Amstrad PCW Drives and Disks
This article is an extract from Malcolm Surl's LuxSoft.co.uk website - thank you Malcolm for your many years of contributing to the Amstrad PCW community!
As mentioned above, the 8256, 8512 and 9512 were supplied with 3" drives which took CF2 discs whereas the 9512+, 9256 and 10 were fitted with 3.5" drives and used standard PC type MF2DD 720k discs (reformatted, of course, to suit the CP/M operating system). Note - some 3.5" drives will also accept MF2HD 1.44mb high density discs (and use them as per 720k's), others will not.
The 1st drive (the one the PCW is booted from) is drive A and any second drive is B. Where a hard disc has been added, the 1st partition is generally C (as on a PC). On single drive machines, addressing the drive as B is permissible as CP/M supports a means of identifying two discs which need to be swapped back and forth as being in drive A or B.
Note that whilst 3" 720k drives can read from (but not write to) 180k format discs, single density drives cannot read 720k discs so programs will report them as "not formatted or faulty".
All the 3" and most of the 3.5" drives are belt driven - the Achilles Heel of the PCW. The drive belts - glorified rubber bands - perish with the passage of time and tend to set in one shape if not used for some time. "It worked when I put it up in the loft 5 years ago" is an oft heard boast, but more often than not it won't when subsequently retrieved and an attempt made to start it. Merely fitting a new belt is not the panacea for all ills claimed by many because rotational speed of the disc is crucial for correct reading & writing and the belt may well have failed because of partial or total seizure of the main bearing. The bearings were lubricated for five year life ... but in many cases that was 20 years ago! Moreover, fitting a new belt can destroy the head alignment. Alignment to the concentric disc tracks is equally important for successful operation; misalignment generally arising from worn sled motor bearings (the motor which moves the read head across the tracks) or physical damage arising from trying to load a damaged disc or withdrawing a disc before the motor has stopped turning.
1.2.2 - The Discs
3.5" discs only fit into their drives one way up but 3" CF2's will fit either way because the same physical discs are designed to suit all PCW models. Care must therefore be taken as they may be used in two different ways:
Single density 180k format - PCW 8256 & 8512's 'A' drives - have just one read/write head so you have to turn the disc over to access the other side. By convention, these are called the 'A' and 'B' sides and each can hold 173k of data after formatting. The 40 tracks per side are numbered 0 to 39.
Double density 720k format - 9512 and 8512 'B' drives - not only pack double the data into the same space but also read BOTH sides at once so the capacity is four times as much - 720k (706k for data after formatting). Once a 3" disc has been formatted as double density, the whole of the disc has been used so DO NOT turn it over in the hope of another 720k on the other side! If you do, you'll wipe the lot so make a habit of using the side marked 'A' (or '1') uppermost (9512)/to the left (8512) and mark the spine label accordingly by barring out the B or 2. The 160 tracks are numbered 0 to 159.
Some early manuals recommended using only of 'CF2DD' discs for the 720k Double Density format. Don't worry - any 'decent' CF2 should be able to be successfully DD formatted but, as with any disc formatting, it is wise to Verify the disc before use.
However, beware relying on sub-standard discs! When the leading disc manufacturers ceased production of CF2's, a mass of horrid discs were imported from southern Europe and sold under a variety of names, including (wrongly & illegally) Amsoft. These are prone to sudden and complete failure and can be recognised by having criss-cross hatchings on the disc case and white write-protect levers in the leading edge of the disc (white showing through the two smaller holes when the disc is laid flat). Maxell discs are plain surfaced (no hatching) with red levers whilst proper genuine Amsoft ones are hatched but write protection is by a white slider tab on the rear left of the upper surface of the disc (so when laid flat, there's a hole showing white on the right but a rectangular slider tab on the left).
Whilst talking of write protection, it is worth noting that most program master discs were manufactured without write-permit tabs so their complete absence does not indicate a damaged disc.
180k discs have 40 tracks and 720k's have 160. Each track is divided into 8 sectors, the sector being the unit of transfer to/from the disc.
The first track of the disc - track 0 - is devoted to system information, notably the format type and number of tracks. On boot (start-up) discs this is followed by the boot program - the one which starts the snowball of intelligence gathering in motion by loading an embryo operating system and telling it to go looking for the next stage - the loading of an EMS program (or EMT on 3.5" models), eg CP/M or LocoScript.
Track 1 contains the all-important directory, or in 720k format, the first part of it. Each 32 character entry contains details of the name of the file, the group it is in, it's size and the location on the disc of the constituent parts of the file. As on PC's, the constituent parts may not be consecutive data 'blocks' - on well-used discs they may be scattered all over the (fragmented) disc. A single directory entry can only accommodate files up to 16k so, for files larger than this, 'continuation' entries are needed for each 16k of data. Each sector of the directory contains 16 entries so, if a disc read error occurs here, up to 16 files may be 'lost'.
The rest of the disc is available for data storage and is addressed by means of a unique 'block' number. On 173k discs these blocks are 1k in size but on larger discs (720k and drive M) these are 2k in order to keep the block number within the permitted numerical range. This explains why files copied from 173k to 720k (or drive M) can apparently increase in size to the next even number of k and, conversely, files copied to 173k may appear to reduce in size.
1.3.1 - Formatting Quirk - Upside down & mixed formats
It was stated above that turning a 720k disc upside down and formatting the B side in the hope of gaining a further 720k of space on the disc would result in loss of the initial formatting, and therefore all the data. This is not quite totally true.
A 'naked' disc in fact has space for around 168 tracks, not 160, and due to the slight offset of the read/write heads, the first 8 tracks of the 'upside down' (B side) format will use space not used by the A side formatting and, by the same token, leave the last 8 tracks untouched. When turned back over to the A side, these untouched tracks are its tracks 0 to 7, which includes the directory. Hence, when inspected in (say) LocoScript, all the original files of the A side format will be listed and the attempt to gain a further 720k will appear to have been successful. Wrong! Any attempt to access any of the listed files will be met with "Disc Address Mark Missing" unless the whole file happens to reside within tracks 2-7.
So do be careful when formatting -
Don't be greedy by trying to get 1440k out of a 720k disc !
Smell a rat if a disc verifies perfectly up to track 7 then collapses in a heap with Disc Address Mark Missing on every single sector of Track 8 onwards.
The same formatting quirk applies when re-formatting a former 180k disc as 720k - the first 8 tracks of the B side won't be touched so loading it B side up will give the appearance that it is a 180k disc at the same time as the A side reports it as 720 !
Don't mix formats if you have confidential or sensitive info you want to get rid of because, if you do mix, some files from the previous format may still be readable.
If you have both 180k and 720k drives, always load discs of unknown type into the latter and, if apparently conflicting formatting info appears, use Disckit to determine which is the complete format.
If the B drive of your 8512 has given up the ghost but you are still soldiering on with just drive A, beware that 720k formatted discs loaded into drive A will report that they are "not formatted or faulty" so tempt you to overwrite potentially valuable data.
Don't rely on what the disc label says is on the disc !
Drive M (or simply M:) is the 'Memory Drive' - part of the PCW's RAM memory given over to acting just like a physical disc drive. Anyone who has ever tried copying a file from one disc to another on a single floppy drive PC will readily appreciate this PCW concept - just copy A to M, switch discs then copy back M to A.
Two other important facets of this are speed and flexibility. Speed because drive M works at electronic speeds rather than the mechanical speed of a revolving disc, flexibility because copying frequently used files or programs to M at the start of a session saves much disc swapping later on. Hence, when loading, Loco loads printer driver files, templates and the LocoSpell dictionary into M so that they are available whatever disc is loaded and, similarly, the standard CP/M start-up loads several useful programs into M. The downside of this that Start-up is prolonged and/or valuable working space is taken on M by files that are not required during the session.