Architecture
The storage word length was 16-bits. The registers had the same width as the address bus. Their length varied between 16, 18 and 20 bits depending on the amount of storage installed. A particular interrupt level had eight registers. Register zero was the program counter which gave the address of the next instruction to be executed; the other seven were accumulators. The four odd-numbered accumulators could be addressed as eight single-byte accumulators.
Instructions were fairly simple. Most were register-to-register or register-immediate instructions which executed in a single memory cycle. There were eight storage reference instructions which required two or three storage cycles to complete. The only shift capability was to shift right one or to add a register to itself.
Special hardware assisted in the calculation of a cyclic redundancy check for detection of transmission errors. Both CRC-16 CCITT and CRC-16 IBM were supported. Assuming the running value was maintained in storage, the execution time to accumulate one more byte was five storage cycles (three instructions).
Rapid context switching was a design objective. The register file was divided into four sections. The three commonly used interrupt levels and the background level had distinct sets of registers. Therefore entry into most interrupt levels did not require saving the registers of the interrupted program. The infrequently used level which processed program and hardware errors shared registers with the next highest level and thus had to save and restore registers.
The five program levels were:
- Error processing
- Communication line events
- Channel adapter events
- Service requests from other levels
- Background level
The 3705 had a flashing light style control panel which was superior to most other IBM panels of that style. The two unusual (for IBM) features were:
- Storage (and some external registers) could be displayed while the program was executing.
- The hex input switches and the two register-sized displays could be used by the program while the machine was running. This capability derived from the objective of emulating the IBM 2703 which had a fairly rich control panel.
Read more about this topic: IBM 3705 Communications Controller
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