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Model Groups 1 & 2 announced Sep 15, 1983 and withdrawn Feb 11, 1986.
Model Group 3 announced Oct 25, 1984 and withdrawn Feb 11, 1986.
Model Groups 11, 12, 13 & 14 announced Feb 11, 1986.
Model Groups 21, 22, 23 & 24 announced May 19, 1987 and withdrawn Aug 19, 1992.

The following is the text of an undated IBM product brochure.

The IBM 4381 Processor is one of the most powerful and versatile intermediate system processors ever produced by IBM. Its system capabilities enable the 4381 to address an expanded range of user requirements in addition to providing growth for users of smaller, intermediate system processors.

•  The 4381 Model Groups 1 and 2 are uniprocessors, whereas the Model Group 3 is a dual processor, consisting of two integrated instruction processing units operating under a single control program, each having its own channels and high-speed buffer and sharing processor storage.
•  The 4381 Model Group 1 in System/370 mode provides:
  
  • Internal throughput rates 1.4 to 1.6 times as fast as the 4341 Model Group 2 for commercial workloads.
  • Internal throughput rates up to 1.7 times as fast as the 4341 Model Group 2 for scientific workloads.
•  The 4381 Model Group 2 in System/370 mode provides:
  
  • Internal throughput rates 1.7 to 2.3 times as fast as the 4341 Model Group 2 for commercial workloads.
  • Internal throughput rates 2.4 to 3.0 times as fast as the 4341 Model Group 2 for scientific workloads.
•  The 4381 Model Group 3 provides internal throughput rates up to 1.7 times as fast as the 4381 Model Group 2 for commercial workloads, and up to 1.9 times as fast for scientific workloads.
•  A 4381 Model Group 1 can be upgraded in the field to a 4381 Model Group 2, and the Model Group 2 can be field upgraded to a Model Group 3.
•  Eight-byte parallel data flow occurs within the processor as well as between the processor and the high-speed buffer storage and channels.
•  Sixteen-byte parallel data flow occurs between the high-speed buffer and processor storage.
•  There are assists available for engineering and scientific applications. These include the Multiply/Add Facility, the Square Root Facility (for all model groups), and the Mathematical Function Facility (for the 4381 Model Groups 2 and 3 only).
•  The 4381 utilizes a unique air-cooling approach, and the upright design saves floor space.
•  The compact packaging and the superior internal performance of the 4381 Processor are made possible through the use of a new module-on-board packaging technique that replaces the card-on-board packaging in the instruction processing unit. Impingement aircooling, a new concept in processor cooling ensures adequate cooling of the high-density modules using only room-temperature air as the cooling medium.
•  I/O device attachment capability is improved: Up to 18 channels are available in the maximum configuration of the Model Group 3. Of these, eight are 3MB per second data streaming channels.
•  Support for both System/370 and System/370 extended architecture (XA) operating systems enables the 4381 to meet the growth and operating system requirements of the smallest System/360, System/370 and 4300 users, while being compatible with the most powerful 308X processors offered by IBM.

* Processor storage sizes of 24MB and 32MB run in 4K page mode only.

• 68 nanosecond internal processor cycle time.
• Aggregate data transfer rate of up to 32MB per second in data streaming mode in the maximum channel configuration with the Model Group 3.
• Supported by MVS/370 and VM/SP in S/370 mode; and MVS/XA and VM/XA Migration Aid in S/370-XA mode. Supported by DOS/VSE with VSE/Advanced Functions, and OS/VS1 with BPE, in S/370 mode, natively in 4381 Model Groups 1 and 2, and as a VM/SP guest operating system only in 4381 Model Group 3.
• Extended Control Program Support (ECPS) for MVS/370 and VM/370.
• Remote Support Facility (RSF) and the Remote Operator Console Facility (ROCF).
• Enhancements to the Problem Analysis Facility, including support for five or six languages other than English (depending on the console type).
• Mathematical Function Facility (4381 Model Groups 2 and 3 only) reduces processor busy time by up to 65 percent for the assisted functions.
• Multiply/Add Facility and Square Root Facility.
• Enhancements to ECPS: MVS for MVS/System Product Version 1 Release 3 and MVS/SP Version 2 Release 1.1 (includes cross memory services, the page fault assist function, and the ADDFRR instruction).
• Preferred machine assist to improve performance of MVS/370 running as a guest under VM/SP High Performance Option.

The performance range of the 4381 Processor and its features enable the 4381 to satisfy a variety of computing needs.

•  As growth options from 4331, 4361, 4341, 3158, 3031, 3168, 3032, or equivalent processors
•  As loosely coupled processors to 308X complexes
•  As S/370-XA migration, test, or production vehicles
•  As application processors for:
  
  • Information centers
  • DP professionals
  • Business professionals
  • Special applications
  • Engineering/Scientific

Twelve models of the 4381 Processor are available, offering three computing power options and various sizes of processor storage.

• Model Group 1: Models L1, Ml and P1 with 4, 8 and 16MB of processor storage respectively.
• Model Group 2: Models L2, M2, P2, Q2 and R2 with 4, 8, 16, 24 and 32MB of processor storage respectively.
• Model Group 3: Models M3, P3, Q3 and R3 with 8, 16, 24 and 32MB of processor storage respectively.

Processor storage upgrades can be made in the field.

• Virtual storage capability by dynamic address translation
• Byte multiplexer channel
• Data Streaming mode
• Channel command retry on block multiplexer channels
• Byte-oriented operands
• Clock comparator and CPU timer
• Control registers
• Error checking and correction in processor storage
• Extended precision floating-point
• Interval timer
• Machine check handling
• Support processor
• Instruction retry
• Program event recording
• Reloadable control storage
• Storage protection (store and fetch)
• S/370 universal instruction set
• Two system diskette drives
• Time-of-day clock
• PSW key handling
• Compare and swap and compare double and swap
• Clear I/O
• Clear channel
• External signals
• Unit power off
• Fast release for the start I/O fast instruction
• Hardware reconfiguration in high-speed buffer, swap buffer, control storage, and channel data buffer
• Recovery Extension Facility
• Remote Support Facility (RSF)
• Remote Operator Console Facility (ROCF) when RSF is installed

Channel-to-channel adapter (#1850): One channel-to-channel feature is optionally available to interconnect two channels (4381 Processor, 4300 Processor, S/360, S/370, 30XX). One of processors requires this feature and three control unit positions on each of the connected channels. Maximum: one. Field Installation: yes. Prerequisite: one #4631 must be specified on the primary 3278 Display Console Model 2A or 3279 Color Display Console Model 2C. The 3205 does not require this feature.

Block multiplexer channels, additional (#1870) (Model Groups 1 and 2 only): An optional group of six block multiplexer channels. Enables simultaneous operation of high-speed devices a data rate of 2.0MB per second for two channels and 1.0MB per second for four channels. Maximum: one. Field Installation: yes.

Block multiplexer channels, additional (#1871) (Model Group 3 only): Two optional groups of three block multiplexer channels (six total channels). Permits simultaneous operation of high-speed devices at a data rate of 2.0MB per second for two channels and 1.0MB per second for one channel for each group. Maximum: one Field Installation: yes.

Each 4381 Processor requires one IBM 3278 Display Console Model 2A, 3279 Color Display Console Model 2C or 3205 Display Console which is equipped with an operator console keyboard and an operator control panel. A second 3278, 3279 or 3205 display console, although not required, will enhance system operations for most customers. For the Model Group 3, it is recommended that the second console be channel attached to the second processor. 3205s cannot be mixed with 3278s or 3279s.