CP/M on the +3

One advantage of buying a Sinclair Spectrum +3 is that it will run CP/M which may still available (at the time of writing). It is a superb piece of software and well work the £30 asking price. But if +3 DOS offers CP/M like commands why bother? Because +3DOS cannot run programs written for CP/M of which there are thousands and most of them Public Domain.

The programs supplied are on a single 3 inch disk with a nice hefty manual. For your money you get CP/M and most of it's standard utilities (with the exception of help) along with Mallard BASIC (a superset of MBASIC) and a text editor written in Mallard BASIC called RPED. Installation is easy. Ensure the disk is write protected (see the +3 manual) and pop it in the drive. Choose the loader option from the main menu and press RETURN. That's it! Of course you will need to make a copy of the disk (using DISCKIT - see the CP/M manual) and keep the original in a safe place away from heat or electrical appliances. The copy process takes a while and needs lots of disk swapping but it is essential that you do it.

For those familiar with the Amstrad PCW there are several things to be noted about this version of CP/M. The RAMdisk is called the C: drive not the M: drive (why?) which is very confusing. The RAMdisk C: drive is also very small at only 11k which is a little disappointing and explains why DISCKIT takes so many disk swaps. The +3 cannot use graphics in any way so that GSX and DRLOGO won't work and so many programs written specifically for the PCW won't work (especially Locoscript) although CP/M only programs will. HELP is missing from the disk but the PCW version seems to work fine.

The biggest difference is the size of the screen which is restricted to 51 columns. Now CP/M needs at least 80 columns which can be accessed but only by flipping the screen to show the other side (see the manual) but most packages can be configured for the 51 column screen if required.

RPED is slightly different but still familiar with slightly different key combinations. SETKEYS also uses different key configurations so those old PCW ones will have to be rewritten. Mallard BASIC remains unchanged but PCW listings using machine code may well not work as the hardware is different. The CP/M utilities also remain unchanged and work very well if a little slowly without the aid of a large M: drive.

The bottom of the screen is reserved for system prompts although this facility can be switched off. It shows the current drive (A: or B:) and has a rather nice scrolling prompt to insert the drve B: disk if one is not physically attached. One strange anomally seems to be that the +3DOS FORMAT command can format disks to CP/M standards but seems to produce a disk that is slow to write to when using CP/M. DISCKIT does not seem to suffer from this problem and it is much more adept at recovering dodgy tracks. It is interesting to note that disks are formatted on the CPC computers using DISCKIT under CP/M - they lack their own internal FORMAT command.

As the disk fromat for the +3 is the same as the PCW, CP/M software supplied for that machine can be read and written to easily despite the differing hardware. This is testimony to the design of CP/M and the computers. CPC disks are not compatible but CP/M programs written for the CPC can still be read. This means that all of the software available on 3 inch disk will work with +3 CP/M provided they are NOT graphical in nature. Programs written specifically for the PCW will also not work so be careful what you buy.

Software is still available from CP/M libraries and from some small suppliers. The second hand market can also be a good source as well as PCW user groups of which there are many. Most of it (but not all of it) is also PD therefore FREE apart from the price of the disk and postage. This makes using CP/M much cheaper than using DOS on the PC.

The utilities supplied with CP/M are of a very high quality and can be used for program development provided you don't mind using assembler. The disk housekeeping utilities are also very good with the possibility of users extending the operating system command suite using SUBmit files similar to BATch files in DOS. I will be covering assembler and submit files in later issues.

This version of CP/M is a classic piece of software and both AMSTRAD and LOCOMOTIVE SOFTWARE are to be congratulated for having used it to such good effect across the range of computers including the humble SPECTRUM. Without such support it would not have achieved the success it deserves.