Doctor Dark’s Diary –4

(An every day story of simple programming folk)

Issue five of the INMC News arrived at Zilog Villas just in time for Christmas – this was clearly the result of a prodigious effort by both the committee and the post office. Having suggested to our illustrious editor that he should print both the episodes I had sent in at once if there was a shortage of material, I was caught with my pencil down. The pressure is on now, no more playing with the piranhas until I have typed this episode out.

Making M5 Run Bigger Programs

If you have run Microdigital’s M5 interpreter, you will know that it can only run programs of about 230 bytes or less, depending on how much use is made of the stack. On an expanded Nascom, only a few changes need to be made to the M5 interpreter to move the program store to the additional memory. The changes are as follows:

0DE1 : 00 10
0E06 : F0 0E
0H2F : FF 0F
0FSA : FF 0F
0EB3 : 00 10
0EDB : FF 0F

As well as changing the program store location, I have also changed the variable stores from the top line of the screen, which was rather unsightly, and put them where the program used to be. There is still plenty of space for the stack to work in, it would take about a hundred consecutive pushes to cause problems – if your calculation is that complex, you should be using Fortran! Should you happen to write a program in M5 that uses 32K of RAM, you may find jumps from the low end of memory to the high end are fairly slow. Serve you right for writing such long programs.

Tiny Disassembler

When I first used M5, I wanted to find out how it worked, so I decided I would disassemble it. Hand disassembly is incredibly tedious, and Personal Computer World still have not printed the end of Parkinson’s Revas (a Nascom disassembler, which it just happens you can buy if you have the money). Half way between the two alternatives is a program written by R. M. Lucas, and published in Computing Today, October 1979. This program is called Intab (short for Intelligent Tabulate) and breaks a program up into its separate instructions – much easier to understand than a rectangular block of code. As Intab is probably protected by all sorts of copyright laws, I have taken the diabolical liberty of writing an improved version, which is fully relocatable, and (sorry!) only works with T4.