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IBM PC XT/370

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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.[1][2]

IBM PC XT/370
One of the three 370PC emulation cards from an IBM XT/370 — modified 68000s + 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 (PC XT chassis with three add-in 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DiscontinuedApril 1987 (withdrawn simultaneously with the IBM AT/370)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
StorageStandard 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
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 (XT chassis) with the 370PC-P, 370PC-M, and PC3277-EM cards

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The XT/370 was withdrawn in April 1987, simultaneously with its big sibling the IBM AT/370.[3] Its eventual successor was the IBM 7437 VM/SP Technical Workstation (1987–1988), then the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 $8,995 to $12,000 for a complete configuration including the XT base unit.[4]

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

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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)

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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.[5][6]
  • 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)

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  • 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.[7]

PC3277-EM (3277 Emulation Adapter)

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  • 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).[8]

Performance

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  • 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.[9]

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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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.
 
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 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

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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 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

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

  1. IBM 5100 / IBM 5110 / IBM 5120 (1975–1980) — PALM microcode S/360 emulator running APLSV.
  2. IBM PC XT/370 (1983) — modified 68000 + modified 8087 hardware S/370 emulator 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. ↑ https://en.wikipedia.org/wiki/PC-based_IBM_mainframe-compatible_systems
  2. ↑ Kozuh, F. P., Livingston, B., Spillman, R. J. "System/370 capability in a desktop computer." IBM Systems Journal 23(3):245, 1984.
  3. ↑ Computer Business Review, "IBM Gives Up on the Personal XT/, AT/370", April 1987.
  4. ↑ IT History Society, "IBM Personal Computer XT/370".
  5. ↑ https://www.cpushack.com/2013/03/22/cpu-of-the-day-ibm-micro-370/
  6. ↑ https://thechipletter.substack.com/p/motorola-intel-ibm-make-a-mainframe
  7. ↑ Mueller, S. Upgrading & Repairing PCs, 2nd edition, 1992, pp. 73–75, 94.
  8. ↑ IBM SA38-0037-00 Personal Computer Family Service Information Manual, July 1989, §6-17.
  9. ↑ Kozuh et al., 1984, IBM Systems Journal.
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