80-Bus News

The EPROM holding the NAS-SYS BOOT software also holds the ‘MENU’, key selection and ‘copy across’ software for the selected program. In my case this is in a 2716, and there is over 1K still free so I have incorporated a number of ‘utility’ routines, some of which are described below. The program required to copy ‘SIMON’ across is very short and the remainder of the EPROM may be used to hold whatever other software the user desires. (This may be software not relating to CP/M, such as more utilities). SIMON is the BOOT software for the Gemini CP/M 2.2 implementation which may be overwritten once it has done it’s job. Users of CP/M 1.4 could copy the relevant software across, but it has to be available the whole time, and so the top 4K of RAM would have to be left free.

Whilst it is normally the practice to copy programs across to RAM, it is quite possible to ‘page in’ the EPROM card and run software on it (provided it’s address is correct), but if there is any quantity of software on the card, this soon becomes impracticable since all software on the card will be on at the same time.

The Gemini G809 CP/M system, once booted, needs no further interaction with the EPROM card, since all the software including the operating systems are on disk. The NAS-SYS system needs frequent access to the card. Indeed the speed of access and clean memory map are amongst the main reasons for using the system. It is therefore necessary to be able to access the ‘MENU’ easily. One method is to use the ‘E’ command to access the card via a small ‘turn on’ routine. A better method is to change the allocation of a command key. This can be done either by modifying the EPROM card copy of NAS-SYS , or by adding a little routine to the software in the BOOT EPROM that alters the copy of NAS-SYS that it has just copied into RAM. I decided to use key ‘D’ to access the MENU since the ‘cold start’ of ROM ZEAP is now redundant, so location 0789H needs to be changed to 08H. (NS1 users change 078E and 078F to 00H, 08H respectively). The software needed to turn on and jump to the MENU is only 7 bytes long (p44 INMC80-4). I placed this routine in the VIDEO margin at 0800H – 0806H, and have not had any problems by so doing.

To give an idea of the versatility of the system, the ‘utility’ routines that I can access from the MENU at present are:

1. Handshake for the Imp printer (see below).
2. Memory test (see below).
3. Basic renumber.
4. Set PIO ports.
5. EPROM programmer (home-brew).
6. Block compare.
7. Find bytes.
8. Improved cassette read.

(The last three were described in recent issues of the Micropower mag.)

Firmware that can be instantly down loaded includes at present, T4 (so that the old favourites like Lollipop Lady can be run to amuse the juniors), ZEAP (RAM version), NAS DIS-DEBUG, and Nascom ROM BASIC. In the near future I hope to add Polydos 2 to the firmware; and a real time clock and auto line numbering for BASIC to the utilities.

Hard­ware Implementation

Refer to the circuit diagram. It will be seen that the wiring is very simple. In my equipment, a three pole three way switch (obtainable from most electronics catalogues) is used to perform three functions: