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

Revision as of 17:23, 14 April 2022

Revision as of 11:39, 29 April 2022

Line 13:

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}}

[[File:IBM XT370 board (1).jpg|thumb|IBM XT/370 board and diagnostic diskette]]

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=https://priorart.ip.com/IPCOM/000059679# |archive-url=https://web.archive.org/web/20151211060121/http://priorart.ip.com/IPCOM/000059679 |url-status=dead |archive-date=2015-12-11 |title=Implementation of IBM System 370 Via Co-Microprocessors/The Co-Processor... - IPCOM000059679D - IP.com |publisher=Priorartdatabase.com |date= |accessdate=2020-07-23 }}</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 {{val|256 |ul=KiB}} ([[motherboard]]) memory could be used to move data to the {{val|512 |u=KiB}} expansion card. The expansion memory was dual ported, and provided an additional {{val|384 |u=KiB}} to the XT Machine bringing the total RAM on the XT side to {{val|640 |u=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 {{val|416&nbsp;|ul=KB}} of usable RAM for S/370 applications, the XT/370 also supported up to {{val|4 |ul=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 {{US$12,000|long=no|12000}} so compared favorably with IBM's own mainframes on a $/MIPS basis; for example, an [[IBM 4341]] delivered {{val|1.2&nbsp;|ul=MIPS}} for {{US$500,000|long=no|500000}}. 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}}

Similarly to the mainframe version of [[VM/CMS]], the VM/PC also created the illusion of [[Logical disk|virtual disks]], but on the PC version these were maintained as PC DOS files, either on floppy or hard disk. For example, the CMS virtual disk belonging to user FRED at device address 101 was stored as the DOS file FRED.101. The CMS IMPORT and EXPORT commands allowed extraction of files from these virtual drives as well as [[ASCII]]/[[EBCDIC]] conversion.<ref>BYTE Guide to the IBM PC, fall 1984, pp. 44-46</ref>