SYS-EX   A 1K extension program for NAS-SYS monitors.
Part I

by David G. Johnson

Over the next few issues of the Nascom Newsletter, a description of the development and the contents of a 1K extension to NAS-SYS will be given. The monitor extension is Galled SYS-EX and is fully compatible with both NAS-SYS 1 and NAS-SYS 3. A full hex dump of SYS-EX will be printed in a future issue of the magazine.

So what is SYS-EX and what does it do? In addition to the normal NAS-SYS keyboard commands, SYS-EX uses twenty one of the lower case letters of the alphabet to extend the range of commands available. These commands plus an additional range of routines may be called from assembler programs in a very similar way to NAS-SYS routines. An added bonus for the users of Nascom 8K Basic, is the set of routines which allow you to use program names up to forty two characters in length. The forty two character names are completely compatible with the standard one character names.

Wherever possible within SYS-EX, the door has been left open to allow further additions to the available facilities. Indeed, without making any changes to the SYS-EX code or tables, it is quite possible to define your own commands which can be called by typing a single letter in the same way as normal NAS-SYS commands. SYS-EX uses its own internal table of routine addresses which could, if required, be amended to ‘customize’ the monitor extension further. SYS-EX is written entirely in relocatable code and the address table entries are all relative addresses. SYS-EX does not require any of its own workspace, although certain routines do make use of some of the fields within the standard NAS-SYS workspace.

The monitor extension can be loaded from tape and run in RAM whenever required. However, the author has found it extremely convenient to put SYS-EX into EPROM ( a single 2708 is ideal ) and set the hardware reset jump ( Nascom 2 ) to cold start SYS-EX at switch on.

Here is a full list of the SYS-EX additional keyboard commands. A brief description of each command is included and a full description of how to use each command will follow at a later date.

a

Display the 10 NAS-SYS arguments. (ARG1 to ARG10)

b

Tabulate memory in double byte format. (i.e. ‘Hilo’ instead of ‘Lo space Hi’ with the NAS-SYS Tabulate command.)

c

Swap the contents of two specified areas of memory.

d

Convert a decimal number in the range −32768 to +65535 to hexadecimal.