SYS-EX   A 1K extension program for NAS-SYS monitors – Part 4.

by David G. Johnson

Errata to Part 2, (Vol.3, No.2)

1) Page 6
ADD L ) ADD 30H turn into Ascii code
should read
ADD L
ADD 30H ) turn into Ascii code

2) Page 6
.ROUT print character from A CP 28H Z flag if final space
should read
.ROUT ) print character from A
CP 20H ) Z Flag if final space

In the last issue we completed the detailed description of the SYS-EX keyboard commands and promised that a description of the SYS-EX BASIC routines would follow in the next issue. Well, here is the next issue and we begin with the SYS-EX BASIC routines.

To recap briefly, SYS-EX provides an alternate set of commands to the 8K BASICs CSAVE, CLOAD and CLOAD?. These commands allow program names to be up to 42 characters long. The commands allow access to tape labels written using the SVS-EX “l” command. The commands are fully compatible with CSAVE, CLOAD and CLOAD? and programs saved with one set of commands can be recognised and read using the other set of commands.

The SYS-EX BASIC named file and label commands are called by using the USR function. Prior to using USR, the start of the SYS-EX BASIC routines should be DOKE’d into memory. The BASIC entry point in SYS-EX is located 01E0H beyond the start of SYS-EX. When the monitor extension is installed at the recommended memory address (B000H to BFFFH) the required BASIC statement is:

DOKE 4100,-20000 (ie. B1E0H = -20000 decimal)

As SYS-EX is fully relocatable, you may have decided to put it at a different memory address. If so, just add 01E0H to the hex start of SYS-EX then convert this to decimal using the “h” command. You won’t get the nice easy to remember number (-20000), but it will work just the same.

The routines may be called by a statement such as PRINT USR(n) or ?USR(n) or I=USR(n). Even though a PRINT statement or an assignment statement is used, nothing will be printed or assigned to a variable on completion of the SYS-EX commands. This is because SYS-EX warm starts BASIC when it finishes command.​(ie. it does not just return to BASIC)