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Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Lynn Wheeler <lynn@garlic.com> Newsgroups: comp.arch Subject: Re: Architectural implications of locate mode I/O Date: Mon, 08 Jul 2024 08:01:22 -1000 Organization: Wheeler&Wheeler Lines: 56 Message-ID: <87wmlvhifh.fsf@localhost> References: <v61jeh$k6d$1@gal.iecc.com> <1bed88y8na.fsf@pfeifferfamily.net> <v69jgf$1lg1$1@gal.iecc.com> <87frsnwbd5.fsf@localhost> <v6g5b0$phk7$1@dont-email.me> MIME-Version: 1.0 Content-Type: text/plain Injection-Date: Mon, 08 Jul 2024 20:01:24 +0200 (CEST) Injection-Info: dont-email.me; posting-host="74b0132dd1e204cda76b6de035d21fd0"; logging-data="1034919"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18610g/wXvHEhnNenUwBEDvUXBBo8sx3SA=" User-Agent: Gnus/5.13 (Gnus v5.13) Cancel-Lock: sha1:0hl3IPKXPC4kDSkmkRc2hUVcG8g= sha1:IJY6JVYZlGU04I450GhKhwuQl6w= Bytes: 3288 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