Nascom News­let­ter

to tape. This was to be achieved in a similar way to the naming of program files from the 8K Basic. The required routines were written and provided many advantages over the facilities within Basic. (e.g. forty two character names and tape label processing.) However, a big problem remained. How could the new routines be easily used? The aim was to find a method of calling the new routines by using a single letter followed by the necessary arguments – that is, in the same way as you might use NAS-SYS ‘W’ or ‘R’. After much head scratching and may reems of assembler coated paper, the problem was solved. It was decided to use the lower case letters of the alphabet as the command letters (‘w’ for named file write, ‘r’ for named file read, ‘v’ for named file verify, etc.) Part of the head scratching had resulted in a method of ‘hooking into’ the main input loop of NAS-SYS, thereby providing a means of branching to SYS-EX processing whenever a lower case command letter was keyed. This method did not require any changes to NAS-SYS, so there was no non-standard EPROM blowing to do. During the development of SYS-EX, the author upgraded his Nascom 2 from NAS-SYS 1 to NAS-SYS 3. As a result of this, the ‘hooking into” routine was amended to make the whole thing compatible with all present versions (and maybe future versions too?) of NAS-SYS. As the author did not posess an assembler of any description (pencil, paper, Z80 manual and NAS-SYS ‘M’, being the available tools), and as it was not known where in memory the code would end up, it was decided to make the routines fully relocatable. (i.e. no absolute jumps or calls and no table data containing or referenced by absolute memory addresses.) This policy of creating entirely relocatable code was continued throughout development – sometimes at great personal risk to the author’s mental health. The result is that the final article can be run at any available memory location without need for re-assembly.

Having created the mechanism to call new commands as if they were normal NAS-SYS commands, it seemed a shame to stop development with only the named file commands present. After all, there were many useful features that the author had always hankered after, which could now be easily tagged on to NAS-SYS. Besides which, there were still some twenty unused lower case letters just begging to be used. All that was now needed was to write the routines’ After a good deal more head scratching, many more reams of assembler coated paper and a partially seized brain, (counting backwards in hex for those relative jumps is detrimental to health) out popped the final version.

“What shall we call this thing?” the author asked himself. (Talking to himself was now a regular occurrence. – Total madness was not far away.)

“SYS-EX”, he replied to himself. The monitor extension (and insanity) were complete.