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	Density	Tracks	Capacity	Directory entries	Block size	Blocks
00	Floppy	3.5" / 5.25"	Single	Single	40	80KB	64	1024	73	Never seen in the wild
01	Floppy	3.5" / 5.25"	Double	Single	40	160KB	64	1024	153	Never seen in the wild
02	Floppy	3.5" / 5.25"	Single	Double	40	160KB	64	1024	153	Never seen in the wild
03	Floppy	3.5" / 5.25"	Double	Double	40	320KB	64	2048	156	Common (FDX 5.25" and SDX 3.5")
04	Floppy	3.5" / 5.25"	Single	Single	80	160KB	64	1024	153	Never seen in the wild
05	Floppy	3.5" / 5.25"	Double	Single	80	320KB	64	1024	156	Never seen in the wild
06	Floppy	3.5" / 5.25"	Single	Double	80	320KB	64	2048	156	Never seen in the wild
07	Floppy	3.5" / 5.25"	Double	Double	80	640KB	128	2048	315	Common (later SDX 3.5")
10	Floppy	8"	Single	Single	77	250KB	64	1024	243	For compatibility with MDS? Skew applies.
11	Floppy	8"	Double	Single	77	500KB	128	2048	246	Never seen in the wild
12	Floppy	8"	Single	Double	77	500KB	128	2048	245	Never seen in the wild
13	Floppy	8"	Double	Double	77	1MB	128	4096	247	Common (SM1 and FDX)
18	SD/CF card	N/A				8MB	512	4096	2046	** Partition 0
19	SD/CF card					8MB	512	4096	2046	** Partition 1
1A	SD/CF card					8MB	512	4096	2046	** Partition 2
1B	SD/CF card					8MB	512	4096	2046	** Partition 3
1C	SD/CF card					8MB	512	4096	2046	** Partition 4
1D	SD/CF card					8MB	512	4096	2046	** Partition 5
1E	SD/CF card					8MB	512	4096	2046	** Partition 6
1F	SD/CF card					8MB	512	4096	2046	** 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
23	HD D-519	5.25"				15MB
23	HD D-526	5.25"				20MB
30	HD SA1002	8"				4MB
31	HD SA1004	8"				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	1024	249
41	Silicon					512KB	128	2048	252
42	Silicon					768KB	128	2048	380
43	Silicon					1MB	128	4096	254
44	Silicon					1.25MB	256	4096	318
45	Silicon					1.50MB	256	4096	382
46	Silicon					1.75MB	256	4096	446
47	Silicon					2MB	256	4096	510
48	Silicon					2.5MB	256	4096	638
49	Silicon					3MB	256	4096	766
4A	Silicon					3.5MB	256	4096	893
4B	Silicon					4MB	512	4096	1022
4C	Silicon					5MB	512	4096	1278
4D	Silicon					5MB	512	4096	1534
4E	Silicon					7MB	512	4096	1790
4F	Silicon					8MB	512	4096	2046
50	RAM					256KB	64	1024	249
51	RAM					512KB	128	2048	252
51	RAM					480KB	128	2048	236	* After S2R 2, or in Italy
51	RAM					144KB	128	2048	68	* After S2R 23, for MTX Series 2
52	RAM					320KB	64	2048	156	***

* = 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.

Note that some of the hard disk sizes are >8MB in size. Presumably the CP/M partition size was capped to 8MB. Perhaps a mechanism existed to tell CP/M to use a particular part of the total disk space. I have a vague memory of Ed Hollingshead mentioning something like this.

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.

If you create ROM Disc, this will typically appear as type 03 and 07.

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.