NAS-SYS Monitors

By J. Haigh

Command Input

While waiting for you to enter a command the Nascom is in a routine called INLIN, sitting in a loop in which it scans the keyboard and the serial input, prints any character received, and continues until it a Newline (£0D) is obtained. It then sets the DE register pair to the address of the start of the line that the cursor was on when the Newline was received, and returns from INLIN to the main monitor routine, which is called PARSE. Here it tests the character at the start of the line entered; if this character is a space it looks in the workspace at address £0C2B, which is a location called ARGX, to discover what the previous command entered was. Normally a line with a space at the start is ignored, that is, the monitor recalls the INLIN routine and waits for further input; however, if the previous command was S (single step), the monitor behaves as if this command had been re-entered, so that you can continue to single step through a program merely by pressing Newline.

If the line does not start with a space the first character is tested to find if it is an upper case letter; for any character outside the range A – Z an error message is produced, and INLIN is then recalled. When an upper case letter is found this is stored at ARGX and also at £0C0A (ARGC), and the monitor proceeds to read any hexadecimal values entered on the line, storing them in the workspace in ten locations, ARG1 – ARG10. the number of values found is stored at £0C0B (ARGN). If any of the values found are invalid (contain illegal characters, or too many character), or if more than 10 values are entered, once again an error message is produced and INLIN is recalled.

If all the tests are passed, the first three hexadecimal values on the input line are loaded into HL, DE and BC, HL is saved on the stack, and the location of the subroutine address is calculated by adding twice the value of the ASCII code for the command letter to the command table base address – £0706 for Nas-Sys 1, £700 for Nas-Sys 3. Thus for command A (ASCII £41) the routine address is stored in the two bytes £0788, £0789 for Nas-Sys 1, and at £0782, £0783 for Nas-Sys 3. As usual, the low byte of the address is stored in the first location, the high byte in the second. The routine address is transferred to HL, and the HL register pair is the exchanged with the top two bytes of the stack. This restores ARG1 to HL, and leaves the address of the routine to be executed on the stack. The program then performs a RET (return from subroutine) instruction, which sets the program counter to a value POPed from the top of the stack; the net result is that the machine enters the command routine with any hexadecimal values specified in the workspace , and the first three values in HL, DE and BC.

You should note that on RESET the region of the workspace ARG1 – ARG10 is set to zero. When you enter a command with a given number of arguments only these argument locations are changed; the remainder are unaltered. Thus if you