SYS-EX   A 1K extension program for NAS-SYS monitors. Part 5

by David G. Johnson

This is the final part of the series of articles on the 1K monitor extension to NAS-SYS. All of the major facilities of SYS-EX have been described in the earlier magazine issues. All that now remains is to tidy up a few remaining details and to print the promised hex dump of the monitor extension itself.

SYS-EX is written in relocatable code and will operate correctly at any memory location above 1000H. However, the Nascom memory map allocates locations B000H to B7FFH as the recommended memory locations for extensions to monitor facilities. SYS-EX may conveniently occupy the first half (B000H to B3FFH) of this available space and at the same time provide facilities for the easy use of further extensions from B400H onwards.

Should SYS-EX be located at B000H, two useful and important facilities are available.

1. The hardware reset jump may be set to cold start SYS-EX automatically at switch on and on pressing the RESET button.
2. NAS-SYS 3 provides the “Y” command which jumps to address B000H. The “Y” command can be used directly to cold start SYS-EX.

Cold starting SYS-EX

A cold start is required if the monitor has not already been initialised. e.g. at switch on or after a program has made non standard use of the memory reserved for NAS-SYS workspace.

The execution address for cold starting SYS-EX is 5 bytes on from the start of SYS-EX. e.g. If SYS-EX resides between 1000H and 13FFH, it may be cold started by the NAS-SYS command:

E 1005

If, and only if, SYS-EX resides between 8000H and B3FFH, it can also be cord started at B000H.

Warm starting SYS-EX

A warm start is used when the initialisation of the NAS-SYS workspace and the clearing of the screen memory are not required.