g802

g802r

This is a 64kB dynamic RAM card. Up until the release of these, the best that could be managed were the standard Nascom cards. This version finally beat the dreaded “RAM plague” that had caused so many problems in the past. It also introduced the potential to add incredible (at the time) amounts of RAM to the system.

The more alert of you (sit up at the back there!) may have realised that, since the Z80 is an 8-bit processor, 64kB is the maximum addressable memory – so where does the EPROM and video RAM fit into the space? The secret lies in this card’s ability to be paged in and out of memory space in chunks. At the right-hand side is a soldered header marked SK1. This is used to allocate four 16-kB RAM areas to four outputs from an address “latch” circuit. In addition to this the board can be selected to one of four “pages” via a slide switch in the bottom row of ICs. Thus the user can have four of these cards on a system to give a maximum of 256kB of RAM. Not too bad for a Z80!

Switching was controlled by controlling bits on port 0xFE.

Bit Function if set
7 Write enable page 3
6 Write enable page 2
5 Write enable page 1
4 Write enable page 0
3 Read enable page 3
2 Read enable page 2
1 Read enable page 1
0 Read enable page 0

Each page is one memory board. On initialisation 0x11 was written to the port, enabling read & write on page 0.

There is a problem with this system though. Although it appears flexible it is very tricky to use more than one card. Software control was restricted to switching a complete board in and out of memory space. That required some very careful programming around a specific setup of memory cards. This limitation was overcome in Gemini’s Extended Addressing mode which was available on later memory cards.

I think this was the first Gemini product for the Nasbus.

GM802_Manual_SecondIssue

GM802_Circuit_1   GM802_Circuit_2

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