Micro­power

per line can be controlled by a fourth argument; if this argument is nn, there will be 8+nn bytes per line. In addition to the hexadecimal data, Nas-Sys 3 outputs the ASCII or Graphics codes of the data (codes in the ranges £00 – £1F, £7F – £9F, and £FF are output as “.”). A fifth argument, hhll, may be entered to suppress either the hexadecimal (if hh is not zero) or ASCII (if ll is not zero) part of the listing.

If you wish to edit a tabulated listing you must quit the Tabulate command by typing ‘Escape’, and then enter M to get into the Modify routine. You will now be able to move the cursor with the control keys and edit the tabulation. If you are using Nas-Sys 3 it is best to suppress ASCII part of the listing. As this will interfere with the modify command, either by producing error messages or, if the ASCII section contains a full stop, by terminating the modify routine.

User Input/​Output U

Input and output is accessed via pointers to tables which list the routines to be called. With the pointers, which are stored in the workspace at £0C73 (output) and £0C75 (input), set to normal values, as on power-up or after a RESET or N command, input scans the keyboard and serial port while output is sent to the screen. The U command resets the pointers so that routines provided by the user are called before the input or output is performed. The user routines can reside anywhere in memory; the start address of the input routine must be stored in the workspace at £0C7B, that of the output routine at £0C78. These locations normally contain the address of a return instruction in the monitor, so that using the U command without providing the addresses of your routines has no effect.

Although the I/O procedure also uses the remaining routines in the tables, if for any reason you do not wish these routines to be called (for example, you may wish to suppress the screen output), you merely have to set the carry flag in the user routine; the remaining routines will then be skipped. This can have unfortunate consequences – if a printer routine carries out tests which leave the carry flag set for certain characters, these characters will not appear on the screen.

VERIFY V

The address stored in the command table for the VERIFY command is the same as that for the READ command. The two commands use the same code, except that as each data byte is obtained the value stored at location £0C2B, which contains the last command letter entered,​is tested to see if it an R. If it is not, the data bytes are not stored in memory, and faulty data cannot corrupt data already in store. If you are calling the READ routine from a program you must store an R at £0C2B or data will not be loaded.