Micro­power

the program returns to this point the register display will be repeated.

But if the restart code is inserted when the Execute command is entered, why doesn’t the program stop immediately when you try to continue after a breakpoint? In fact the £E7 is not inserted until one instruction has been carried out, as the following simple experiment shows. Enter the following code at £0C80:

Press reset, then enter BC81 followed by EC80. The register display will show the contents of HL to be 0000; this is because the execute command picks up the values in the register save area of the workspace, and these bytes are zeroed by a reset. Now press reset, enter BC80 and then EC80. HL will be 0001; the restart was not inserted until the first instruction (NOP) had been executed, so HL was incremented before it came back to the breakpoint. If you insert £E7 directly in the program at £0C80 using the modify command, the program will break immediately on execution. The main use of this restart is in ‘debugging’ – tracing faults in programs that don’t work properly. You can also use it as a single byte end to a program when you want to know the contents of the registers.

The next restart will be familiar to all Nascom users, as it has remained virtually unchanged from the earliest monitor. It is the ‘print string’ routine, £EF, which outputs the characters following the code until a zero is reached. The routine can handle control codes and graphics characters as well as the standard ASCII codes. The bytes in the string are loaded successively into the A register, and are then output by restart £F7. At the end of the routine the A register always contains 00, the byte that marks the end of the string.

The output restart, £F7, operates in a similar way to the subroutine called by the input restart; that is, it accesses a table of routines, the address of which is stored at £0C73. On initialisation this is set to use only the standard CRT routine, but the user can extend the table with the U and X commands, or can provide his own routine table. As in the case of the input restart, if one of the routines in the table sets the carry flag subsequent routines will not be used.

Finally, restart £FF produces a delay depending on the value in the A register when the restart is called. When A is one, the delay at 4Mhz, including the call and return, is 5.2 microseconds, while the maximum delay, produced when A is zero, is 2.7 milliseconds. Of course, the times are doubled with a 2Mhz clock rate.

That completes the Z80 restarts; in the next article I shall deal with some of the individual subroutine calls.

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