Micro­power

The solution is simple – after testing the argument the routine should reset ARG1 to zero. Alternatively, you can POKE the value to zero from Basic. That brings us to the end of the Nas-Sys commands, but there are many more addresses in the subroutine call table – 34 in Nas-Sys 1 and 37 in Nas-Sys 3. These SCALs cannot be accessed directly from the keyboard like the command letters. Many of them correspond to lower case letters, but if you try to enter them as commands you will just get an error message, because the routine which accepts commands test the input character to see if it lies in the range A – Z. Of course they are there to be used in programs.

SCAL MRET DF 5B

This is the normal way to return to the monitor from a program. The routine resets the monitor and user stacks, prints the monitor message (— NAS-SYS 3 —, or whatever you have substituted) on the screen, restores the byte replaced by any breakpoint that has been set, and then waits for an input. The screen is not cleared, and the monitor message will appear at whatever point the cursor was left by the program – for tidyness you should shift the cursor to the left of the screen by a carriage return if it has been moved. If you want to clear the screen on return to the monitor you can use RST 0 (£C7); this will re-initialise the workspace and clear the screen before jumping to MRET.

SCAL SCALJ DF 5C

This enables you to access any Nas-Sys subroutine by storing the subroutine ‘number’ at ARGC (£0C0A). The routine saves the HL, AF and DE registers, picks up the routine number from ARGC, and then jumps to section of code in the subroutine call restart where the call address is calculated from the subroutine number. Of course, any requirements of the normal call must be met if a subroutine is accessed ‘indirectly’ by SCALJ; for example, to call the READ command via SCALJ you would have to store £52 (“R”) at £0C0A, but you would also have to place £52 at £0C2B, or the routine would only ‘verify’ the tape.

SCAL TDEL DF 5D

This routine calls the ‘delay’ £FF restart, RDEL (RST £38, £FF) 512 times. As each RDEL takes rate 2.7 msec. with a clock rate of 4 Mhz, the total delay in TDEL is 1.38 seconds. Obviously, at 2 Mhz the above times are doubled. Registers A and B are both set to zero on return from this routine.