Micro­power

  

Volume 1, Number 3 – November 1981

Page 10 of 33

the values used by Nascom’s Basic (Tokens are the single byte codes used for reserved words, e.g., PRIN is stored as £9E in the Nascom, as £B2 in the TRS-80). Any token which cannot be converted is changed to a REM, and its memory location, line number (in decimal) and TRS-80 value are displayed on the screen. The first two bytes of each line point to the start address of the next line. As the TRS-80 Basic starts at £42E9 and Nascom 2 Basic at £10FA, £31EF must be subtracted from the value read from the tape. Finally location £10F9 is set to zero, and the address of the end of the Basic program is stored at £10D6, 10D8 and £10DA. The conversion routine then returns to the monitor.

To use the program, first enter Nascom Basic with a cold start by command J, then exit by entering MONITOR or by pressing RESET. Next load the tape reading/​conversion program, with the correct delay values for your clock speed. Run the program by entering E0C80, connect your cassette recorder to the input port via the given circuit, and start the TRS-80 program tape. After loading is completed and control has returned to Nas-Sys, re-enter Basic with a warm start. The TRS-80 program can now be listed, and may even run.

There are several factors which can prevent the Basic program running. Some of the commands used in TRS-80 Basic are not available in the Nascom Basic, and the program may have to be modified to carry out these instructions in some other way. From the listed program and the table of token values given at the end of this article it should be possible to find out what the program is doing. Two useful articles are “Whose Basic Does What”, BYTE, January 1981, page 318, and “TRS-80 Program Recovery”, INTERFACE AGE, December 1980, page 100. “The Basic Handbook”, by David A. Lien, published in the U.S.A. by Compusoft, is an invaluable book.

Peek and poke addresses may also vary. In particular, the TRS-80 screen consists of 16 lines of 64 characters, and is located from £3C00 to £3FFF. The decimal values of the screen locations run from 15360 at the top left, 15423 top right, to 16320 bottom left, 16383 bottom right. Remember that the Nascom top line is not scrolled, and that it is located in memory after the bottom line. You will find that the volume and tone settings of the cassette recorder are very critical. If nothing appears on the screen, re-run the program with £008E set to 00 to stop sync checking, and £0C80 set to 00 to prevent the program ending on reading the first null characters, (the program will have to be terminated by pressing the RESET button, and the second part of the program, token conversion, will not have been executed.) Adjust the volume and tone settings until a display appears. Probably the display will not make sense, because the characters will be out of sync. Replace the original values at £0C8E and £0CC0, and try again.

If you load a program and find that when you LIST it starts correctly but then produces rubbish, the line address pointers have probably been misread. Try adjusting the volume or tone slightly and reloading.


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