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Deep technical XT/370 + 3270 PC family page with verified sources (IBM SA38-0037-00 Service Info Manual, IBM GA33-3141-0, IBM 1502336, Kozuh/Livingston/Spillman IBM Systems Journal 1984, seasip.info, Wikipedia)
 
Fix refs so they render; IBM fits TWO cards not three - PC/370-P in slot 4 and PC/370-M in slot 3, with PC/3277-EM only on older machines and the 3278/79 or Remote Display Terminal adapter as alternatives; add IBMs model numbers (5160 Models 568/588/589), the 10 MB vs 20 MB fixed-disk configurations and IBMs unit specifications; host link is coax not twinax
 
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| name          = IBM PC XT/370
| name          = IBM PC XT/370
| image        = [[File:IBM XT 370 board.jpg|260px]]
| 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
| caption      = A 370PC emulation card from an IBM XT/370. Modified 68000s and a modified 8087 implement IBM System/370 instructions in hardware
| developer    = IBM Entry Systems Division, Boca Raton, in cooperation with IBM mainframe groups
| developer    = IBM Entry Systems Division, Boca Raton, in cooperation with IBM mainframe groups
| manufacturer  = IBM
| manufacturer  = IBM
| type          = Desktop System/370 workstation (PC XT chassis with three add-in cards)
| type          = Desktop System/370 workstation — a 5160 PC XT with '''two''' System/370 adapter cards
| release date  = October 1983
| release date  = October 1983
| discontinued  = April 1987 (withdrawn simultaneously with the IBM AT/370)
| discontinued  = April 1987 (withdrawn simultaneously with the IBM AT/370)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'''
| 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
| 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
| storage      = 5.25-inch 360 KB diskette drive. IBM configures it either as a '''10 MB system''' (one fixed disk in the system unit) or a '''20 MB system''' (two fixed disks in a 5161 Expansion Unit)
| display      = IBM 5151 monochrome or IBM 5153 CGA — same as a stock XT
| display      = IBM 5151 monochrome or IBM 5153 CGA — same as a stock XT
| sound        = PC speaker
| sound        = PC speaker
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| predecessor  = IBM 5100 family (PALM microcode-based S/370 emulation)
| 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)
| 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
| model        = '''5160 Models 568, 588 and 589'''. IBM's card names are '''PC/370-P''' (processor, slot 4) and '''PC/370-M''' (512 KB memory, slot 3); older machines may also carry a '''PC/3277-EM''' card
}}
}}


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 '''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. IBM's own description is "an extended version of the 5160 XT with 256Kb of memory on the system board", carrying '''two''' XT/370 adapter cards that execute most System/370 instructions in hardware and let the machine run a single-user version of VM/CMS called '''VM/PC'''.<ref name="sim">IBM, ''IBM Personal Computer Family Service Information Manual'', SA38-0037-00, July 1989 &mdash; hosted on this wiki as [[IBM Personal Computer Family Service Information Manual]]. Chapter 6, "5160 PC XT": the 5160 model feature chart, which lists the PC/370-M, PC/370-P and PC/3277-EM cards against particular 5160 models; the "5160 XT/370" pages, which give the card complement, the slot assignments, the fixed-disk configurations and the unit specifications; and the 5160 power supply description, which gives the 130 W rating, the per-rail currents and tolerances, the existence of separate 120 V and 220/240 V supplies, and the automatic shutdown on dc overcurrent or overvoltage. Chapter 9 covers the AT/370; chapter 18 lists the card part numbers.</ref><ref name="kozuh">Kozuh, F. P., Livingston, B., and Spillman, R. J., "System/370 capability in a desktop computer", ''IBM Systems Journal'' 23(3):245, 1984, DOI 10.1147/sj.233.0245. The engineering paper by the team that built it, and the source for the performance figures quoted here. ''This wiki has not read the full text'' &mdash; it is behind a paywall &mdash; so those figures are recorded as attributed rather than verified.</ref>
 
'''It is two cards, not three.''' IBM names them and gives their slots: the '''Processor card (PC/370-P) in expansion slot 4''' and the '''512 KB Memory card (PC/370-M) in expansion slot 3'''.<ref name="sim" /> A third card is often counted in descriptions of this machine, but IBM lists the '''PC/3277-EM''' as an ''option'', notes only that "older models of the PC XT/370 might contain" one, and offers the standard '''3278/79 Emulation Adapter''' or a '''Remote Display Terminal Device Emulation Adapter''' (which can go in slot 2) as the alternatives.<ref name="sim" /> Counting three cards as standard will send you looking for a card a given machine was never built with.
 
The cards are listed as standard features of '''5160 Models 568, 588 and 589''' in IBM's 5160 model feature chart.<ref name="sim" />


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 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).
The XT/370 was '''withdrawn in April 1987''', at the same time as the IBM AT/370.<ref name="cbr87">''Computer Business Review'', "IBM Gives Up on the Personal XT/, AT/370", April 1987, at <code>techmonitor.ai</code>.</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 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>
Launch pricing was approximately '''$3,790 for the three emulation cards alone''', or about '''$8,995 to $12,000''' for a complete configuration.<ref name="iths">IT History Society, "IBM Personal Computer XT/370", at <code>ithistory.org</code>. Secondary; the only source located for the launch pricing.</ref>
 
''BYTE'' (Fall 1984, Ernest Sabine) called it "a qualified success", and ''Computerworld'' of 25 November 1985 reported sales below IBM's expectations.<ref name="cw85">''Computerworld'', 25 November 1985, page 25 (Google Books <code>ZUMIDEwANHgC</code>). Secondary.</ref> The XT/370 was '''withdrawn in April 1987''' along with the AT/370.
 
== The XT/370 Cards ==
 
IBM's own list, with slots:<ref name="sim" />
 
<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:90%;"
|+'''IBM XT/370 — cards and slots'''
! Card !! Slot !! Status
|-
| '''PC/370-P''' — Processor card || Expansion slot '''4''' || Standard
|-
| '''PC/370-M''' — 512 KB Memory card || Expansion slot '''3''' || Standard
|-
| Remote Display Terminal Device Emulation Adapter || May be in expansion slot '''2''' || Option
|-
| IBM 3278/79 Emulation Adapter || &mdash; || Option
|-
| '''PC/3277-EM''' || &mdash; || Only on older machines: "older models of the PC XT/370 ''might'' contain a PC/3277EM card"
|}
 
Most secondary accounts, including the earlier version of this page, name the cards '''370PC-P''' and '''370PC-M'''. '''IBM writes them the other way round''' — PC/370-P and PC/370-M — in both the feature chart and the XT/370 description.<ref name="sim" /> Either form will be understood, but IBM's is the one on the parts lists.
 
=== PC/370-P (Processor Card) ===
 
* '''Two modified Motorola 68000s''' — the microcode was rewritten to execute most System/370 fixed-point and non-floating-point instructions directly. An opcode the modified part does not recognise traps to a software interpreter running on the unmodified 68000. The custom part is usually called the '''Micro/370'''.<ref name="micro370">[https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro-370/ The CPU Shack &mdash; IBM Micro/370] and [https://thechipletter.substack.com/p/motorola-intel-ibm-make-a-mainframe The Chip Letter &mdash; Motorola, Intel, IBM Make A Mainframe in a PC]. Community sources for the modified-68000 and modified-8087 story and for the "Micro/370" name. IBM's service manual names neither part.</ref>
* '''A modified Intel 8087''' implementing System/370 floating point — excess-64 hexadecimal, not the IEEE 754 binary format a standard 8087 does.<ref name="micro370" />
* Bridging logic to the host ISA bus and a dedicated card-to-card connection to the memory card.
 
'''IBM's service manual names neither the 68000 nor the 8087.''' It calls the card "the Processor card (PC/370-P)" and stops there.<ref name="sim" /> The chip-level detail above comes from community teardowns and from the engineering literature, and is recorded here as such.


''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.
=== PC/370-M (Memory Card) ===


== The Three 370PC Cards ==
* '''512 KB''', per IBM's own name for the card: "The 512Kb Memory card (PC/370-M)".<ref name="sim" />
* Dual-ported between the processor card and the host.
* Of the 512 KB, about '''416 KB''' is usable by System/370 applications, the rest being emulator runtime.<ref name="kozuh" />
* The card also brings the host's memory up from the 256 KB on the system board.<ref name="sim" /><ref name="mueller">Mueller, S., ''Upgrading and Repairing PCs'', 2nd edition, 1992, pages 73&ndash;75 and 94. Secondary, for the host-memory contribution.</ref>


The XT/370 is defined by three full-length 8-bit ISA cards that occupy three of the XT's eight expansion slots:
=== PC/3277-EM and the emulation alternatives ===


=== 370PC-P (Processor Card) ===
The XT/370 needs a terminal-emulation path to a host to load VM/PC and to reach mainframe files. IBM offers three ways of getting one, and which one a given machine has depends on when it was built:<ref name="sim" />


The processor card carries:
* '''PC/3277-EM''' — the original, on older machines only.
* '''IBM 3278/79 Emulation Adapter''' — the same card the [[IBM 3270 PC]] uses.
* '''Remote Display Terminal Device Emulation Adapter''' — which IBM notes can sit in expansion slot 2.


* '''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>
All of them attach to the control unit over '''coaxial''' cable, not twinax.<ref name="sim" />
* '''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) ===
== Fixed Disk Configurations ==


* '''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.
IBM offers two, and the difference is where the second drive lives:<ref name="sim" />
* 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) ===
* '''10 MB system''' — one fixed disk drive in the system unit.
* '''20 MB system''' — two fixed disk drives in a '''5161 Expansion Unit'''. The expansion unit has its own 130 W PC XT power supply and its own eight extension slots.<ref name="sim" />


* '''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.
== Unit Specifications ==
* '''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>
 
IBM's figures for the XT/370 system unit:<ref name="sim" />
 
{| class="wikitable styled-table" style="width:70%;"
|+'''IBM XT/370 system unit'''
! Item !! Value
|-
| Height || 142 mm (5.5 in)
|-
| Length || 500 mm (19.6 in)
|-
| Depth || 410 mm (16.1 in)
|-
| Weight || '''14.5 kg (32 lb)'''
|-
| Power cable || 1.83 m (6 ft), 18 AWG
|-
| Heat output || '''650 BTU/hr'''
|-
| Noise level || 49.5 dBA, system unit with monochrome display
|}


== Performance ==
== Performance ==


* '''S/370 performance''' — approximately '''0.1 MIPS''' sustained, when the working set fits in the 416 KB of usable S/370 RAM.
* '''System/370 performance''' — around '''0.1 MIPS''' sustained when the working set fits in the usable System/370 memory.<ref name="kozuh" />
* 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.
* 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.<ref name="kozuh" />
* 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 ==
== Operating Environment: VM/PC ==
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== Hardware Configuration ==
== Hardware Configuration ==


The XT/370 is a stock IBM PC XT (5160) chassis with:
A 5160 PC XT chassis with '''256 KB on the system board''', the two XT/370 cards, a 5.25-inch 360 KB diskette drive, one or two fixed disks, an IBM 5151 monochrome or 5153 colour display, and the XT keyboard.<ref name="sim" /> Everything outside the two cards is stock XT.
 
* 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)]]
[[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)]]
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== Service Documents ==
== 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 Personal Computer Family Service Information Manual]] — '''SA38-0037-00''', July 1989, '''hosted here'''. Chapter 6 covers the XT/370, chapter 9 the AT/370, chapter 18 the parts.<ref name="sim" />
* '''IBM 6137739''' — ''Virtual Machine/Personal Computer User's Guide'' (December 1984).
* '''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.
* '''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.
Line 98: Line 148:


* '''RIFA mains-suppression capacitors''' in the PSU — vent and produce smoke / fish odour. Replace immediately as a preventive measure.
* '''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.
* '''Tantalum bypass capacitor shorts''' on the planar — these pull the +5 V rail down and stop the machine posting.
* '''Dallas DS1287 RTC battery''' on the planar (where fitted).
* '''There is no real-time clock on a 5160 planar.''' Any battery in the machine belongs to an add-in card.


XT/370-specific faults:
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.
* '''The XT/370 cards are effectively unobtainable.''' The modified 68000s and the modified 8087 are custom parts that nobody is making again. A machine with both cards present is museum-grade.
* '''Card-to-card backplane connector''' between 370PC-P and 370PC-M can develop oxidation; cleaning restores function in most cases.
* '''Check slots 3 and 4 before concluding a card is missing.''' IBM's slot assignment is fixed: processor in 4, memory in 3. A card found elsewhere has been moved.
* '''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.
* '''Card-to-card connection''' between the processor and memory cards — oxidation here produces a machine that posts normally as an XT and then fails to start VM/PC. Reseat before suspecting the cards.
* '''VM/PC system diskettes''' — rare, and the original installation route needed a working emulation card attached to a control unit. Image any set you find.


== Architectural Significance ==
== Architectural Significance ==


The XT/370 sits in IBM's long lineage of '''S/360- and S/370-emulation-in-a-personal-computer''' products:
The XT/370 sits in IBM's long line of System/360 and System/370 emulation in a personal computer:<ref name="wppc">Wikipedia, "PC-based IBM mainframe-compatible systems". Tertiary; used for the product chronology and the succession from the XT/370 through the 7437, P/370 and R/390.</ref>


# '''[[IBM 5100]] / [[IBM 5110]] / [[IBM 5120]]''' (1975–1980) — PALM microcode S/360 emulator running APLSV.
# '''[[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 XT/370''' (1983) — modified 68000 plus modified 8087, running VM/PC.
# '''IBM PC AT/370''' (1984) — 16-bit cards (370PC-P2, 370PC-M2) on the AT chassis.
# '''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.
# '''IBM 7437 VM/SP Technical Workstation''' (1987–88) — separate enclosure, channel-attached.
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== References ==
== References ==


* [https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compatible_systems PC-based IBM mainframe-compatible systems — Wikipedia].
<references />
* 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].


{{Navbox-IBMComputers|state=collapsed}}
{{Navbox-IBMComputers|state=collapsed}}

Latest revision as of 22:08, 24 September 2026

IBM PC XT/370
A 370PC emulation card from an IBM XT/370. Modified 68000s and a modified 8087 implement IBM System/370 instructions in hardware
Specifications
DeveloperIBM Entry Systems Division, Boca Raton, in cooperation with IBM mainframe groups
ManufacturerIBM
TypeDesktop System/370 workstation — a 5160 PC XT with two System/370 adapter cards
ReleasedOctober 1983
DiscontinuedApril 1987 (withdrawn simultaneously with the IBM AT/370)
CPUHost: 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
MemoryHost (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
Storage5.25-inch 360 KB diskette drive. IBM configures it either as a 10 MB system (one fixed disk in the system unit) or a 20 MB system (two fixed disks in a 5161 Expansion Unit)
DisplayIBM 5151 monochrome or IBM 5153 CGA — same as a stock XT
SoundPC speaker
OS / FirmwarePC 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
PredecessorIBM 5100 family (PALM microcode-based S/370 emulation)
SuccessorIBM 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 no.5160 Models 568, 588 and 589. IBM's card names are PC/370-P (processor, slot 4) and PC/370-M (512 KB memory, slot 3); older machines may also carry a PC/3277-EM card

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. IBM's own description is "an extended version of the 5160 XT with 256Kb of memory on the system board", carrying two XT/370 adapter cards that execute most System/370 instructions in hardware and let the machine run a single-user version of VM/CMS called VM/PC.[1][2]

It is two cards, not three. IBM names them and gives their slots: the Processor card (PC/370-P) in expansion slot 4 and the 512 KB Memory card (PC/370-M) in expansion slot 3.[1] A third card is often counted in descriptions of this machine, but IBM lists the PC/3277-EM as an option, notes only that "older models of the PC XT/370 might contain" one, and offers the standard 3278/79 Emulation Adapter or a Remote Display Terminal Device Emulation Adapter (which can go in slot 2) as the alternatives.[1] Counting three cards as standard will send you looking for a card a given machine was never built with.

The cards are listed as standard features of 5160 Models 568, 588 and 589 in IBM's 5160 model feature chart.[1]

The XT/370 is the architectural successor to the IBM 5100 / IBM 5110 / IBM 5120's use of the IBM 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, at the same time as the IBM AT/370.[3] 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

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Launch pricing was approximately $3,790 for the three emulation cards alone, or about $8,995 to $12,000 for a complete configuration.[4]

BYTE (Fall 1984, Ernest Sabine) called it "a qualified success", and Computerworld of 25 November 1985 reported sales below IBM's expectations.[5] The XT/370 was withdrawn in April 1987 along with the AT/370.

The XT/370 Cards

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IBM's own list, with slots:[1]

IBM XT/370 — cards and slots
Card Slot Status
PC/370-P — Processor card Expansion slot 4 Standard
PC/370-M — 512 KB Memory card Expansion slot 3 Standard
Remote Display Terminal Device Emulation Adapter May be in expansion slot 2 Option
IBM 3278/79 Emulation Adapter — Option
PC/3277-EM — Only on older machines: "older models of the PC XT/370 might contain a PC/3277EM card"

Most secondary accounts, including the earlier version of this page, name the cards 370PC-P and 370PC-M. IBM writes them the other way round — PC/370-P and PC/370-M — in both the feature chart and the XT/370 description.[1] Either form will be understood, but IBM's is the one on the parts lists.

PC/370-P (Processor Card)

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  • Two modified Motorola 68000s — the microcode was rewritten to execute most System/370 fixed-point and non-floating-point instructions directly. An opcode the modified part does not recognise traps to a software interpreter running on the unmodified 68000. The custom part is usually called the Micro/370.[6]
  • A modified Intel 8087 implementing System/370 floating point — excess-64 hexadecimal, not the IEEE 754 binary format a standard 8087 does.[6]
  • Bridging logic to the host ISA bus and a dedicated card-to-card connection to the memory card.

IBM's service manual names neither the 68000 nor the 8087. It calls the card "the Processor card (PC/370-P)" and stops there.[1] The chip-level detail above comes from community teardowns and from the engineering literature, and is recorded here as such.

PC/370-M (Memory Card)

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  • 512 KB, per IBM's own name for the card: "The 512Kb Memory card (PC/370-M)".[1]
  • Dual-ported between the processor card and the host.
  • Of the 512 KB, about 416 KB is usable by System/370 applications, the rest being emulator runtime.[2]
  • The card also brings the host's memory up from the 256 KB on the system board.[1][7]

PC/3277-EM and the emulation alternatives

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The XT/370 needs a terminal-emulation path to a host to load VM/PC and to reach mainframe files. IBM offers three ways of getting one, and which one a given machine has depends on when it was built:[1]

  • PC/3277-EM — the original, on older machines only.
  • IBM 3278/79 Emulation Adapter — the same card the IBM 3270 PC uses.
  • Remote Display Terminal Device Emulation Adapter — which IBM notes can sit in expansion slot 2.

All of them attach to the control unit over coaxial cable, not twinax.[1]

Fixed Disk Configurations

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IBM offers two, and the difference is where the second drive lives:[1]

  • 10 MB system — one fixed disk drive in the system unit.
  • 20 MB system — two fixed disk drives in a 5161 Expansion Unit. The expansion unit has its own 130 W PC XT power supply and its own eight extension slots.[1]

Unit Specifications

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IBM's figures for the XT/370 system unit:[1]

IBM XT/370 system unit
Item Value
Height 142 mm (5.5 in)
Length 500 mm (19.6 in)
Depth 410 mm (16.1 in)
Weight 14.5 kg (32 lb)
Power cable 1.83 m (6 ft), 18 AWG
Heat output 650 BTU/hr
Noise level 49.5 dBA, system unit with monochrome display

Performance

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  • System/370 performance — around 0.1 MIPS sustained when the working set fits in the usable System/370 memory.[2]
  • 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.[2]


Operating Environment: VM/PC

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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

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A 5160 PC XT chassis with 256 KB on the system board, the two XT/370 cards, a 5.25-inch 360 KB diskette drive, one or two fixed disks, an IBM 5151 monochrome or 5153 colour display, and the XT keyboard.[1] Everything outside the two cards is stock XT.

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

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  • IBM Personal Computer Family Service Information Manual — SA38-0037-00, July 1989, hosted here. Chapter 6 covers the XT/370, chapter 9 the AT/370, chapter 18 the parts.[1]
  • 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

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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 capacitor shorts on the planar — these pull the +5 V rail down and stop the machine posting.
  • There is no real-time clock on a 5160 planar. Any battery in the machine belongs to an add-in card.

XT/370-specific faults:

  • The XT/370 cards are effectively unobtainable. The modified 68000s and the modified 8087 are custom parts that nobody is making again. A machine with both cards present is museum-grade.
  • Check slots 3 and 4 before concluding a card is missing. IBM's slot assignment is fixed: processor in 4, memory in 3. A card found elsewhere has been moved.
  • Card-to-card connection between the processor and memory cards — oxidation here produces a machine that posts normally as an XT and then fails to start VM/PC. Reseat before suspecting the cards.
  • VM/PC system diskettes — rare, and the original installation route needed a working emulation card attached to a control unit. Image any set you find.

Architectural Significance

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The XT/370 sits in IBM's long line of System/360 and System/370 emulation in a personal computer:[8]

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

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 IBM, IBM Personal Computer Family Service Information Manual, SA38-0037-00, July 1989 — hosted on this wiki as IBM Personal Computer Family Service Information Manual. Chapter 6, "5160 PC XT": the 5160 model feature chart, which lists the PC/370-M, PC/370-P and PC/3277-EM cards against particular 5160 models; the "5160 XT/370" pages, which give the card complement, the slot assignments, the fixed-disk configurations and the unit specifications; and the 5160 power supply description, which gives the 130 W rating, the per-rail currents and tolerances, the existence of separate 120 V and 220/240 V supplies, and the automatic shutdown on dc overcurrent or overvoltage. Chapter 9 covers the AT/370; chapter 18 lists the card part numbers.
  2. ↑ 2.0 2.1 2.2 2.3 Kozuh, F. P., Livingston, B., and Spillman, R. J., "System/370 capability in a desktop computer", IBM Systems Journal 23(3):245, 1984, DOI 10.1147/sj.233.0245. The engineering paper by the team that built it, and the source for the performance figures quoted here. This wiki has not read the full text — it is behind a paywall — so those figures are recorded as attributed rather than verified.
  3. ↑ Computer Business Review, "IBM Gives Up on the Personal XT/, AT/370", April 1987, at techmonitor.ai.
  4. ↑ IT History Society, "IBM Personal Computer XT/370", at ithistory.org. Secondary; the only source located for the launch pricing.
  5. ↑ Computerworld, 25 November 1985, page 25 (Google Books ZUMIDEwANHgC). Secondary.
  6. ↑ 6.0 6.1 The CPU Shack — IBM Micro/370 and The Chip Letter — Motorola, Intel, IBM Make A Mainframe in a PC. Community sources for the modified-68000 and modified-8087 story and for the "Micro/370" name. IBM's service manual names neither part.
  7. ↑ Mueller, S., Upgrading and Repairing PCs, 2nd edition, 1992, pages 73–75 and 94. Secondary, for the host-memory contribution.
  8. ↑ Wikipedia, "PC-based IBM mainframe-compatible systems". Tertiary; used for the product chronology and the succession from the XT/370 through the 7437, P/370 and R/390.


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