New wiki page: Emulation related file formats

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Por Grauw

Ascended (10174)

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02-01-2018, 23:57

Off-topic: there isn’t a time limit on edits but I think once someone presses the quote button (or reply?), the editing is locked.

On-topic: if file size of empty sectors is a concern, there’s always gzip Smile.

Por TomH

Champion (327)

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03-01-2018, 14:52

Grauw wrote:

On-topic: if file size of empty sectors is a concern, there’s always gzip Smile.

Alas there is not enough MSX software in the world to make storage of it a problem. Must correct.

Re: cartridges and taking guesses at their hardware, has anybody ever proposed a file format that just tells you the answer? Or is dump-of-the-ROM all there's ever been? I notice that a few here and there aren't multiples of 8kb in size, but in every case it just looks like a minor dumping error.

Por TomH

Champion (327)

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03-01-2018, 15:04

Manuel wrote:

The size tells a lot, but not all.
Size of 720kB must be a double sided double density disk. But a 360kB image can be either double sided single density or single sided double density. You can't tell. A 180kB image is probably single sided, single density. To make things simpler, on MSX single density is not used, so that helps Smile
Then there's also special things like extra sectors or extra tracks. An extra track (e.g. 81 tracks) fits in the format, but it does make the total size different from 360kB or 720kB.

Delayed follow-up on this: surely a single-density disk encoded as DSK fails to make sense because a track is no longer nine 512kb sectors? You might squeeze five, especially with a slow-end-of-tolerance drive and/or tiny inter-sector gaps, but if you were to store that in a DSK it seems like it's undefined whether you'd store shorter tracks, or should use only four or five sectors out of nine, or possibly something else?

The only single density drive I can find any evidence of Microsoft ever having supported anywhere is 8" floppies in that other OS, MS-DOS, on platforms other than the IBM PC. Where 21 128kb sectors were deployed.

Por Manuel

Ascended (18254)

imagem de Manuel

03-01-2018, 15:53

Single density is FM encoded instead of MFM. I think it's only 40 tracks. Old disk ROMs offer you to format them as such. E.g. the National CF-3300:

Por TomH

Champion (327)

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03-01-2018, 17:48

I'm aware of what single density is — it's a clock bit between every data bit rather than, in double density, one only if neither neighbouring data bit is already set. So in the worst case it's twice as many flux transitions. So you write it at half the clock rate, because proximity of flux transitions is the bottleneck on magnetic media. Which halves the density. (Also: different encoding of syncs, and four different sync patterns rather than two, but that's getting into the weeds)

The question was, essentially: if a single density disk were encoded in DSK format, how many sectors and what sector size would you expect per track? If I'm an emulator or a disk-writing tool and I want to read the first complete track of information from the DSK file, how much am I reading, and how am I dividing it?

The screenshot doesn't quite answer the question, sadly, as "double track" is descriptive of the amount of bytes you can fit, but tells me nothing about the idiomatic decision over sector sizing and quantity.

(EDIT: and, hopefully obviously, I meant 128-byte sectors in the previous post. Sector size of 0 in IDAM terms. 21 sectors at 128kb would be quite something)

Por ricbit

Champion (438)

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03-01-2018, 19:09

Manuel wrote:

About the mapper stuff: openMSX was actually the first to use a checksum-based method (like a database) to look up mapper types.

Actually BrMSX had it since Sep 13, 1998.

Por ricbit

Champion (438)

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03-01-2018, 19:15

TomH wrote:

The question was, essentially: if a single density disk were encoded in DSK format, how many sectors and what sector size would you expect per track? If I'm an emulator or a disk-writing tool and I want to read the first complete track of information from the DSK file, how much am I reading, and how am I dividing it?

I have some DSK which are single density (40 tracks, 9 sectors of 512 bytes / track). If you have a .dsk which is 180kb, this is most likely the format used.

Por ricbit

Champion (438)

imagem de ricbit

03-01-2018, 19:19

Out of curiosity, here in Brazil most "single density" drives are 5 1/4", and most "double density" drives are 3 1/2", the difference between them from a DSK point of view is the number of tracks (40 vs 80). It gets ambiguous for 80 tracks/1 side or 40 tracks/2 sides, as both are 360kb. But, at least in brazilian dumps, most of time 360 means 40/2.

Por tvalenca

Paladin (747)

imagem de tvalenca

03-01-2018, 20:08

ricbit wrote:

Out of curiosity, here in Brazil most "single density" drives are 5 1/4", and most "double density" drives are 3 1/2", the difference between them from a DSK point of view is the number of tracks (40 vs 80). It gets ambiguous for 80 tracks/1 side or 40 tracks/2 sides, as both are 360kb. But, at least in brazilian dumps, most of time 360 means 40/2.

This takes me back to 1990's when I got several 3.5" disks from my uncle and a few of them were single-sided... it took quite long to figure out why some software (namely a utility named "ZZELEXEC.BIN") worked on these disks but didn't worked on the others...

It happened because that particular program had the FAT address hardcoded and both 3.5" Single sided and 5.25" Double sided disks had FAT on the same track number...

Por TomH

Champion (327)

imagem de TomH

03-01-2018, 20:15

Oh, right. Smaller density, bigger disk. That was the missing part of the puzzle! Obviously 9 sectors at 512 bytes wouldn't fit in single density on a 3.5". But doesn't that imply something crazy like a 150RPM drive?

It seems strange to me that anybody with an MSX would use single-density format though: I guess disk interfaces with a different controller? It looks like if you went all the way back to the WD FD1781 you could get a compatible command set but possibly provide only plain FM in the external modulator? Or perhaps I'm assuming too much: maybe somebody had a stockpile of old 8271s and just wrote their own disk ROM?

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