Editing IBM PC XT/370

{{Infobox computer
| name          = IBM PC XT/370
| image         = [[File:IBM XT 370 board.jpg|260px]]
| caption       = One of the three 370PC emulation cards from an IBM XT/370 — modified 68000s + modified 8087 implement IBM System/370 instructions in hardware
| developer     = IBM Entry Systems Division, Boca Raton, in cooperation with IBM mainframe groups
| manufacturer  = IBM
| type          = Desktop System/370 workstation (PC XT chassis with three add-in cards)
| release date  = October 1983
| discontinued  = April 1987 (withdrawn simultaneously with the IBM AT/370)
| cpu           = Host: Intel '''8088 @ 4.77 MHz'''. S/370 side: '''two modified Motorola 68000s''' (microcode rewritten to directly execute most S/370 fixed-point and non-floating instructions; unrecognised opcodes trap to a program running on the unmodified 68000) plus a '''modified Intel 8087''' that handles S/370 floating-point. The custom 68000 is sometimes called the '''Micro/370'''
| memory        = Host (XT side): up to 640 KB with the 370PC-M card contributing 384 KB. S/370 side: 512 KB dual-ported RAM on the 370PC-M card (416 KB usable for S/370 applications); 4 MB virtual memory via hard-disk paging
| storage       = Standard IBM PC XT (5160) — 5.25" 360 KB DSDD floppy + 10 MB ST-412 hard drive. IBM 5161 expansion chassis supports a second hard drive
| display       = IBM 5151 monochrome or IBM 5153 CGA — same as a stock XT
| sound         = PC speaker
| os            = PC DOS 2.10 boots first, then loads '''VM/PC''' (Virtual Machine/Personal Computer) — IBM's modified VM/CMS for a single user. VM/PC v2 (November 1985) added page-cache support
| predecessor   = IBM 5100 family (PALM microcode-based S/370 emulation)
| successor     = '''IBM AT/370''' (1984); then '''IBM 7437 VM/SP Technical Workstation''' (1987–88); then '''P/370''' MCA card (~1989); then '''R/390''' RISC; eventually software emulators (Hercules, zPDT)
| model         = 5160 (XT chassis) with the 370PC-P, 370PC-M, and PC3277-EM cards
}}

The '''IBM PC XT/370''' is a desktop System/370 workstation released by IBM in '''October 1983''' as a follow-on to the [[IBM 5100]]'s PALM-microcode-based mainframe emulation. The XT/370 is mechanically an [[IBM PC XT (5160)]] but with three large add-in cards that emulate IBM System/370 mainframe instructions in hardware, allowing the XT to run a modified single-user version of '''VM/CMS''' called '''VM/PC'''.<ref>https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compatible_systems</ref><ref>Kozuh, F. P., Livingston, B., Spillman, R. J. "System/370 capability in a desktop computer." ''IBM Systems Journal'' 23(3):245, 1984.</ref>

The XT/370 is the architectural successor to the [[IBM 5100]] / [[IBM 5110]] / [[IBM 5120]]'s use of the IBM [[IBM PALM processor|PALM]] processor's System/360 emulator microcode to run an IBM-mainframe-flavoured interpreter (APLSV) as a self-contained personal computer.

The XT/370 was '''withdrawn in April 1987''', simultaneously with its big sibling the IBM AT/370.<ref>Computer Business Review, "IBM Gives Up on the Personal XT/, AT/370", April 1987.</ref> Its eventual successor was the '''IBM 7437 VM/SP Technical Workstation''' (1987–1988), then the '''P/370''' MCA card for PS/2 systems (~1989), then the '''R/390''' for RS/6000 (mid-1990s).

== Launch and Reception ==

Launch pricing was approximately '''$3,790 for the three emulation cards alone''', or '''$8,995 to $12,000 for a complete configuration''' including the XT base unit.<ref>IT History Society, "IBM Personal Computer XT/370".</ref>

''BYTE'' (Fall 1984, Ernest Sabine) called it "a qualified success." It was a slow seller — ''Computerworld'' (25 November 1985) reported sales below IBM's expectations. The XT/370 was '''withdrawn in April 1987''' along with the AT/370.

== The Three 370PC Cards ==

The XT/370 is defined by three full-length 8-bit ISA cards that occupy three of the XT's eight expansion slots:

=== 370PC-P (Processor Card) ===

The processor card carries:

* '''Two modified Motorola 68000s''' — IBM had Motorola rewrite the 68000 microcode to '''directly execute most System/370 fixed-point and non-floating-point instructions'''. When the 68000 encounters an opcode it does not recognise as native S/370, the instruction traps and is handled by a software interpreter running on the unmodified 68000. The custom 68000 is sometimes called the '''Micro/370'''.<ref>https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro-370/</ref><ref>https://thechipletter.substack.com/p/motorola-intel-ibm-make-a-mainframe</ref>
* '''A modified Intel 8087''' — IBM had Intel modify the 8087 floating-point unit to implement '''IBM System/370 floating-point arithmetic''' (excess-64 hexadecimal floating-point, not the IEEE 754 binary floating-point the standard 8087 implements).
* Glue logic to bridge the 370PC-P to the host XT's ISA bus and to the 370PC-M memory card via a dedicated back-edge connector.

=== 370PC-M (Memory Card) ===

* '''512 KB of dual-ported RAM''', shared between the 370PC-P card and the XT host through a back-edge connector unique to the 370PC card pair.
* Of the 512 KB, '''416 KB is usable for S/370 applications'''; the remainder is reserved for the emulator runtime.
* The card also '''contributes 384 KB to the XT host''', bringing the host's RAM from 256 KB to the full 640 KB.<ref>Mueller, S. ''Upgrading & Repairing PCs'', 2nd edition, 1992, pp. 73–75, 94.</ref>

=== PC3277-EM (3277 Emulation Adapter) ===

* '''3270 terminal emulation adapter''' — required for the XT/370 to download VM/PC system software from a host mainframe, and for connecting CMS to mainframe-resident files.
* '''Late-production XT/370s replaced this with the standard 3278/79 Emulation Adapter''' (same card used by the [[IBM 3270 PC]]).<ref>IBM SA38-0037-00 ''Personal Computer Family Service Information Manual'', July 1989, §6-17.</ref>

== Performance ==

* '''S/370 performance''' — approximately '''0.1 MIPS''' sustained, when the working set fits in the 416 KB of usable S/370 RAM.
* When the working set exceeds 416 KB, VM/PC pages out to the XT's hard disk, supporting up to '''4 MB of virtual memory''' but with a heavy performance penalty.
* In contemporary terms, the XT/370 is comparable to a low-end System/4341 mainframe channel of the era for compute-intensive single-stream CMS workloads.<ref>Kozuh et al., 1984, ''IBM Systems Journal''.</ref>

== Operating Environment: VM/PC ==

The XT/370 boots '''PC DOS 2.10''' first, then loads '''VM/PC''' (Virtual Machine/Personal Computer). VM/PC presents a single-user CMS environment that closely mirrors mainframe VM/CMS:

* '''CMS "virtual disks"''' (minidisks) are stored as PC DOS files. User FRED's minidisk 101 becomes the PC DOS file '''FRED.101'''.
* '''EXPORT''' and '''IMPORT''' commands transfer files between CMS and PC DOS, with automatic EBCDIC↔ASCII conversion.
* CMS commands, EXEC scripts, and most CMS applications run unchanged.
* No multi-user support — VM/PC is single-user only.

VM/PC version 2 (November 1985) added '''page-cache support''' to reduce hard-disk paging overhead.

The IBM AT/370 ships an updated VM/PC for PC DOS 3.0, with the wider 16-bit 370PC-P2 and 370PC-M2 cards (still 512 KB on the M2 card, with 32 KB reserved for microcode).

== Hardware Configuration ==

The XT/370 is a stock IBM PC XT (5160) chassis with:

* 8088 host CPU at 4.77 MHz.
* 256 KB on planar (XT standard), expanded to 640 KB by the 370PC-M card.
* 5.25" 360 KB DSDD floppy.
* 10 MB ST-412 hard drive.
* IBM 5161 Expansion Chassis support for a second hard drive.
* IBM 5151 monochrome or IBM 5153 CGA display.
* IBM XT 83-key Model F keyboard.

[[File:IBM XT 370 board.jpg|center|thumb|480px|One of the three 370PC emulation cards from an IBM XT/370. The full-length 8-bit ISA cards carry modified 68000s (Micro/370), modified 8087, and 512 KB dual-ported RAM. (Image: Wikimedia Commons, CC BY-SA 3.0)]]

== Service Documents ==

* '''IBM SA38-0037-00''' — ''Personal Computer Family Service Information Manual'' (July 1989), Chapter 6 covers the XT/370, Chapter 9 covers the AT/370.
* '''IBM 6137739''' — ''Virtual Machine/Personal Computer User's Guide'' (December 1984).
* '''Kozuh, F. P., Livingston, B., Spillman, R. J.''' — "System/370 capability in a desktop computer," ''IBM Systems Journal'' 23(3):245, 1984. The canonical engineering paper.

== Common Faults ==

The XT/370 chassis is identical to the [[IBM PC XT (5160)]], so chassis faults are:

* '''RIFA mains-suppression capacitors''' in the PSU — vent and produce smoke / fish odour. Replace immediately as a preventive measure.
* '''Tantalum bypass cap shorts''' on the planar — pull the +5 V rail and prevent POST.
* '''Dallas DS1287 RTC battery''' on the planar (where fitted).

XT/370-specific faults:

* '''The three 370PC cards are effectively unobtainium.''' The modified 68000s, the modified 8087, and the dual-ported memory backplane are not reproducible by current technology. A surviving XT/370 with all three cards is a museum-grade machine.
* '''Card-to-card backplane connector''' between 370PC-P and 370PC-M can develop oxidation; cleaning restores function in most cases.
* '''VM/PC system diskette''' — the install diskettes are increasingly rare and the install procedure requires a working PC3277-EM card connected to a 3174 / 3274 controller, which is itself increasingly rare.

== Architectural Significance ==

The XT/370 sits in IBM's long lineage of '''S/360- and S/370-emulation-in-a-personal-computer''' products:

# '''[[IBM 5100]] / [[IBM 5110]] / [[IBM 5120]]''' (1975–1980) — PALM microcode S/360 emulator running APLSV.
# '''IBM PC XT/370''' (1983) — modified 68000 + modified 8087 hardware S/370 emulator running VM/PC.
# '''IBM PC AT/370''' (1984) — 16-bit cards (370PC-P2, 370PC-M2) on the AT chassis.
# '''IBM 7437 VM/SP Technical Workstation''' (1987–88) — separate enclosure, channel-attached.
# '''P/370''' (~1989) — MCA card for PS/2 Model 60/70/80.
# '''R/390''' (mid-1990s) — RISC PCI card for RS/6000.
# '''S/390 Integrated Server''' (1998).
# Eventually superseded by software emulators (Hercules, zPDT) on commodity hardware.

== Related Pages ==

* [[IBM PC XT/370 Maintenance Guide]]
* [[IBM PC XT/370 Troubleshooting Guide]]
* [[IBM PC XT/370 Capacitor Replacement Guide]]
* [[IBM PC XT (5160)]] — base chassis
* [[IBM 3270 PC]] — contemporary XT-derived mainframe-connect workstation
* [[IBM 5100]] — direct PALM-era ancestor
* [[IBM PALM processor]] — the earlier emulation approach

== References ==

* [https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compatible_systems PC-based IBM mainframe-compatible systems — Wikipedia].
* Kozuh, F. P., Livingston, B., Spillman, R. J. "System/370 capability in a desktop computer." ''IBM Systems Journal'' 23(3):245, 1984. DOI [https://doi.org/10.1147/sj.233.0245 10.1147/sj.233.0245].
* [https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro-370/ The CPU Shack — IBM Micro/370].
* [https://thechipletter.substack.com/p/motorola-intel-ibm-make-a-mainframe The Chip Letter — Motorola, Intel, IBM Make A Mainframe in a PC].
* [http://bitsavers.org/pdf/ibm/pc/SA38-0037-00_Personal_Computer_Family_Service_Information_Manual_Jul89.pdf IBM SA38-0037-00 Personal Computer Family Service Information Manual (July 1989)].
* [https://techmonitor.ai/technology/ibm_gives_up_on_the_personal_xt_at370 Computer Business Review — "IBM Gives Up on the Personal XT/, AT/370"].
* [https://www.ithistory.org/db/hardware/ibm/ibm-personal-computer-xt370 IT History Society — IBM Personal Computer XT/370].
* [https://books.google.com/books?id=ZUMIDEwANHgC&pg=PA25 Computerworld 25 November 1985].

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