Let's sort out this STZ/R65C02 thing once and for all (R Brooksby, issue 99; various correspondents, issue 102).

1. The STZ opcode is termed an 'undocumented opcode', which means that the opcode has been left out of the official documentation because it was not 100 per cent reliable when the 6502 was launched. However, the bugs may well have been fixed and the opcode made reliable in later versions of the processor.
2. The CMOS 6502 (the R65C02) does have a STZ instruction along with a number of others. The processor is available as a drop-in replacement, and Mr. Brooksby or anyone else with a 6502-based machine, should be able to change their processors for an R65C02 and use these new instructions. Anyone interested should contact:

   Solidisk Technology
   17 Sweyne Avenue
   Southend-on-Sea
   Essex SS2 6JQ

   (Tel: 0702-354674)

Solidisk sells the R65C02 for £12 including VAT, postage and packing.

Solidisk also does some software on disk which adds the new opcodes into the assembler on the BBC Micro.

I hope this clears up any discrepancies which may have come up over the past few weeks.

Simon N. Taylor, RCS Microsystems