New wiki page: Emulation related file formats

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Por TomH

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03-01-2018, 22:37

To be more explicit about that point:

When writing with single density encoding, a fixed-rate clock pushes a 250 kbps bit stream. That's clock bits and data. It's 125kbps of data.

Standard floppy disk rotation speed is 300RPM. That's 5 rotations per second.

Therefore 125/5 = 25kbps of data can be written in one rotation. So 25kps is one track.

25kpbs / 8 gives 3,125 bytes of data per track, unformatted capacity. Which is an often cited figure.

Even if there weren't a need for IDs and marks and inter-sector gaps and CRCs, you'll notice that 3,125 bytes is not enough space to fit nine 512 byte sectors, as 9*512 = 4,608 bytes.

I count 58 byte sizes of space of overhead per sector with the recommended gap sizes*. So at 512 bytes/sector that's five sectors at the absolute most.

You'd need to spin the disk more slowly and produce a higher-density disk surface to fit more than that on a track from a standard single density encoder. Nine sectors at 512 bytes/sector doesn't fit in single density.

(* pre-address bytes: 6 bytes; address: 7 bytes, being one byte for the address mark, four for the location, two for the CRC; post-address bytes: 11; pre-data bytes: 6; data mark: 1 byte; [data here]; CRC: 2 bytes; post-data bytes: 27 — data and address marks in FM are completely distinct, unlike in MFM where each is a sync mark then a byte to explain the sync, so each occupies only a single byte of space)

Por Manuel

Ascended (18254)

imagem de Manuel

03-01-2018, 23:17

Isn't single density (FM) 256 bytes per sector?

An SVI disk image is even more interesting, quoting from svi2dmk:

Quote:

The SVI image is expected to have to following format:
- 1st track (track 0, side 0) contains 18 sectors of 128 bytes.
- All other tracks contain 17 sectors of 256 bytes.
- There are 40 tracks on the disk.
- The disk can be either single or double sided.
This means that the size of the input image should be exactly
172032 bytes for single sided disks or 346112 bytes for double
sided disks.

Por NYYRIKKI

Enlighted (5889)

imagem de NYYRIKKI

04-01-2018, 00:09

Manuel wrote:

Isn't single density (FM) 256 bytes per sector?

An SVI disk image is even more interesting, quoting from svi2dmk:

Quote:

The SVI image is expected to have to following format:
- 1st track (track 0, side 0) contains 18 sectors of 128 bytes.
- All other tracks contain 17 sectors of 256 bytes.
- There are 40 tracks on the disk.
- The disk can be either single or double sided.
This means that the size of the input image should be exactly
172032 bytes for single sided disks or 346112 bytes for double
sided disks.

No, the sector size does not have anything to do with density... On SVI actually only track 0 on side 0 is single density (18*128 bytes in FM)... Rest of the disk is formatted to double density (17*256 bytes in MFM) The disk can be either 40 track or 80 track and it can have 1 or 2 sides. (Not sure though if 80 track 2 side disks were supported by any version of SV-BASIC or CP/M)

Por ricbit

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04-01-2018, 05:26

TomH wrote:

You'd need to spin the disk more slowly and produce a higher-density disk surface to fit more than that on a track from a standard single density encoder. Nine sectors at 512 bytes/sector doesn't fit in single density.

Well when I was younger I used to have a "dual density" 5 1/4" drive. It had a switch in front where I could select 360kb disks (40/2), or 720kb disks (80/2). At the time I didn't know what exactly the switch did on the hardware side, but from your description it might be changing the rotation speed.

Por ricbit

Champion (438)

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04-01-2018, 05:34

I just remembered about a program called HELLO which could measure the drive rotation. It would display 300rpm in both densities, so if this program was correct disk speed didn't change.

I wonder if "single density" in the msx world actually relates to density between tracks, not between bytes (so it would not be a matter of modulation, but instead of moving the drive head with more precision). I really don't know about the hardware implementation and I'm curious if anyone has a more definitive answer.

Por NYYRIKKI

Enlighted (5889)

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04-01-2018, 16:35

ricbit wrote:

I wonder if "single density" in the msx world actually relates to density between tracks, not between bytes (so it would not be a matter of modulation, but instead of moving the drive head with more precision). I really don't know about the hardware implementation and I'm curious if anyone has a more definitive answer.

Yes, I think these understanding problems partly originate from mixed up SS/DS and SD/DD acronyms. (Single Side / Double side and Single Density / Double Density) also "Density" is a bit bad word as it can either mean bit density (that the SD/DD originally means) or track density (40/80 tracks). In coordinate system you might think the bit density as X-density and track density as Y-density.

AFAIK any MSX can't handle SD as that requires compatible chip (FM capable), selectable custom clock for the chip, SD compatible drive as well as signal between chip and drive to indicate correct density. MSX can't handle HD or ED disks (High Density / Extra-high Density) either. These formats also use 80-track disks. How ever MSX can use 40 or 80 track disks as these differences are mechanical and do not really need any features from the chipset it self.

Por TomH

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04-01-2018, 20:13

Right, I've probably been dense again here. The WD177x, which is the one with that instruction set that seems to have been the last one standing since it's in the Atari ST and Acorn Archimedes, has an input that selects between FM and MFM operation, and it was common to provide a programmatic way to set that. But it's not part of the command set or anything. So an easier way to proceed probably would have been to consult the technical documentation on whatever the standard disk ROMs provide as additional control. But I didn't really think to inspect that side of things, as I sat here pondering only the surface of the disk.

Sorry for the waste of everybody's time. I removed the FAT mentions from the wiki page earlier in the conversation, I'll elaborate there on the selection otherwise being 40/80 track single/double sided, with single-sided 80 track and double-sided 40 track being ambiguous.

Por Manuel

Ascended (18254)

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05-01-2018, 00:16

So, NYYRIKKI/RicBit: what do you think the National CF-3300 diskROM was targeting with these 4 options?

Por NYYRIKKI

Enlighted (5889)

imagem de NYYRIKKI

05-01-2018, 01:50

Manuel wrote:

So, NYYRIKKI/RicBit: what do you think the National CF-3300 diskROM was targeting with these 4 options?

I think the options were:
1 - SS / 40-track
2 - DS / 40-track
3 - SS / 80-track
4 - DS / 80-track

SD/DD/HD/ED all define the bit density, not the track density... On MSX it is always DD.

Por ricbit

Champion (438)

imagem de ricbit

05-01-2018, 20:05

Agreed, those were the options on the brazilian microsol/ddx interfaces as well.

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