Nascom News­let­ter

Upon entry to the user routine, various Z80 registers have preset values. These values are: HL = value from ARGI, DE =

Value from @RG2, BC = value from ARGS, – SP = OCSFH “within the NAS-SYS stack. Forty tno bytes are available on the stack for use by the user routine. Calls to | NAS-SYS routines or to SYS-EX routines, from within a user routine, Mill require a number of stack levels to be available. y

Call user routine eumber 2

Calls a user routine whose execution address has been stored in the NAS-SYS argument ARG?

The command is identical to the “x"command except that the argument ARG? is used in place of ARGS.

Call user routine number 5

Calls a user routine whose execution address has been stored in the NAS-SYS argument ARGIO.

The command is identical to the “x"command except that the argument ARGIO is used in place of ARGS.

The three user routine calls, ‘x”, ‘y’ and ‘2’, provide an easy method of attatching individually tailored Commands ta the monitor. Uniike the NAS-SYS Execute command, these commands allow a return ta the monitor (SYS-EX) with a single RET instruction. Also, the “Error? message can be output under user control by manipulating the carry flag prior to the RET. The commands are ideally suited. to. calling individual monitor type functions. e.g. – Display the character represented by the ASCII code entered as ARGI -, or – Print screen contents of attatched printer Tt might even be possible to directly call some of the functions within Basic.

In addition to “x”, “y” and “2”, there is another method of calling user routines.” The commands “9”, "37s “my “oO” and “pr, which are not used directly by SYS-EX, all Call a meaory address 0400H (1K) beyond the start “of SYS-EX “U-e-immediately after SYS-EX). If you put your owr code here, these commands can be used in a simlar way to “x”, “y” and 2’. The register values on entry are the same as’ for “x", ty’ “and “2”. A word of warning here – if you haven’t put any cade beyond SYS-EX, don’t use “9”, "3”, "m, “a” or “p’ as the results will be unpredictable.

That completes the description of the keyboard commands. In the next issue of the magazine we will look at, amongst other things, how the GK Basic can make use of forty two character program names.