Micro­power

  

Volume 1 · Number 2 · September 1981

Page 12 of 33

MONITOR.COM

By Chris Blackmore

THE SCENARIO

You have just added a disc drive or two and CP/M to your Nascom 2. After a few minutes gloating, you try the Digital Research editor, ED.COM) and find that it is a pig to use. Then you discover that the assembler provided only knows 8080 codes, instead of speaking Z80; even worse, it uses weird mnemonics designed to confuse anyone who is used to the Zilog set.

THE ANSWER

You could buy Diskpen and Macro-80 to solve your problem – if you have enough money left after lashing out on the discs. If you did, you still won’t be able to run all your old programs. In this article I provide instructions that will enable you to produce a CP/M program that will pretend to be Nas-Sys 1. I also provide some machine code which enables the new version of Nas-Sys to read from and write to the disc.

To avoid possible problems with copyright, not to mention space in the magazine, the Nas-Sys code is not given. If you got your Nascom by legal means, you will find it in your manual.

THE METHOD

  1. Initialise a disc, and copy DDT.COM onto it. This dynamic debugger is the closest CP/M comes to a monitor program like Nas-Sys; it can do some clever tricks, but is not exactly “user friendly".
  2. Type DDT followed by Enter. This, of course, runs DDT.COM. In future I will put Enter so-and-so for anything that you have to type with Enter after it.
  3. Clear some memory to work in, otherwise things can get very untidy, and you won’t be sure if you typed the code you find when you come back from a tea break.
    Enter F100, B00 ,0.
  4. Type in the section of Nas-Sys 1 from 0108H to 07FFH in it’s proper addresses in memory. Enter S108 to do this. Notice that it isn’t as easy to type in hexadecimal code with this system as it is with Nas-Sys. Also notice that, where the listing should show all of the strings following EF, it only shows the first byte. Fortunately, the machine code in the listing does show what should be there.
  5. Type in the start of Nas-Sys 1, normally found between 0000H and 0107H, at addresses 0800H to 0907H, using the S command instead of DDT again.
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