80-Bus News

RP/M is one of the two ROM monitor programs supplied with Gemini GM811 and GM813 CPU cards. It is designed for use with or without disks and is particularly suited to users who wish to develop and test software under CP/M which is to be placed in ROM and used in dedicated systems which do not have disks. It is however able to boot a floppy disk based system and is therefore suitable as a general purpose boot ROM. (It can boot a hard disk based system via a floppy disk, but cannot directly boot a hard disk.) It also has a variety of simple commands which allow many hardware problems to be analysed if the disk system fails to operate.

The other ROM monitor program supplied by Gemini is SIMON which is specifically designed for use in disk based systems and has the ability to boot both floppy and hard disk drives. It too is designed to allow diagnosis of hardware problems.

RP/M was originally issued as version 0.1 and this was later replaced by version 2.0, which had many improvements. See 80-BUS NEWS Vol 2 Issue 1 for details. This issue also specifies a small patch to convert version 2.0 to version 2.1.

This article provides a series of patches to version 2.0 or 2.1 so that if you have the ability to program 2732 type EPROMS then you can upgrade your copy of RP/M to an enhanced version, called 2.3. (To avoid confusion with CP/M V2.2 there is no RP/M version 2.2.)

Version 2.3 allows the Boot command to be followed by an optional value which makes RP/M attempt to read in and execute a boot sector which is on the first track of any disk in the system. It is possible to have the boot sector on a 5.25 inch or an 8 inch floppy disk. It may be either single or double density. Also the sector number may be specified. For example a normal Gemini format 48 or 96 tpi double density system disk with a 512 byte boot sector in physical sector 0 in drive B could be booted. So could a standard single density 8 inch disk with a 128 byte boot sector in physical sector 1.

A particular advantage of this ability is to check out the correct operation of the disks should there be a problem with booting the system. Before this if the system failed to boot with a “Bad disk” message then it was impossible to determine if the problem was in the disk controller or the disk drive without dismantling the drives and altering their physical addresses.

Note that since the boot sector on any disk is likely to contain code which loads in CP/M from drive A, it is still not possible without a special version of CP/M to actually boot CP/M and use it if drive A is out of operation. Even if the boot sector is patched so that CP/M is loaded from the other drive, as soon as it executes it will attempt to log in drive A.

Also note that RP/M loads and executes only the first 128 bytes of a boot sector, regardless of its length. For this reason it is not possible to load a double density boot sector from an 8 inch disk, for timing reasons associated with the Z80 code. Also, as before, if the boot sector does not start with the Gemini identifying charaters then the “Wrong disk” message is displayed.

RP/M will still attempt as now to boot a standard Gemini disk in drive A on power-on if there is a disk card in the system. Also the normal B command with no values entered after it works exactly as before.