[zxspectrum] Re: Per chi ha il GOTEK
- From: Malantrucco Carlo <carlo.malantrucco@xxxxxxxxx>
- To: zxspectrum@xxxxxxxxxxxxx
- Date: Fri, 12 Jan 2018 16:44:24 +0100
Inviato da iPhone
Il giorno 12 gen 2018, alle ore 09:56, Enrico Maria Giordano
<e.m.giordano@xxxxxxxxxxxxxxx> ha scritto:
Default ('standard') format is 40 tracks, 18 sectors per track, 256 bytes
per sector, skew 5 and interleave 13.
QuickDOS default format is 40 tracks, 18 sectors per track, 256 bytes per
sector, skew 13 and interleave 1.
Non ricordo bene cosa significa skew e interleave. Probabilmente si poteva
fare anche nella ROM standard.
Anch’io avevo difficoltà a capire bene il significato di skew e interleave, poi
oggi ho trovato questo documento che lo spiega bene:
http://www.hermannseib.com/documents/floppy.pdf
Sectors may not be recorded ordered, but with a interleave and a skew.
Interleave means not to store sectors in directly ascending order. The neutral
interleave is called 0 or 1:1. Sectors would be ordered 1, 2, 3, 4, 5. The next
is interleave 1 or 2:1, with which the order would be 1, 4, 2, 5, 3. A higher
interleave gives the computer more time to process a sector before the next
sectors passes by. A wrong interleave factor may slow down accesses very bad,
up to the number of sectors per track. Skew means a skew between the sector
numbers of tracks which are next to each other. The neutral skew is 0, for
which the sectors of two neighbour tracks (note that for simplification a
neutral interleave is assumed further) would be ordered like 1, 2, 3, 4, 5 and
1, 2, 3, 4, 5. A skew of 1 would shift the sectors of the second track to be 5,
1, 2, 3, 4. A wrong skew can slow down accesses at most with the factor two,
because once the first sector has been found all further sectors can be read
without waiting. Skew and interleave can be combined. The best interleave and
skew factors are hardware dependent, so keep in mind that speeding up
sequential access e.g. for backups may lead to very slow restoring if you are
forced to restore files on a machine with a slower floppy.
Nelle floppy tables dell’Opus Discovery (SDC Newsletter n. 3) si legge:
SECTOR SKEW: The number of sectors between each consecutively numbered sector
on a track. An 18 sector track with a skew of 13 has the sectors in the order,
0,7,14,3,10,17,6,13,2,9,16,5,12,1,8,15,4,11.
Having read sector 0 the drive head will be positioned over sector 1 by the
time DOS is ready to read it. Useful for optimising access speed. Used by
format routine.
TRACK SKEW: The skew between consecutive tracks. Useful for optimising access
speed, though effect is minimal. Used by format routine.
Mi pare di capire che l’interleave sia il sector skew e lo skew sia il track
skew. Sbaglio?
Perché modificare questi valori acceleri o rallenti le operazioni di SAVE/LOAD
lo spiega bene Wikipedia:
https://en.m.wikipedia.org/wiki/Interleaving_(disk_storage)
The primary purpose of interleaving was to adjust the timing differences
between when the computer was ready to transfer data, and when that data was
actually arriving at the drive head to be read.
Interleaving was used to arrange the sectors in the most efficient manner
possible, so that after reading a sector, time would be permitted for
processing, and then the next sector in sequence is ready to be read just as
the computer is ready to do so. Matching the sector interleave to the
processing speed therefore accelerates the data transfer, but an incorrect
interleave can make the system perform markedly slower.
Carlo
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