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used in place of the upper case letters. During the operation of SYS-EX, all of the NAS-SYS keyboard commands remain available for use in the normal way. With the exception of the ‘d’, ‘h’ and ‘u’ commands, all of the SYS-EX commands use the arguments ARG1 through ARG10 in the same way as does NAS-SYS. The ‘d’ command is a special case in that the NAS-SYS arguments ARG1 through ARG10 are neither updated or used by the command. Also, the single required argument for the ‘d’ command may be in a format which is normally unsuitable as a command argument. (i.e. a minus sign and up to five digits are permitted.) The ‘h’ command updates ARG1 through ARG10 in the normal way, however, the command itself will only recognise an argument which is entered on the same line as the command letter. The ‘u’ command places value in ARG8, ARG9 or ARG10 without the necessity of entering 8, 9 or 10 values on a command line.

Certain SYS-EX commands require further input parameters to be supplied after entry of the command line. The find (‘f’) command requires the entry of a find mask, while the remainder of commands in this category require the entry of a file or a tape label name.

Single step and non-maskable interrupt processing is in no way altered by SYS-EX. However, after each of these facilities has been used, control of keyboard input returns exclusively to NAS-SYS. i.e. the SYS-EX keyboard commands become temporarily unavailable until a SYS-EX warm or cold start is executed. The same situation applies when a program is ended by a breakpoint or by a NAS-SYS MRET instruction. (.. or by the action of the RESET button unless the hardware reset jump is set to the beginning of SYS-EX.)

A number of the features within SYS-EX have been designed specifically with the Basic programmer in mind. Perhaps the most notable amongst these features are the routines which provide enhanced alternatives to Basic’s CLOAD, CLOAD? and CSAVE commands. The SYS-EX routines not only permit program names up to forty two characters in length, but also allow access to tape labels written using the SYS-EX ‘l’ command. Program files which are written using the SYS-EX routine will be successfully recognised and read by the standard Basic commands. Similarly, program files which are written using the standard Basic command will be successfully recognised and read by the SYS-EX routines.

The routines which convert between decimal and hexadecimal numbers are particularly useful when using the PEEK, POKE, DEEK and DOKE commands in Basic. Both for memory addresses and for the values contained in particular memory locations, Basic uses the decimal notation. However, much of the useful documentation quotes values in hexadecimal. The SYS-EX ‘d’ and ‘h’ commands provide easy methods of converting in either direction.

Other features will also prove useful to the Basic programmer. For example, the SYS-EX ‘k’ command will in many circumstances be invaluable in suppressing the spurious input