80-Bus News


July–August 1984 · Volume 3 · Issue 4

Page 31 of 43

Polydos File Update

by M. J. R. Gibbs

This is a program for directly updating files on disk for a Gemini GM809/​GM815 system with Polydos 2.0.

This allows the user to update copies of a file stored on several disks without having to enter all the normal commands which can cause problems especialy if you are like me and sometimes forget to use the ‘NEW’ command (I have spent many hours trying to sort out the chaos that this causes). The required program/​data is loaded into RAM between £1000 and £C000 either by using the Polydos ‘Read’ command or by Assembling the program directly into RAM.

This program is loaded into RAM at £0C80 and executed at £0C80, the user is asked for the file name, file extension and the RAM location for the new code. The Program asks you to insert the disks and press the ‘Enter’ key, the disk directory is read and the filename checked and, should it exist, the file is replaced with the new version. The old file is completely overwritten and this means that the number of sectors used by the old file are replaced by the program stored in RAM. The user should be careful in ensuring that the new version is not going to require more sectors on disk than the original otherwise the whole of the new program will not be saved. I normally save programs that are common to several disks with a few sectors more than required to allow for expansion, this does not normally cause a problem as I have found that the maximum number of fifty files means that the disk is not often full before the directory is used up. This program can update a disk with a full directory because the directory is left unaltered.

Should the program not exist on the disk inserted a suitable message is printed out telling the user and the disk is left unaltered.

The Program informs the user of the start disk sector and the number of sectors updated and the RAM location used for the update. Should there be any disk errors then a message is printed out and the program repeats the inital menu asking the user to insert a new disk. The one thing that I have found is that it is easy to forget to remove any write protect tabs and this does not cause a problem as you simply take the disk out remove the tab and reinsert the disk (do not forget to replace the tab afterwards).

A word of warning when testing this program especially when entering it from the dump listing, which is always prone to errors, you are strongly advised to test it on a copy of an existing disk because it can overwrite irreplaceable portions of disk if anything goes wrong.

I suggest that you proceed as follows:–

  1. Enter the program and save it on disk using Polydos. Use the ‘Read’ routine to load a file from the disk to say £1000 in RAM.
  2. Examine and make a note of the number of sectors that this file uses.
  3. Use SZAP to examine the next file on the disk and make a note of the first few bytes.
  4. Change the first few bytes of the file loaded into RAM with the NAS-SYS ‘M’ (modify command).
  5. Run this program to update the disk and check the user information for the correct sector location, number of sectors and RAM location.
  6. Use SZAP to verify that the original file has been updated correctly. Also that the file following is not corrupted.

Below is a full Assembly listing, a sorted symbol table and a dump of the program using a modified version of the disk dump published in Vol.1 iss. 2 of 80-BUS NEWS (the numbers on the left hand side are the RAM locations).

Page 31 of 43