80-Bus News

CP/M Operating System

The Galaxy uses the familiar CP/M version 2.2 as its operating system. Little need be said here about CP/M and its utility programs as these have all been adequately covered elsewhere. However, the Galaxy features a number of small but significant improvements related to the BIOS. Many, criticisms have been levelled at CP/M as an operating system, not the least of which refer to its ‘unfriendliness’. Some criticism is justified, as is probably the case for any operating system, but CP/M comes in for a lot of unjustified criticism, which is not the fault of Digital Research, but the fault of the programmers who wrote the BlOSes for various machines. Gemini amply demonstrate what can be done to confound the critics, and deserve the highest praise for both the orderly approach and the manner in which they have removed much of the supposed unfriendliness of CP/M without touching a byte of CP/M itself.

Perhaps the worst areas of CP/M are its very bald and unhelpful disk error trapping. Now as the disk access routines are in the BIOS and as stated, the BIOS is the province of the system programmer and not Digital Research, scope exists to improve things. With the Galaxy the days of ‘BDOS ERROR ON DRIVE x’, a very unhelpful statement, are gone. In the event of a disk read/​write error, the Galaxy BIOS traps it and attempts to reread it four times, if that fails, then the heads are drawn back to track 0, repositioned, and the read attempted another four times. Only after this is an error reported with a message indicating the type of error, the track and sector where the error occurred and a polite request to the user to either try again, accept the error anyway or boot the system. In the odd instances where disk errors have occurred and been reported, the fault can often be cleared by removing and replacing the disk in the drive and then requesting a retry.

Another notable improvement is the ‘on screen’ editing using the cursor controls. An invaluable feature, particularly for careless two finger typists such as the author. The CP/M input works normally until the EDIT key is depressed, where upon the cursor changes from its normal underline symbol to a solid inverse video block, indicating edit mode. The cursor can then be moved around the screen at will and incorrect input lines edited. This feature was found to be particularly useful with MBASIC, as most editing could be achieved without recourse to the somewhat cumbersome internal Microsoft editor. The screen editing suffers from two limitations, the edit line must not exceed one displayed line, 80 characters, and it can only be used with programs that use the CP/M buffered line input mode. These features are all under the control of the programmer who wrote the original BIOS, and not a function of the Galaxy hardware, so it is pertinent to ask why these features do not seem to have appeared on other machines?

In common with most implementations of CP/M, there are a number of default modes, whether to run a program on start up, what type of output device to select, etc, which, whilst selectable by the user, are often poorly documented (if at all) and usually hard to find. Likewise the hardware also has a number of defaults which need to be declared on start up, such as the speed of the UART, etc. Gemini overcome this problem by supplying a small utility program, CONFIG.COM, which allows all the defaults to be set in the most simple manner. This program is a delight to use and works by using a menu of the options available and then allowing the particular option to be set using the screen editing. The program is self documenting and explains the consequences of any particular default set up.