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Announced June 30, 1970 and withdrawn December 23, 1977.
The IBM System/370 Model 155 was a high-performance data processing system that provided the reliability, availability, and convenience demanded by business and scientific users, as well as by users with applications in communications or control.
The Model 155 included the advantages, characteristics, and functions pioneered by the IBM System/360. The high performance of the Model 155 can be attributed to:
- Access to 16 main-storage bytes in parallel.
- Local storage used for CPU (general and floating-point registers) and I/O applications.
- High-speed buffer storage that stored currently used sections of main storage for faster accessing during processing.
- Read-only storage (ROS); this control storage contained the microprograms (ROS control words) that control CPU and I/O operations.
- Overlap, where possible, of the instruction and execution portions of CPU operations.
- Overlap, where possible, of CPU and I/O operations.
- Retry facilities at the CPU, channel, and control unit level.
- An optional alter/display feature that provided an easy method to store small program loops or to make changes to programs already in storage.
Programming support for the System/370 Model 155 was provided by System/360 Operating System (OS), MFT and MVT, as well as System/360 Disk Operating System (DOS). The Model 155 took advantage of the gains possible because of this support.
The Model 155 had a major machine cycle time of 115 nanoseconds. Main-storage data flow was 16 bytes (one quadword). Main-storage cycle time was 2.07 microseconds. However, the high-speed buffer storage operated to make the effective system storage cycle time one-third to one-quarter of the actual main-storage cycle time.
Seven capacities of main storage were available, with error checking and correction (ECC) implemented in the processor storage units.
For input/output operations, the system could have one byte multiplexer channel and as many as five block multiplexer channels, or two byte multiplexer channels and as many as four block multiplexer channels.
The major components in a System/370 Model 155 were the IBM 3155 Processing Unit, which contained arithmetic and logical circuits, read-only storage, local storage, and at least three input/output channels; and the IBM 3360 Processor Storage. Input/output devices were attached to the channels by control units.
The standard features for any IBM 3155 Processing Unit included:
- System/370 universal instruction set
- Storage protection (store and fetch)
- Byte-oriented operand feature
- High-speed buffer storage
- Error checking and correction (processor storage)
- Time-of-day clock
- Interval timer
- Instruction retry
- Channel retry
- Command retry
- First byte multiplexer channel (Ch 0)
- Block multiplexer channel (Ch I)
- Block multiplexer channel (Ch 2)
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