Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Lynn Wheeler Newsgroups: comp.arch Subject: Re: What is an N-bit machine? Date: Fri, 29 Nov 2024 17:42:25 -1000 Organization: Wheeler&Wheeler Lines: 40 Message-ID: <871pytpeu6.fsf@localhost> References: MIME-Version: 1.0 Content-Type: text/plain Injection-Date: Sat, 30 Nov 2024 04:42:29 +0100 (CET) Injection-Info: dont-email.me; posting-host="b36869ee031d7ab86f526253a47debc6"; logging-data="1586998"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/QFBxcma6ghHHgiN4HFvkH2D3auZ7A6PA=" User-Agent: Gnus/5.13 (Gnus v5.13) Cancel-Lock: sha1:W6Z7EF1/q4V0Qsc66ITPf88RLWM= sha1:QCJg27/ruBQ7NvKHSp0Dt1R2YPQ= Bytes: 3173 Lawrence D'Oliveiro writes: > IBM had legendary market power, all the way up to monopoly status. > Whatever it decreed, its market had to follow. In the wake of the Future System mid-70s implosion, the head of POK (high-end mainframe) also convinced corporate to kill the (virtual machine) VM370 product, shutdown the development group and transfer all the people to POK to work on MVS/XA (i.e. a lot of XA/370 changes were to address various bloat&kludges in MVS/370). Come 80s, with 3081 and MVS/XA, customers weren't converting as planned, continuing to run 3081 370-mode with MVS/370. Amdahl was having more success, it had developed microcode hypervisor/virtual machine ("multiple domain") support and able to run both MVS/370 and MVS/XA concurrently on the same (Amdahl) machine (note Endicott did eventually obtain VM370 product responsibility for the mid-range 370s, but had to recreate a development group from scratch). Amdahl had another advantage, initially 3081 was two processor only and 3081D aggregate MIPS was less than the single processor Amdahl machine. IBM doubles the processor cache sizes for the 2-CPU 3081K, having about same aggregate MIPs as Amdahl single CPU .... however at the time, IBM MVS documents had MVS two-processor (multiprocessor overhead) support only getting 1.2-1.5 times the throughput of single processor (aka Amdahl single processor getting full MIPS throughput while MVS two processor 3081K loosing lots of throughput to multiprocessor overhead). POK had done a rudementary virtual software system for MVS/XA testing .... which eventually ships as VM/MA (migration aid) and then VM/SF (system facility) ... however since 370/XA had been primarily focused to compensate for MVS issues ... 3081 370/XA required a lot of microcode tweaks when running in virtual machine mode and 3081 didn't have the space ... so switching in and out of VM/MA or VM/SF with virtual machine mode, had a lot of overhead "paging" microcode. It was almost a decade later before IBM was able to respond to Amdahl's hypervisor/multiple-domain with 3090 LPAR & PR/SM support. -- virtualization experience starting Jan1968, online at home since Mar1970