If I am not mistaken, if you allocate on an SMS-managed volume
and specify enough DCB attributes so that SMS knows that it is
intended to be a SAM data set, SMS will write an EOF at the
beginning of the first track and correspondingly set DS1LSTAR.
I seem to recall this as one of the "features" that SMS provided
when it first came out.
Art, I believe you are right, but I can never remember the exact rules
to get the EOF written. I searched the FMs once, but I don't think it
is documented.
but actually DS1LSTAR is still zero in that case. the EOF is record 1,
so the last "data block" is "record 0". You can't tell the difference
from LSTAR.
As for Gil's suggestion that the access method should not allow you to
read past LSTAR, in the past it was not uncommon for LSTAR to be
inaccurate. It is updated at CLOSE but if CLOSE fails or system crashes
or whatever, the DSCB may not get updated. There may be lots of valid
data past the LSTAR. Not so common today but can happen.
--
Bruce Black
Senior Software Developer
Innovation Data Processing
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