Scor­pio News


January–March 1987 – Volume 1. Issue 1.

Page 30 of 63

missed out by the FDC. In fact, when sending the A1H byte, there is no clock pulse sent between bits 4 and 5. Similarly, when sending the 0C2H byte, the clock pulse between bits 3 and 4 is missed.

In single density, various other clock pulses are missed when sending control bytes to the disc. In the table below, when sending a data byte, if all 8 clock pulses associated with that byte are present then the clock can be considered having the value 0FFH. A missing clock in any position may be represented by a zero bit in the clock byte. The table also shows the values written to the disk for the values sent to the FDC.

Byte sentSingle DensityDouble Density

00 to F4Write 00 to F4 with Clk-FFWrite 00 to F4
F5Not allowed*Write A1, preset CRC
F6Not allowed**Write C2
F7Generate 2 CRC bytesGenerate 2 CRC bytes
F8 to FBWrite F8 to FB, Clk=C7, preset CRCWrite F8 to FB
FCWrite FC with Clk=D7Write FC
FDWrite FD with Clk=FFWrite FD
FEWrite FE, Clk=C7, preset CRCWrite FE
FFWrite FF with Clk=FFWrite FF

*Missing Clock transition between bits 3 and 5.
**Missing clock transition between bits 3 and 4.

File sizes, disk sizes and directories

Having well and truly taken a disk to pieces we can at last return to CP/M and the questions asked at the beginning of this article. (Can anyone remember what they were?) Well, you’ll have to wait until the next episode.

A Look at MultiNet 2

by P.A. Greenhalgh

MultiNet Design Philosophy

The aim of Gemini’s MultiNet networking system is to provide computing facilities to a number of people for the minimum possible cost. As a very significant proportion of the cost of any system is in the mass storage and hard copy devices, the overall cost of a multiple system installation can be dramatically reduced by allowing a number of users to share these facilities. ‘Share’ in this instance means the ability for any user to be able to of the mass storage device, but not, in general, the sharing of the stored data.

CP/M Compatibility

As Gemini MultiBoard systems are all capable of running the CP/M operating system, and given the amount of applications software available for that operating system, MultiNet is designed to provide a CP/M ‘compatible’ environment, the major difference being that the user need not have physical disk drives present at his Workstations but the MultiNet software refers all disk requests to a Fileserver.

To achieve this, software has to be written that looks to the applications program as though it is CP/M, but in reality this software contains no disk driver or file handling routines, but instead refers these to the Fileserver that is controlling the mass storage. To achive this the relevant CP/M documentation is used to write software that meets the given specifications as closely as possible, given the major premise that there are no. physical drives present. Unfortunately, in practice, it is found that certain programs make use of certain ‘quirks’ of ‘undocumented features’ of CP/M, and so the emulation Software has to be modified in order to provide as identical an environment as possible. It is thus extremely difficult, if not impossible, to achieve 100 per cent compatibility.

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