Re: Architectural implications of locate mode I/O

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

Did IMS have a locate mode as well?

channel programs were built by filesystem library running as part of
application or directly by applicatiion code .... and then executes
system call, EXCP/SVC0 to invoke the channel program. With MVS and
virtual memory its in application virtual address space.

QSAM the library code data is to/from library buffers and then
either copies to application buffers or "locate" mode passing
pointers in QSAM buffers.

For IMS has data buffer cache directly managed (aware of whether data record
is aleady in cache or must be read ... and/or is changed in cache and
must be written) ... also transaction log)

With transition to virtual memory, the channel programs passed to
EXCP/SVC0 now had virtual addresses and channel architecture required
real addresses ... so EXCP/SVC0 required making a copy of the passed
channel programs replacing virtual addresses with real addresses (as
well as pinning the associated virtual pages until I/O completes, code
to create channel program copies with real addresses and managing
virtual page pin/unpin initially done copy crafting virtual machine CP67
"CCWTRANS" into EXCP).

for priviliged apps that had fixed/pinned virtual pages for I/O buffers,
a new EXCPVR interface was built ... effectively the original EXCP w/o
(CCWTRANS) channel program copying (and virtual page pinning/unpinning).

IMS "OSAM" and "VSAM" (OSAM may use QSAM
https://www.ibm.com/docs/en/ims/15.3.0?topic=sets-using-osam-as-access-method
IMS communicates with OSAM using OPEN, CLOSE, READ, and WRITE macros. In
turn, OSAM communicates with the I/O supervisor by using the I/O driver
interface.

Data sets

An OSAM data set can be read by using either the BSAM or QSAM access method.

... snip ...

IMS Performance and Tuning guide page167
https://www.redbooks.ibm.com/redbooks/pdfs/sg247324.pdf
* EXCPVR=0
Prevents page fixing of the OSAM buffer pool. This is the correct choice
these days

... snip ..

START_Input/Output
https://en.wikipedia.org/wiki/Start_Input/Output
EXCPVR
https://en.wikipedia.org/wiki/Execute_Channel_Program_in_Real_Storage

--
virtualization experience starting Jan1968, online at home since Mar1970