fluxengine/doc/disk-ibm.md

Disk: Generic IBM

IBM scheme disks are the most common disk format, ever. They're used by a huge variety of different systems, and they come in a huge variety of different forms, but they're all fundamentally the same: either FM or MFM, either single or double sided, with distinct sector header and data records and no sector metadata. Systems which use IBM scheme disks include but are not limited to:

• IBM PCs (naturally)
• Atari ST
• late era Apple machines
• Acorn machines
• the TRS-80
• late era Commodore machines (the 1571 and so on)
• most CP/M machines
• etc

FluxEngine supports reading these. However, some variants are more peculiar than others, and as a result there are specific decoders which set the defaults correctly for certain formats (for example: on PC disks the sector numbers start from 1, but on Acorn disks they start from 0). The IBM decoder described here is the generic one, and is suited for 'conventional' PC disks. While you can read all the variant formats with it if you use the right set of arguments, it's easier to use the specific decoder.

The generic decoder is mostly self-configuring, and will detect the format of your disk for you.

Reading disks

Just do:

fluxengine read ibm

...and you'll end up with an ibm.img file. This should work on most PC disks (including FM 360kB disks, 3.5" 1440kB disks, 5.25" 1200kB disks, etc.) The size of the disk image will vary depending on the format.

Configuration options you'll want include:

• --ibm-sector-id-base=N: specifies the ID of the first sector; this defaults to 1. Some formats (like the Acorn ones) start at 0. This can't be autodetected because FluxEngine can't distinguish between a disk which starts at sector 1 and a disk which starts at sector 0 but all the sector 0s are missing.

• --ibm-ignore-side-byte=true|false: each sector header describes the location of the sector: sector ID, track and side. Some formats use the wrong side ID, so the sectors on side 1 are labelled as belonging to side 0. This causes FluxEngine to see duplicate sectors (as it can't distinguish between the two sides). This option tells FluxEngine to ignore the side byte completely and use the physical side instead.

• --ibm-required-sectors=range: if you know how many sectors to expect per track, you can improve reads by telling FluxEngine what to expect here. If a track is read and a sector on this list is not present, then FluxEngine assumes the read failed and will retry. This avoids the situation where FluxEngine can't tell the difference between a sector missing because it's bad or a sector missing because it was never written in the first place. If sectors are seen outside the range here, it will still be read. You can use the same syntax as for track specifiers: e.g. 0-9, 0,1,2,3, etc.

Writing disks

FluxEngine can also write IBM scheme disks. Unfortunately the format is incredibly flexible and you need to specify every single parameter, which makes things slightly awkward.

The syntax is:

fluxengine write ibm -i input.img <options>

The format of input.img will vary depending on the kind of disk you're writing, which is configured by the options. There are some presets, which you will almost certainly want to use if possible:

• --ibm-preset-720: a standard 720kB DS DD 3.5" disk, with 80 cylinders, 2 sides, and 9 sectors per track.
• --ibm-preset-1440: a standard 1440kB DS HD 3.5" disk, with 80 cylinders, 2 sides, and 18 sectors per track.

These options simply preset the following, lower-level options. Note that options are processed left to right, so it's possible to use a preset and then change some settings. To see the values for a preset, simply append --help.

• --ibm-track-length-ms=N: one disk rotation, in milliseconds. This is used to determine whether all the data will fit on a track or not. fluxengine rpm will tell you this; it'll be 200 for a normal 3.5" drive and 166 for a normal 5.25" drive.
• --ibm-sector-size=N: the size of a sector, in bytes. Must be a power of two.
• --ibm-emit-iam=true|false: whether to emit the IAM record at the top of the track. The standard format requires it, but it's ignored by absolutely everyone and you can fit a bit more data on the disk without it.
• --ibm-start-sector-id=N: the sector ID of the first sector. Normally 1, except for non-standard formats like Acorn's, which use 0.
• --ibm-use-fm=true|false: uses FM rather than MFM.
• --ibm-idam-byte=N: the sixteen-bit raw bit pattern used for the IDAM ID byte. Big-endian, clock bit first.
• --ibm-dam-byte-N: the sixteen-bit raw bit pattern used for the DAM ID byte. Big-endian, clock bit first.
• --ibm-gap0-bytes=N: the size of gap 0 in bytes (between the start of the track and the IAM record).
• --ibm-gap1-bytes=N: the size of gap 1 in bytes (between the IAM record and the first sector record).
• --ibm-gap2-bytes=N: the size of gap 2 in bytes (between each sector record and the data record).
• --ibm-gap3-bytes=N: the size of gap 3 in bytes (between the data record and the next sector record).
• --ibm-sector-skew=0123...: a string representing the order in which to write sectors: each character represents on sector, with 0 being the first (always, regardless of --ibm-start-sector-id above). Sectors 10 and above are represented as letters from A up.

Mixed-format disks

Some disks, usually those belonging to early CP/M machines, have more than one format on the disk at once. Typically, the first few tracks will be low-density FM encoded and will be read by the machine's ROM; those tracks contain new floppy drive handling code capable of coping with MFM data, and so the rest of the disk will use that, allowing them to store more data.

FluxEngine copes with these fine, but the disk images are a bit weird. If track 0 is FM and contains five sectors, but track 1 is MFM with nine sectors (MFM is more efficient and the sectors are physically smaller, allowing you to get more on), then the resulting image will have nine sectors per track... but track 0 will only contain data in the first five.

This is typically what you want as it makes locating the sectors in the image easier, but emulators will typically require a different format. Please get in touch if you have specific requirements (nothing's come up yet). Alternatively, you can tell FluxEngine to write a .ldbs file and then use libdsk to convert it to something useful.

One easy option when reading these is to simply read the two sections of the disk into two different image files.

FluxEngine can write these too, but in two different passes with different options. It's possible to assemble a flux file by judicious use of -D something.flux --merge, which can then be written in a single pass with fluxengine writeflux, but it's usually not worth the bother: just write the boot tracks, then write the data tracks, possibly with a script for automation.