Alternative method of upgrading a B+ 64K to 128K
Here's an alternative method of upgrading a B+ 64K to 128K.
Its simpler than the Acorn upgrade, and 100% compatible.
You now have a three high ROM module, with 4 'ezihooks' attached.
- Stack up 3 43256, 62256 or equivalent devices (e.g. Farnell part number 209-7992 or 209-7986)
- Fold pins 1 (A14) and 27 (WE) up on all three devices.
- Fold pin 20 on the top device up, and on the middle device out horizontally.
- Solder all pins on all devices together (except the pin 20s)
- Attach an 'ezihook' (or equivalent) to the pin 1 group, the pin 27 group, and the two extra pin 20s.
Photo of installed module, click for a bigger version
To use it:
Job done, and you've got an extra 32K of sideways RAM too!
- Place the module into a spare ROM socket
- Connect the pin 1 ezihook to a source of the LSB of the ROM select bus (The east-most pin of link L9, L11, L12, L15 or L18 is the obvious choice)
- Connect the pin 27 ezihook to a source of a write strobe (e.g. pin 24 of the 7002)
- Connect one of the extra pin 20s to pin 9 of IC46 (the chip select for ROMs 12 and 13)
- Connect the other extra pin 20 to the spare pin of L13 (CS for either roms 0/1 or 14/15, depending on where you want BASIC)
- It does not matter which of the pin 20 ezihooks you connect to which location - this would just swap which of the two piggybacked RAM chips was in which banks
Close up of location of pin 20 ezihook connections, click for a bigger version
Note that unless you have a version of the DFS ROM with the SRAM commands in, your startup banner will still say "Acorn OS 64K", as the code to include the sideways RAM in the startup banner is in the newer versions of the DFS ROM. The DFS 2.26 ROM (available from the BBC Lives page here) is known to work.
The utility available here will check for Sideways RAM banks.
If your RAM is installed correctly it should detect RAM in banks 0,1,12,13 and two other banks, depending on which socket you installed the module in.
The official Acorn 64k upgrade fitting instructions at the 8BS website contains an SSD image that will test the SRAM operation, this would be another useful tool to use to check the operation of your upgrade
Also of interest on the 8BS site is the BBC B+ 128k Additional User Information document, which documents the sideways RAM commands and the "BAS128" sideways RAM Basic version
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