Disk Types

The Memotech family of computers (SM1, FDX, SDX) had a variety of disk sizes and formats they supported.

In addition, I've added a few, primarily to support REMEMOTECH, REMEMOrizer and SDX Plus.

I maintain a set of diskdefs, suitable for use with (or inclusion in) cpmtools.

Each Memotech disk format had a type code, which is a 2 digit hex number :-

Type Physical CP/M Logical view Comment
Media Size Sides DensityTracksCapacity Directory entriesBlock size Blocks
00 Floppy 3.5" / 5.25"SingleSingle 4080KB 64 102473 Never seen in the wild
01 Floppy 3.5" / 5.25"DoubleSingle 40160KB 64 1024153 Never seen in the wild
02 Floppy 3.5" / 5.25"SingleDouble 40160KB 64 1024153 Never seen in the wild
03 Floppy 3.5" / 5.25"DoubleDouble 40320KB 64 2048156 Common (FDX 5.25" and SDX 3.5")
04 Floppy 3.5" / 5.25"SingleSingle 80160KB 64 1024153 Never seen in the wild
05 Floppy 3.5" / 5.25"DoubleSingle 80320KB 64 1024156 Never seen in the wild
06 Floppy 3.5" / 5.25"SingleDouble 80320KB 64 2048156 Never seen in the wild
07 Floppy 3.5" / 5.25"DoubleDouble 80640KB 128 2048315 Common (later SDX 3.5")
10 Floppy 8" SingleSingle 77250KB 64 1024243 For compatibility with MDS? Skew applies.
11 Floppy 8" DoubleSingle 77500KB 128 2048246 Never seen in the wild
12 Floppy 8" SingleDouble 77500KB 128 2048245 Never seen in the wild
13 Floppy 8" DoubleDouble 771MB 128 4096247 Common (SM1 and FDX)
18 SD/CF card N/A 8MB 512 40962046** Partition 0
19 SD/CF card 8MB 512 40962046** Partition 1
1A SD/CF card 8MB 512 40962046** Partition 2
1B SD/CF card 8MB 512 40962046** Partition 3
1C SD/CF card 8MB 512 40962046** Partition 4
1D SD/CF card 8MB 512 40962046** Partition 5
1E SD/CF card 8MB 512 40962046** Partition 6
1F SD/CF card 8MB 512 40962046** Partition 7
20 HD ST506 5.25" 5MB Need to see the hard disk driver HDX/HRX systems may have had a 5.25" one
I saw an SM1 with an 8" one
21 HD ST412 5.25" 10MB
22 HD D-507 5.25" 5MB
23 HD D-514 5.25" 10MB
30 HD SA10028" 4MB
31 HD SA10048" 8MB
32 HD Q2010 8" 8MB
33 HD Q2020 8" 16MB
34 HD Q2030 8" 24MB
35 HD Q2040 8" 32MB
40 Silicon N/A 256KB 64 1024249
41 Silicon 512KB 128 2048252
42 Silicon 768KB 128 2048380
43 Silicon 1MB 128 4096254
44 Silicon 1.25MB 256 4096318
45 Silicon 1.50MB 256 4096382
46 Silicon 1.75MB 256 4096446
47 Silicon 2MB 256 4096510
48 Silicon 2.5MB 256 4096638
49 Silicon 3MB 256 4096766
4A Silicon 3.5MB 256 4096893
4B Silicon 4MB 512 40961022
4C Silicon 5MB 512 40961278
4D Silicon 5MB 512 40961534
4E Silicon 7MB 512 40961790
4F Silicon 8MB 512 40962046
50 RAM 256KB 64 1024249
51 RAM 512KB 128 2048252
51 RAM 480KB 128 2048236 * After S2R 2, or in Italy
51 RAM 144KB 128 204868 * After S2R 23, for MTX Series 2
52 RAM 320KB 64 2048156 ***

* = Hacked type 51 codes, explained below.

** = I added these for REMEMOTECH and REMEMOrizer. The CFX system (with Compact Flash media) also uses these same type codes.

*** = I added this primarily for REMEMOTECH, REMEMOrizer and SDX Plus.

All Memotech disk types appear to have 26 128 byte records per logical track. This is what programs see, when using the driver code in high memory, and this is what CP/M sees. Physically, the layout is often different.

The first two logical tracks are reserved for CP/M. This is followed by the directory, and then the data area, as a number of blocks (both directory and data area are in the block area).

When dealing with 3.5" or 5.25" disks, the disk driver code converts the logical track and sector number to what would today be called a logical block address, and then converts it back to the physical track and sector. It also handles the fact the disk may be double sided, double density, or may have double the number of tracks (80 tracks, at 96TPI). 3.5" and 5.25" disks have 16 physical sectors per physical track. In double density modes, these are 256 bytes long, else 128 bytes.

Its interesting to note that 5.25" and 3.5" disks use exactly the same geometry and some limited success has been reported running a 3.5" disk attached to an FDX (I think I did this once) and a 5.25" disk attached to an SDX. I have no experience working with 5.25" disks on Linux.

8" disks did have 26 physical sectors per track. Type 10 disks also used a skew table.

Silicon disks (RAM accessed via I/O ports) and RAM disks (RAM accessed via bank switching) have no physical geometry.

It is interesting to note that with physical disks, the disk geometry reflects an integer number of tracks, each with the full number of sectors per track. With Silicon or RAM disks, the available memory produces a non-integer number of tracks, where the last fractional track contains fewer sectors. This affects how the number of Blocks in the DPB and the table above is calculated. It also means the disk definitions in the cpmtools diskdefs files need to take this into account, and because you advise cpmtools of the number of tracks (as a rounded up integer) this may also mean disk images may end up slightly bigger than the memory they fit into.

Hacked type 51 codes

Normally a type 51 RAM disk would be 512KB. I had a 512KB RAM expansion card in my FDX back in 1984. I therefore had 64KB CP/M memory on the motherboard, plus an extra 512KB which could all be used as RAM Disc.

In Italy (for an unknown reason), it would only be 480KB. Italy has a CONFIG48.COM and SIDISC48.COM programs, which know the smaller size. An alternative would have been to use the normal CONFIG.COM and SIDISC.COM, along with the S2R 2 command to shrink the type 51 code, by 2*16KB to 480KB.

The MTX512 S2 had 256KB DRAM on the motherboard, of which 64KB was visible in RELCPMH=0 mode, and 208KB was visible in RELCPMH=1 mode (due to addressing limitations). Use the S2R 23 command to shrink the type 51 code, by 23*16KB to 144KB.

The S2R command is intended to be used at boot time, in the startup sequence, before anything writes to the RAM Disc.

A better approach would have been to define new type codes for the various reduced sizes of RAM Disc. At the time, a rather poor precedent had been set in the hack for Italy. I also didn't own the source to the boot ROMs, SIDISC or CONFIG.

However, note that REMEMOTECH, REMEMOrizer and SDX Plus do the right thing, with the type 52 code for a 320KB RAM Disc.


This page maintained by Andy Key
andy.z.key@googlemail.com