's a pretty basic part so little to go wrong.

Do you have an LCSC part number (the part number beginning C)?

I think I went with the Diodes Incorporated part because I prefer brand names where there's little difference in stock levels and price.

[benjamink]

Makes sense on preferring the brand names. This is what JLCPCB has: C23380830

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On Wed, Dec 31, 2025 at 9:55 AM Piers Finlayson ***@***.***> wrote: If it's the same part number (still AP2112K-3.3TRG1), almost certainly. It'll just be a cheaper clone, but it's a pretty basic part so little to go wrong. Do you have an LCSC part number (the part number beginning C)? I think I went with the Diodes Incorporated part because I prefer brand names where there's little difference in stock levels and price. — Reply to this email directly, view it on GitHub <#71 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AALD4ZC6X3SN2AKFBOK4TMT4EPPUZAVCNFSM6AAAAACP6TD2EGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKMZYGE3TANA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

-- - Benjamin Krein

[piersfinlayson]

Yes, this part looks fine. Just make sure it's oriented properly when JLC displays them all on the board - sometimes JLC parts disagree with the rotation from kicad.

[benjamink]

Yes, I did have to rotate it 90 degrees. Thanks!

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On Wed, Dec 31, 2025 at 2:37 PM Piers Finlayson ***@***.***> wrote: Yes, this part looks fine. Just make sure it's oriented properly when JLC displays them all on the board - sometimes JLC parts disagree with the rotation from kicad. — Reply to this email directly, view it on GitHub <#71 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AALD4ZGNMRO2YHPNODWHTY34EQQV3AVCNFSM6AAAAACP6TD2EGVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTKMZYGMYTKNA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

-- - Benjamin Krein