IDEDOS - the access to harddisks
The IDEDOS means extra facilities of the +3e operating system, written by Garry Lancaster. For more details about +3e please consult his page at http://www.zxplus3e.plus.com/.
In ZXVGS and CPM22QED the IDEDOS partition table (also called IDEDOS system partition) is implemented.
Why IDEDOS partitionning is the best for 8bit computers?
- Is simple but not so primitive like MS-DOS one and less complicated than Amiga OS one.
- Informations about all partitions is in one place, not spread over the disk like MS-DOS one.
- Floppy diskimages (TR-DOS, MB-02, +D, Sam) can be easy defined, also with information of its (virtual) phisical structure.
- Each partition must have a name, up to 16 charaters long.
- Extra parameters can be assigned to partition. For example, CP/M partitions (also CP/M floppy diskimages) can have XDPB table.
- System files can be stored as partitions. Wide range of computers can be booted from one disk, as every hardware uses different name of boot file.
- For big files, e.g. movies made form series of screenshoots (see BMP2SCR) unfagmented areas (partitions) can be allocated.
- Up to 65535 partitions can be defined.
- Other partition types can be added in future.
- CHS adressing is suitable for older disks.
Technical details
Each partition definition takes 64 bytes. The first entry defines system partition. Partitions are defined in whole tracks - from cylinder/head to cylinder/head. There's no support for LBA - default geometry is always used.
For the diskimage type partitions (type #30..#3F), virtual geometry is defined. Area allocated for diskimage can be rounded up to track boundary or the last track can be shared with next diskimage partrion. When partitions overlap on same track, the latter one must have sector shift defined. For example for 980/5/17 disk, a 800kB partition takes 1600 sectors = 18 cylinders + 4 tracks + 2 sectors, so the shift for next partition is 2, otherwise the last 15 sectors can be unused.
The 64 bytes partition entry
| Offset | Length | Description |
|---|
| +0 | 16 | Partition name (case-insensitive, space-padded). |
| +16 | 16 | Partition definition. |
| +16 | 1 | Partition type (0=free handle) |
| +17 | 2 | Starting cylinder. |
| +19 | 1 | Starting head. |
| +20 | 2 | Ending cylinder. |
| +22 | 1 | Ending head. |
| +23 | 4 | Largest logical sector number. |
| +27 | 5 | Type-specific information.
|
| +27 | 1 | Shift from sector 1 of partition start (0 for no shift). |
| +28 | 1 | Virtual sector size (0-128B, 1-256B, 2-512B, 3-1kB). |
| +29 | 1 | Virtual first sector number (0..240). |
| +30 | 1 | Virtual heads (1, 2, more?). |
| +31 | 1 | Virtual sectors (usually 5, 8, 9, 16, 18). |
| +32 | 32 | Type-specific information. For example XDPB, movie format. |
Partition types
Partition
type
number | Description |
|---|
| #00 | Unused partition/free handle. |
| #01 | System partition. The first partition on a disk, starting at phisical sector 1 (cylinder 0, head 0 or 1), is always the system partition and contains a list of 64-byte partition entries that define all the partitions on the disk (including the system one). Only one partition of this type is permitted on a disk, and this is always the first partition. The name is always "PLUSIDEDOS" (followed by 6 spaces). |
| #02 | Swap partition. |
| #03 | +3DOS partition. The maximum theoretical size for a +3DOS partition is just under 32MB. The XDPB has logical geometry. |
| #04 | CP/M partition with XDPB that reflects phisical disk structure. So if the disk has 17 sectors per track, the LSPT is 68. The partition uses always integer number of cylinders and uses whole cylinder (from head 0). Otherwise (when from not track 0) this is converted to reserved tracks (OFF in XDPB). This is required for DSKHNDLR low level disk drivers. |
| #05 | Boot partition. This is only one file, stored as a partition. Used to boot a hardware. Eg. Timex FDD 3000, extedend with YABUS.TF, will search the IDEDOS partiton table to find "YABUS.TF" partition. If found, the partition contents is loaded into RAM and started. The partition size is usually 8k to 64kB, what gives 1..2 tracks (or 1..8 track for disks with 17 spt). The number of sectors to load is in partition definition. |
| #0F | Movie partition. A sequence of screenshoots and optional sound track. |
| #10 | MS-DOS (FAT16) partition. |
| #20 | UZI(X) partition. |
| #3x | Non-CP/M diskimages, ZX Spectrum. |
| #30 | TR-DOS diskimage. Usual geometry 80/2/16, 640kB. |
| #31 | +D/SAMDOS diskimage (B-DOS record)*. Geometry 80/2/10, 800kB. |
| #32 | MB-02 diskimage. Usual goeometry 82/2/11, 1804kB. | | #FE | Bad disk space. |
| #33 | TOS A.2 diskimage. Usual goeometry 40/1/16, 160kB or 80/2/16, 640kB. |
| #4x | CP/M diskimages. Partition type describes computer that uses CP/M format for non-CP/M purposes. |
| #40 | ZX Spectrum +3 diskimages. |
| #41 | Elwro 800 Junior diskimages. |
| #48 | Amstrad CPC diskimages. |
| #49 | Amstrad PCW diskimages. |
| #FE | Bad disk space. |
| #FF | Free disk space. |
*Caution! SAMDOS diskimage can have swapped L/H bytes due to compatibility with Atom interface.
Searching for IDEDOS structures:
- Read the sector 1 on cylinder 0 and head 0.
- If sector length is 512B (full IDE), go to 5.
- (Simple IDE - sector length is 256B.) If PLUSIDEDOS signature found, go to 10.
- Don't mount the disk at all.
- If PLUSIDEDOS signature found, go to 10.
- If P?L?U?S?I?D?E?D?O?S signature found, switch disk access to 8-bit and go to 10.
- Read the sector 1 on cylinder 0 and head 1.
- If PLUSIDEDOS signature found, go to 10.
- Don't mount the disk. (Or try to analyse MS-DOS partition tables.)
- Analyse IDEDOS partition table.
MBR - MS-DOS partition table
It is recomended to place IDEDOS partition as first on the disk. If you would like to have also a FAT (FAT-16) partion, place it as second. To prevent overwriting, define the IDEDOS partition as CP/M one (type #DB in MS-DOS partition table).
Example partitions data
The disk is Seagate ST351A/X and its structure is 980/5/17 (not the reported in manual 820/6/17).
First track is reserved to allow MBR and MS-DOS partitions table in 0/0/1 sector. The MS-DOS (and other) partitions can start at cylinder 198 as IDEDOS structure ends at cylinder 197 inclusive. IDEDOS structure could also take whole disk and contain MS-DOS partition inside.
IDEDOS structure starts at sector 0/1/1, so 1 * 17 * 512 = 8704 = #2200 bytes are skpipped.
02200 50 4C 55 53 49 44 45 44 4F 53 20 20 20 20 20 20 PLUSIDEDOS
02210 01 00 00 01 00 00 01 10 00 00 00 00 00 00 00 00 ................
02220 C6 00 05 11 55 00 87 00 02 00 00 00 00 00 00 00 O...U...........
02230 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
IDEDOS system partition (type #01 - area that describes other partitions).
Starts at cylinder 0 head 1 and ends at the same one. Partition size is 17 sectors (#00000010). At #02220 disk parameters are defined: 198 cylinders (#00C6, two bytes), 5 heads (one byte), 17 sectors per track (#11, one byte), sectors per cylinder is calculated to 85 (#0055, two bytes). Up to 136 partitions can be defined (#0087 is last number = 1 * 17 * 512 / 64 - 1, two bytes). Area from cylinder 198 and behind is reserved for other systems (e.g. MS-DOS) and not available as IDEDOS partitions.
02240 5A 58 56 47 53 20 20 20 20 20 20 20 20 20 20 20 ZXVGS
02250 04 02 00 00 C5 00 04 13 41 00 00 00 00 00 00 00 ....L...A.......
02260 44 00 04 0F 00 44 10 FF 03 FF FF 00 00 00 00 02 D....D.˙.˙˙.....
02270 03 04 C4 11 01 00 02 00 00 A1 FF 00 00 00 00 00 ..A.......˙.....
Main CP/M partition (type #04). Starts at cylinder 2 head 0 and ends at cylinder 197 (#00C5) head 4 inclusive. Partition has 16660 sectors (#00004113). At #02260 XDPB is provided (#A1 at #02279 is the unit code for HDD partition).
02280 50 4C 33 4D 45 4D 2E 53 59 53 20 20 20 20 20 20 PL3MEM.SYS
02290 05 01 00 03 01 00 04 1F 00 00 00 00 00 00 00 00 ................
022A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
022B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
Boot file (type #05) dedicated for the PL3MEM interface. Starts at cylinder 1 head 3 and ends at cylinder 1 head 4. File size is 32 sectors (#0000001F) - last two are unused.
022C0 46 52 45 45 20 20 20 20 20 20 20 20 20 20 20 20 FREE
022D0 FF 00 00 02 01 00 02 65 00 00 00 00 00 00 00 00 .......e........
022E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
022F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
Some free space (type #FF). Starts at cylinder 0 head 2 and ends at cylinder 1 head 2 inclusive. Has 102 sectors (#00000065). Can be used later to place more boot files.
Links to other pages
| | CP/M
CPC
CPM22QED
Filesystem
MGT
Timex
ZX Spectrum
ZXVGS
|