G833 RAM Disk

An interesting concept for the NASCOM and Gemini systems, this Ram Disk acted just like a 500Kbyte disk drive. This is tiny by today’s standards, but when your total address space was 64k it was HUGE! At the time, so was the price: 450UKP was a *lot* of money in 1983.Data was transferred to and from the drive via I/O ports. On the Z80 microprocessor these do not need to be mapped into memory as they are on some systems, so the only overhead needed when using this board was for two small routines to actually send and receive the data. This meant that the 64k address space was hardly affected. The three ports can be regarded as “track”,”sector” and “data”. Just output the required track & sector addresses then read or write 128 bytes of data to or from the data port. Support for this was built into CP/M and it appeared automatically as drive M:

Of course, using dynamic memory was not very good in some ways. Its complexity needed a memory management IC (TMS4500) to keep it refreshed. Oh, and it lost everything stored in it when you switched off! On the other hand, the system used meant that, short of cutting off the power, the data was relatively safe. System resets had no effect. Up to 16 of these could, in theory, be fitted to a system to give a total of 8MB. However, this was beyond the power supply capacity of the NAS-BUS!

I never actually used one of these (and I don’t know if the sample I have will run yet…) but it was probably extremely useful during software development – even if you had to save all the important stuff at the end of the day!

After a little more research I’ve discovered that this particular card has been modified to increase the capacity to 2MB. All the 4164 chips have been changed to 41256 and the little add-on card in the middle is a multiplexer to provide the additional addressing required. I’m not sure if this is the one, but a complete conversion kit, including the RAM, was available from a company called PBM Systems.

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