A Neat Hardware Solution to fitting CP/M

Yet another article by P. Anderson (by request!!)

This article covers hardware modifications to a standard(??) Nascom 2 to provide the fallowing functions. They don’t sound much but it does provide single bit logical control either by switch or port control (and enables compatibility with EPROM/​RAM A card where fitted).

Features.

1. Switch in CP/M Boot ROM.
2. Switch out Monitor ROM.
3. Move Video/​Work RAM.
4. Automatic Reset on “switchover’
   This avoids over-writing due to a ‘wandering’ CPU and avoids the necessity on doing a Reset on each change-over.
5. Automatic switching of restart address.
   This allows restarts at F000H plus any other Dil switch selected address of your choice.

These modifications have been found useful in fast changing from one system to another without the risk of corrupting the contents of memory so enabling the Nascom to get at CP/M!!

The modifications outlined (which are open to variation) also cover re-arrangement of the use of the workspace RAM and the HASIC ROM positions for these using EPROM cards. These enable a complete CP/M system on N2 & RAM cards whilst still retaining the use of the normal RAM / EPROM sockets for more useful applications than JUST CP/M.

CP/M / Nascom Switching.

Fig. 1 shows the physical wiring arrangements. The 74LS157 is a Quad 2-line-to-1-line data selector and provides 4 ‘change-over’ switch functions (operating active High). These are:

1. Disable Monitor ROM.
2. Enable CP/M ROM.
3. Move Video & Work space.
4. Change Reset address.

In 1 & 2 the Chip Enables are high when not selected, 4 is optional.

Fig. 2 shows the very simple track modifications to enable Reset address changing. It involves cutting two tracks (the one on the upper PCB side is superfluous anyway – being a ground track which is bypassed several times on other parts of the PCB). The other cut isolates the ground connection to the 4