80-Bus News

Stimulated by Dr Dark’s suggestion in the March-April 1984 issue (Vol 3 Issue 2) I decided to rake out this program, first written in January, and submit it for publication.

The Spectrum is a remarkable little machine, but most of the interesting games available are in machine code, and difficult to patch (i.e. cheat!) without decent tools. I was too mean to buy a machine code package and a printer interface for my son’s Spectrum, having perfectly good facilities on the Nascom, and I decided to interface the Spectrum to my Nascom 2 without using any extra hardware so that I could disassemble, list, and generally tinker with code dumped from the Spectrum.

The Spectrum uses one of its ports (0FEH) for output to the MIC socket and to provide the Border colours on the screen, bit 3 for the MIC and bits 0-2 for the three primary colours. Its built-in tape routines use a non-standard format, but it is possible to use the output bit provided in any format you wish, in this case the CUTS standard used in the Nascom, which outputs 1 cycle at 1200 Hz for a zero bit, and 2 cycles at 2400 Hz for a non-zero bit when it is set up for 1200 Baud output. I used 300 Baud values (4 cycles and 8 cycles) initially, but found that 1200 Baud was more reliable!

The crucial routine in the program is labelled SndBit; each bit to be output to tape is passed to this routine in the carry flag, and the MIC bit in port FE is toggled accordingly, with appropriate delays for the frequencies involved. The initial counts for the delay loops have to be calculated from the known clock frequency of the Spectrum and the number of T-states used by each instruction, and are included in the EQUates at the beginning of the listing.

SndBit may be regarded as a software emulation of the Nascom’s cassette interface, and is sent data bytewise by the UART emulator routine BytOut, which is in turn fed by the NAS-SYS “Write” emulator routine, which is the bulk of the remainder of the program. The analogy is not complete, in that the Nascom hardware is continuously sending out a stop bit when the cassette output is not in use. The loop labelled PL0 is used to provide a short burst of this “pilot tone”.

For convenience, the start and end+1 addresses are passed to the program at 8000 and 8002 (ie 32768 and 32770) and the program itself runs at 8004 (RANDOMIZE USR 32772). I use a second copy assembled at F000 if code in the range 8000 to 80C0 is of interest. You may wish to write a short BASIC program to prompt for the necessary addresses and remind you to start the tape, but remember that this may encroach on code under examination.

The output follows exactly the format of a “W XXXX YYYY” command under NAS-SYS with all the usual checksums, and may be saved on cassette and then loaded directly into memory on the Nascom using the “R” command. If NAS-SYS 3 is available, it may be loaded at any address by giving the appropriate offset as an argument. With suitable amplification it may be connected directly into the tape input on the Nascom.

Important Note: This program will not operate properly at addresses below 8000 because the Video circuitry is constantly reading this block of RAM, and upsets the timing loops. As the Spectrum keyboard is read under interrupt, this must also be disabled while the program is run (and enabled afterwards of course!).

I have also written code using similar techniques which uses the Spectrum LLIST and LPRINT commands to speak to the Nascom through the cassette interface, using the Nascom as a printer buffer to my Epson. It doesn’t handle screen dumps yet, but one of these wet Sundays…