PC-based IBM mainframe-compatible systems: Difference between revisions

Revision as of 14:04, 28 March 2020

Revision as of 08:52, 28 April 2020

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In October 1983, IBM announced the IBM Personal Computer XT/370. This was essentially a three-in-one product. It could run [[PC DOS]] locally, it could also act as [[3270]] terminal, and finally—its most important distinguishing feature relative to an [[IBM 3270 PC]]—was that it could execute S/370 instructions locally.{{r|byte1984fall}}

The XT/370 was an [[IBM Personal Computer XT]] (System Unit 5160) with three custom 8-bit cards. The processor card (370PC-P),<ref>{{cite web |url=http://priorartdatabase.com/IPCOM/000059679# |title=Implementation of IBM System 370 Via Co-Microprocessors/The Co-Processor... - IPCOM000059679D - IP.com |publisher=Priorartdatabase.com |date= |accessdate=2012-06-07 }}{{Dead link|date=April 2020 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> contained two modified [[Motorola_68000_series|Motorola 68000]] chips (which could emulate most S/370 fixed-point instructions and non-floating-point instructions), and an [[Intel 8087|Intel 8087 coprocessor ]] modified to emulate the S/370 floating point instructions. The second card (370PC-M), which connected to the first with a unique card back connector contained 512 [[Kibibyte|KiB]] of memory. The third card (PC3277-EM), was a 3270 terminal emulator required to download system software from the host mainframe. The XT/370 computer booted into DOS, then ran the [[VM (operating system)|VM]]/PC Control Program. The card's memory space added additional system memory, so the first 256 KiB ([[motherboard]]) memory could be used to move data to the 512 KiB expansion card. The expansion memory was dual ported, and provided an additional 384 KiB to the XT Machine bringing the total RAM on the XT side to 640 KiB. The memory arbitrator could bank switch the second 128 KiB bank on the card to other banks, allowing the XT [[Intel 8088]] processor to address all the RAM on the 370PC-M card.<ref name=Mueller92>Scott Mueller ''Upgrading and Repairing PCs, Second Edition'', Que Books, 1992, {{ISBN|0-88022-856-3}} pages 73-75, page 94</ref> Besides the 416&nbsp;KB of usable RAM for S/370 applications, the XT/370 also supported up to 4 MB of [[virtual memory]] using the hard drive as its paging device.<ref name="killen1984fall">{{cite news | url=https://archive.org/stream/byte-magazine-1984-09/1984_09_BYTE_09-09_Guide_to_the_IBM_PCs#page/n33/mode/2up | title=IBM Forecast / Market Dominance | work=BYTE | date=Fall 1984 | accessdate=18 March 2016 | author=Killen, Michael | pages=30–38}}</ref>

IBM claimed the XT/370 reached 0.1 [[Instructions per second|MIPS]] (when the data fit in RAM). In 1984, the list price of an XT/370 in its typical configuration was approximately $12,000 so compared favorably with IBM's own mainframes on a $/MIPS basis; for example, an [[IBM 4341]] delivered 1.2&nbsp;MIPS for $500,000. While it theoretically reduced demand on customers' mainframes by offloading load onto the smaller computer, as customers purchased more XT/370s they likely increased the overall load on the mainframes, increasing IBM's mainframe sales.{{r|killen1984fall}}