A formal description of SYSTEM/360

for assistance in the early stages; to IBM SYSTEM/~~O architectsG. A. Blaauw and A. Padegs for the patient exposition and checkingof countless points; to J. C. McPherson, Director of the IBMSystems Research Institute for an opportunity to present thematerial in a course; and to members of the Institute class forhelpful comments and corrections.CITED REFERENCES AND FOOTNOTES1. IBM SYSTEM/360 Principles of Operation, Form A22-6821-0, IBM GORP.,1964.2. The present description is carefully checked against the specifications ofIBM SYSTEM/~~O available in August, 1964. It is, however, not official;because of normal revisions of the system and possible errors in thepresent paper, the machine manuals issued by IBM will, as usual, be thefinal arbiters of system specifications.3. The programs and tables have been reprinted on large display sheets tofacilitate study. Copies may be obtained on request to the ZBM SystemsJournal.4. K. E. Iverson, A Programming Language, John Wiley and Sons, 1962.5. A. D. Falkoff, “Algorithms for Parallel-Search Memories”, Journal ofthe ACM 9, No. 4, 488, (October, 1962).6. K. E. Iverson, “Programming notation for systems design”, ZBM SystemsJournal 2, 117, (June, 1963).7. To provide complete freedom in the attachment of devices in SYSTEM/~~O,the hardware for the interface has been standardized with respect to bothelectrical characteristics and logical behavior. The correspondence betweenthis hardware and the representation used here is as follows. The hardwarehas a separate bus and separate tags for each direction of data flow,and therefore has no need for V;. There is an exact correspondencebetween physical control lines and each of e, $, 6, and PC. The hardwarehas, in addition, two lines not represented by iF or PC. A line called“hold-out” is not needed here because its function has been incorporatedin the treatment of PC, which differs in detail from the hardware selectout;and a line called “request-in” appears in lines 37 and 38 of CH onlyas a logical function of the variables Vs and VI2 which are set by thecontrol units.8. The hardware analog of this formulation is derived by identifying witheach channel number available to a control unit a wire from it to theappropriate channel. Within each channel all such wires are ored together.Putting a number in Vy2 and setting V: then corresponds to selecting anoutgoing wire in the control unit and signalling on it.9. “If [a] device is currently communicating over the I/O interface, thedevice immediately disconnects from the channel. Data transfer and anyoperation using the facilities of the control unit are immediately terminated,and the 1/0 device is not necessarily positioned at the beginningof a block. Mechanical motion not involving the use of the control unit,such as rewinding magnetic tape or positioning a disk access mechanism,proceeds to the normal stopping point, if possible. The device remainsunavailable until the termination of mechanical motion or the inherentcycle of operation, if any, whereupon it becomes available. Status informationin the device and control unit is reset, and no interruption conditionis generated upon completing the operation.” (Ref. I, p. 90).10. Like machine checks in other parts of the system, which depend uponsensing the parity in various machine registers and date paths, thegeneration of Sg is not shown in this description.1 11. This reset signal is detected only by a device that is connected to thechannel at the time, and will cause this device and its control unit toreact exactly as for a system reset (see note 9), except that an interruptioncondition m y be generated when a mechanical operation is completed.FORMAL DESCRIPTION OF SYSTEM/360 239