Jump to content

IBM PC XT/370

From RetroTechCollection
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DiscontinuedApril 1987 (withdrawn simultaneously with the IBM AT/370)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

[edit | edit source]

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

[edit | edit source]

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)

[edit | edit source]
  • 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)

[edit | edit source]
  • 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

[edit | edit source]

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

[edit | edit source]

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

[edit | edit source]

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

[edit | edit source]
  • 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

[edit | edit source]

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

[edit | edit source]

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

[edit | edit source]
  • 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

[edit | edit source]

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

[edit | edit source]

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.
[edit | edit source]

References

[edit | edit source]
  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.


Retrieved from "https://wiki.retrotechcollection.com/index.php?title=IBM_PC_XT/370&oldid=12919"