If the frustration of debugging your programs threatens to drive you into the madhouse, try this handy machine code solution from Stuart Nicholls. And it opens up a new avenue of programming possibilities

Spectrum owners who can't face another mammoth Basic program debugging exercise could try this machine code solution. It helps with the debugging and opens up a range of possibilities for machine code programmers.

You'll cover the method of running a Basic program under the control of the operator, stepping through the program a statement at a time, either at predetermined intervals or when a key is pressed, displaying the line and statement number being executed. In other words you'll end up with a TRACE (TRON) facility as found on many other computers.

This routine is *not* interrupt-driven: it diverts the running of a Basic program from the ROM interpreter to our own interpreter in RAM. With this set up, it is a simple matter to insert an extra machine code routine (TRON) into the interpreter so a printout can be given after each statement is executed.

To set up the routine use a hexloader and enter the code as in the hexdump. Don't forget to set RAMTOP to 64499 before entering the code. Then save it using:

SAVE "TRACE" CODE 64500,547

To check the routine you need to know some of the following instructions.

The delay between the execution of each statement is set up in a similar manner to the PAUSE command, in that the unused system variables 2372823729 hold the number of 1/50th seconds delay. For example, a delay of two seconds requires the direct commands:

POKE 23728,100:POKE 23729,0

Similarly a delay of ten seconds requires:

POKE 23729,INT(500/256:POKE 23728,(500-256*PEEK 23729)

An infinite delay that allows a step only when a key is pressed is:

POKE 23728,0:POKE 23729,0

Once this delay is set up and your Basic program is loaded, run it using:

RANDOMIZE USR 64500:RUN

as a direct command. If it's the first line of your program use:

1 RANDOMIZE USR 64500:GOTO NEXT LINE

Your program should now run as normal, but with a printout at 21,0; of the current line and statement in the form [20:5]. All errors are reported when found and the break keys function. What's more, if a bug is found, it can be corrected and the program run without the TRACE using the RUN command in the usual way thus using the ROM interpreter.

Listing 2 is my assembly listing for machine code programmers who are interested in the working of the routine. The main part is a copy of that in ROM but with the addition of the CALL TRON routine at line 0545, i.e. after the correct interpretation of a statement and the checking of the break keys (line 0510)

The diverting of the ROM interpreter is achieved at the beginning of the code by ensuring that the ERR-SP is correctly set up with 1303h and that the machine stack is cleared. In other words, the command RANDOMIZE USR 64500 is assumed completed when the code reaches line 0024, and our interpreter takes over to execute the next statement at STLP line 0040; the next statement, of course, is run and as such [0:2] is the first TRACE output.

Machine code programmers will also notice that it has been necessary to redirect the REM and IF commands to run in our RAM interpreter, as those in ROM will cause the routine to re-enter the ROM interpreter and we would lose our TRACE.

This method of diverting the ROM interpreter to one in RAM opens up the path to rewriting the whole of Spectrum Basic.