In MVT days (i.e. pre-DAT days) Type-3 and Type-4 SVCs (and I think
certain machine check error handling routines, but I'm not sure) were
loaded into a fixed set of 1K buffers. As interrupts would occur, the
SVC code (if any) being used by the interrupted task would be
overlayed in the buffers by SVC code (if any) being used by the
interrupting task. I.e. the code would be "refreshed".
Further, code that had been running in one buffer might be loaded
into a different buffer. So the code needed to be free of location
dependencies (which is different from being self relocating).
So the SVC modules would require the REFR attribute.
It used to be that reentrant code could be self-modifying as long as
it did so only temporarily while the hardware was disabled for
interrupts. Refreshable code was not permitted to be self-modifying
no matter what the circumstances.
Now of course, RENT code is not permitted to be self-modifying under
any circumstances, so the distinction between RENT and REFR has
largely (but not entirely) disappeared.
Asynchronous relocatability is might still be considered a
distinction, but I don't know of any circumstance where the system
performs or supports that.
Nevertheless, these days whenever the Binder sets the RENT attribute,
it also sets REFR automatically.
Why IBM keeps REFR around anymore, I'm not sure.
Dave
REFR as an attribute, does this also imply that the code is
self-relocating so that if the code is reloaded at a different place, it
is still able to continue execution (of the task in flight)? So PRE DAT
days, this would be an overlay type program, right?
But in DAT days where everything is V=V, just what is the purpose of
REFR? What does it really accomplish over RENT
I ask this because in reading the various postings, and IBM's stuff, I'm
getting a little lost.
Later,
Steve Thompson
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