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For many ZX81 owners it would be a welcome relief to have a program to indicate exactly what was going wrong in a program just typed into the machine. Now all that is possible with Trace, a utility for the 16K ZX81.

The program has been written in machine code so it will load above RAMTOP. You can then type-in your Basic program as usual and run it for any errors which may occur.

If the program stops with an error code, you can use Trace to look at the structure of the lines running up to where the program crashed.

To trace through a program, you have to call the utility using either a direct machine code invoke command or using a call in a program.

Invoking the utility in a program has the advantage that you can trace through a few lines which occur in the middle of some code.

When Trace is invoked, you should see the number of the line which is being executed displayed at the bottom of the screen and the effects of the program line on the rest of the screen.

In that way you will gain a better understanding of what each line is doing and how it fits into the structure of the rest of the program.

Trace is an excellent utility which provides a useful addition to any ZX81 owner's programming library. You should find it helpful in dealing with bugs in any Basic program.

Trace can be obtained from Texgate Computers Ltd, 14 Brook Lane, Corfe Mullen, Dorset BH21 3RD. It costs £5.95.