OSRAM-side.jpg Solderless Operating System shadow RAM module for the Acorn BBC Model B


This upgrade for the Model A/B provides shadow RAM in the operating system address space.

It provides extra user memory (2x 15KB memory banks).

Totally transparent to the OS, the extra memory is directly accessible from BASIC or user code.

This adapter is ideal for people who want to keep their BBC as original as possible but expand it to make it more useful. Or people who want a simple plug and play memory expansion for the Model B.

Module Description

This module is designed to fit in the OS ROM socket on a BBC Model A/B. The OS ROM is refitted in the piggy back socket on the module. It has 32kB of surface mount static RAM on the underside and a standard 28 pin ROM socket on the top. It provides 2x 15kB banks of shadow RAM memory where the OS normally resides. The top 1kB is unavailable because of the BBC's memory mapped IO.

The extra RAM is visible from user programs, filesystems or language ROMs like BASIC but it is invisible to the operating system which runs normally. BASIC can make direct use the module for code and/or data by altering PAGE, LOMEM and HIMEM. 6502 code can be executed from the shadow memory allowing larger machine code programs or the ability to replace the OS with a different version.

The module provides RAM in the OS address space and does not shadow the screen memory which is more commonly associated with the phrase shadow RAM on the B+ and Master Acorn machines. This memory behaves more like the Andy memory area on a Master.

The adapters are physically narrow and have a low vertical profile so with the OS ROM is installed in the 28 pin socket there is still clearance to the BBC's keyboard which overlaps the paged ROM socket area (Unlike WE Quest Paint I own which actually hits the keyboard!).

Who is it for?

Anyone who is interested in writing their own software for the BBC in any language. Beginner, intermediate or expert.

A Model B with DFS in graphics mode 0,1 or 2 has less than 6KB of user memory free. With this module you can very simply use one bank for an extra 15kB in your BASIC program - nealy quadrupling the free memory. Easy for beginners to use.

Intermediate programmers can use both 15kB banks from BASIC or their assembly programs adding an extra 30kB of user RAM to their programs while keeping the OS running normally in the background.

Advanced programmers can write machine code to run directly from the module while accessing the other bank for data or more code. With sideways RAM installed the programmer can see 63kB of contiguous RAM on a Model B. In fact 78.5kB of RAM is directly useable in a Model B without any runtime bank switching or paging.


  • Solderless - no permanent modifications required
  • Jumperless - no configuration required
  • Occuies the OS ROM socket on a model B
  • 2x 15kB OS shadow RAM (30kB total)
  • Shadow RAM banks available for data or code
  • RAM is invisible to the OS so machine runs normally with extra RAM enabled
  • Software controlled ROM/RAM access - no switches
  • Ultra low power & heat - negligible stress on the power supply

Kit Contents

Installation is very simple, solderless and completely reversible. The kit contains everything needed.

  • 32 kB OS ROM module
  • 2x IC clips for write strobe (NWDS) and CPU sync (SYN)
  • Wire to daisychain with EEPROM/SRAM modules
  • Printed step by step instructions

Where to buy?

The modules are available for sale on eBay or direct through this website.

Contact me for more information