See page 10 of the 3390 Reference Summary, manual GX26-4577, which can
be found online.
The physical records per track calculation is complex because the
physical space (or emulated space) is divided into 1729 cells of 34
bytes each (the source of the 58,786 value), allocation is in by cells,
is separately allocated for Count, Key and Data (CKD) portions of each
physical record, and includes cells between records and the parts of
records. The physical space taken by the entire record depends on the
Key Length KL (possibly 0) and the Record Length (RL).
The number of cells required for a physical records is given by C + K +
D, where
C= 10
If KL =0, then K = 0; otherwise K = 9 + ceil( (KL + (6 x ceil( (KL +
6)/232 ) + 6) / 34 )
D = 9 + ceil( (DL + (6 x ceil( (DL + 6)/232 ) + 6) / 34 )
where ceil ( x) rounds x upward to an integer value if x is not an integer.
The 1729 max cells already excludes space for a standard R0 record,
which appears to occupy 20 additional cells on each track.
Maximum usable bytes per track occurs when a single record is written
per track with no Key field. The above formula limits the max size of
that record to 56,664 bytes, which is the origin of that value.
Standard I/O places a limitation of 32 KiB on the size of a record,
which means the maximum usable capacity with those access methods is 2
physical records per track. The above formulas limit the max record
size for 2 records per track to 27,998 or 55,996 usable bytes per
track. That's where your third value comes from.
Joel C. Ewing
On 11/23/22 07:42, John Gateley wrote:
Hello
On all the disk volumes I have looked at, the format 4 DSCB field DS4DEVTK
(device track length) has the value 58786.
All the IBM documentation says that there are 56664 bytes in a track on a 3390
drive.
At this link https://www.lascon.co.uk/hwd-3390-disks.php reference is made to
55996 usable bytes out of 56664 per track.
Is there some 'magic' formula that converts the value in DS4DEVTK to either
56664 or 55996, or should I just ignore DS4DEVTK?
Thanks
John
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