I see from another letter that someone out there has been spreading false
rumours about CP/M block sizes and disk sizes: CP/M (in version 2.2) supports
logical drives up to 8Mbytes in size. CP/M can allocate the disk space in block
sizes of 1k, 2k, 4k, 8k, or 16k. The one restriction is that if a block size of
1k is used, then the disk capacity cannot exceed 256 blocks (256k). All the other
block sizes can be used with any disk capacity up to the limit of 8Mbytes. One
point to be aware of is that CP/M has a little bit of intelligence built in. If
the total disk capacity is less than 256 blocks, each directory entry, (which
records where each file is on the disk), holds the block numbers as 8-bit
quantities. If the capacity is greater than 256 blocks, then each block number is
stored as a 16-bit quantity. (i.e. If the capacity of the disk is less than 256
blocks, each directory entry can hold up to sixteen 8-bit pointers, but if the
capacity exceeds 256 blocks each entry only holds up to eight 16-bit pointers.)
This is the reason why a block size of 4k was chosen for the Gemini Galaxy &
As these use Quad density drives (i.e. Double density recording and double
track density – 96tpi), the option of offering both single-sided (400k formatted)
and double-sided (800k formatted) drives was open. If a block size of 2k had been
used then the directory formats of single-sided disks (400k capacity – 200 2k
blocks), and double-sided disks (800k capacity – 400 2k blocks) would have been
incompatible. With a block size of 4k the ‘block capacity’ of both single and
double sided drives are less than 256, thus ensuring that CP/M uses an identical
method of recording block numbers in the disk directories. (A single-sided disk
can be read/written in a double-sided system, and a less-than-half-full double-sided
disk can be read/written in a single-sided system).
PRODUCT PREVIEW TIME
is an 8x8 80-BUS card containing 512kbytes (or 0.5Mbytes,
if you prefer!) of dynamic RAM. This RAM cannot be used as conventional system
memory, but is configured to appear as a disk. It is accessed via three I/O ports
which can be regarded as “track”, “sector” and “data”. To access the memory all
that has to be done is to output “track” and “sector” addresses, and then read or
write 128 bytes of data to/from the data port (perhaps using INIR or OTIR in a
pseudo DMA transfer).
The software interface matches very nicely to CP/M, and is so simple that an
application program in a non-disk based system could drive it directly. Unlike
the “page mode” scheme, or the extended addressing modes of
or the MAP256
card, no complex memory management is involved as the RAM-DISK is completely
independent of the normal system memory.
CALL FOR ARTICLES
Reverting to the earlier theme, to help me answer yet another question
perhaps someone out there (a Dealer even?) would like to produce a summary of the
various DOSs & CP/Ms available for 80BUS cards? The obvious things to cover are
the hardware they require to run. The extra hardware they can support (e.g.
The standard Nascom/Gemini software that
can/can’t be used under it. How easy it is to customise (can you add a printer?).
An alternative might be for YOU to send in a summary for the DOS you use. A
user’s view is perhaps better, as a dealer, though he sees all the products, may
not have the time/inclination to use everything/anything he sells, and so may
miss some of the finer points of the DOSs. Your replies can then be collated,
compared, checked and published. This, (if it ever appears!), will no doubt be
followed by a deluge of your articles on how to modify X to get it to run under
CALL FOR QUESTIONS
As I said at the start, these pages will be fueled by your letters – so send
them in even if it is only one sentence on the back of a postcard. If you want
any of the above replies expanded on say so. (More on CP/M directory formats?).
If I can’t answer your questions I’ll try and find someone who can.