NAS-SYS Monitors

by J.Haigh

LOAD L

ln Nas-Sys 1 this command is used to load data from a paper tape reader. The data must have been stored on the tape in the format used by the standard Nas-Sys 1 tabulate commands i.e., the address of the first data byte, eight data bytes, checksum, all represented in hexadecimal and separated by spaces; the line is terminated by a carriage return. As the tape is read the data is displayed on the screen; when the end of the line is reached (detected by the carriage return) the data is read into the workspace by the same routine which reads the arguments supplied with commands. Thus the loading address is read into ARG1 (£0C0C, £0C0D), the eight data bytes into ARG2 – ARG9 (£0C0E to £0C1D), and the checksum into ARG10 (£0C1E, £0C1F). The routine then totals the values stored in ARG1 – ARG9 and compares the result with the checksum. If the values are identical the eight data bytes are copied from the workspace to the appropriate memory location, the cursor is reset to the beginning of the line, and the next block is read in, overwriting the last line. if a checksum error is detected, or if invalid characters are detected by the routine which reads the data into the workspace, the faulty line is scrolled up the screen and the routine proceeds to the next block.

Data can be written to a paper tape punch in the correct format by routing the output of the Tabulate command to the serial port by means of the External (X) command. However, in Nas-Sys 3 the Load command has been dropped; the address stored in the subroutine table for command L (at £0798) is £0366, the Error subroutine. Because the Nas-Sys 3 tabulate command is not restricted to the – format of a Load command it has been made more versatile in the line lengths it can produce, and also no longer gives a checksum byte.

MODIFY Mxxxx

This permits direct insertion of data into memory from the keyboard. When the command is entered the address xxxx is displayed, followed by the byte currently at that address, the cursor is moved left three spaces after the routine which displays the data byte; since this routine outputs the two digits which represent this byte in hexadecimal followed by a space, this places the cursor on the first character of the byte. Data can now be typed in hexadecimal format, successive bytes being separated by one or more spaces. When the newline key is pressed the monitor interprets the current line; the first number on the line is taken as the address at which data storage is to start. If the first group of characters on the line is not a valid hexadecimal number, that is, does not consist only of the ASCII characters 0 – 9 and A – F representing a hexadecimal number between 0 and FFFF the word ‘Error’ is printed, and the routine restarts at the last valid address.