At 06:54 -0400 on 10/18/2005, Bill Fairchild wrote about Re: MVCIN instruction:
In a message dated 10/18/2005 12:22:15 A.M. Central Daylight Time,
[EMAIL PROTECTED] writes:
It was fun to wrote the code to allow the DASD records to be read
in reverse order(IOW: If there were 5 records on the current track,
read them as 5, 4, 3, 2, 1 before going to the prior track).
HASP accessed its SPOOL data this way until users began to complain
that they had no program that would back up/restore SPOOl volumes to
tape. Then the HASP team made this record alternation an option. The
thought was that accessing every other record in sequence would
provide a little boost in performance. The same technique was (and
maybe still is) used in CMS files. HASP's record alternation option
was removed when HASP was replaced with JES2.
You are confusing two separate schemes. In my case the data was
written in standard order. Since I was using BDAM (it might have been
BSAM where I was supplying the record number I wanted) I just
REQUESTED the records in reverse order (after I used the EXCP CCW
string to find the last record on the current track).
HASP WROTE the records in the order (and numbered as in the Count
Section) 1 4 2 5 3 6 (assuming 6 blocks per track). This allowed it
to read the full track in two revolutions of the drive. If it had
tried to read a normally numbered track the channel was too slow to
catch the next record on that revolution. The
out-of-sequence-interleaved numbering allowed the channel to catch up
and be ready for the next block before it passed the read head. By
JES2 days the channels were fast enough to keep up with the DASD (as
well as there being cached buffering and read-track commands/etc.).
The trick was to read them using BDAM (TTR) after using an EXCP
that issued a CCW Loop that read the CKD Record IDs until it ran
off end of the track thus determining how many records were on the
current track.
This was before CCWs like read track or read multiple CKD were implemented.
It's much easier now to find out what's on a track with only one CCW.
True. This was in 1974-76 (if I remember my work history right).
Bill Fairchild
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO
Search the archives at http://bama.ua.edu/archives/ibm-main.html
