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.
|00 to F4
|Write 00 to F4 with Clk-FF
|Write 00 to F4
|Write A1, preset CRC
|Generate 2 CRC bytes
|Generate 2 CRC bytes
|F8 to FB
|Write F8 to FB, Clk=C7, preset CRC
|Write F8 to FB
|Write FC with Clk=D7
|Write FD with Clk=FF
|Write FE, Clk=C7, preset CRC
|Write FF with Clk=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
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.
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