IBM System/360 architecture: Difference between revisions

Revision as of 19:11, 23 January 2025

Revision as of 10:24, 25 January 2025

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* {{anchor|IBM_Byte_Mux}}A '''byte multiplexer channel''' is capable of executing multiple '''CCW'''s concurrently; it is normally used to attach slow devices such as card readers and telecommunications lines. A byte multiplexer channel could have a number of selector subchannels, each with only a single subchannel, which behave like low-speed selector channels.

* {{anchor|IBM_Sel}}A '''selector channel''' has only a single subchannel, and hence is only capable of executing one channel command at a time. It is normally used to attach fast devices that are not capable of exploiting a block multiplexer channel to suspend the connection, such as magnetic tape drives.

* {{anchor|IBM_Blk_Mux}}A '''block multiplexer channel''' is capable of concurrently running multiple channel programs, but only one at a time can be active. The control unit can request suspension at the end of a channel command and can later request resumption. This is intended for devices in which there is a mechanical delay after completion of data transfer, e.g., for seeks on moving-head DASD. The block multiplexer channel was a late addition to the System/360 architecture; early machines had only byte multiplexer channels and selector channels. The block multiplexer channel wasis an optional feature only on the models 85 and 195. The block multiplexor channel wasis also available on the later [[IBM System/370|System/370]] computers.

Conceptually peripheral equipment is attached to a S/360 through ''control units'', which in turn are attached through channels. However, the architecture does not require that control units be physically distinct, and in practice they are sometimes integrated with the devices that they control. Similarly, the architecture does not require the channels to be physically distinct from the processor, and the smaller S/360 models (through 360/50) have integrated channels that [[Cycle stealing|steal cycles]] from the processor.

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Control units are assigned an address "capture" range. For example, a CU might be assigned range 20-2F or 40-7F. The purpose of this is to assist with the connection and prioritization of multiple control units to a channel. For example, a channel might have three disk control units at 20-2F, 50-5F, and 80-8F. Not all of the captured addresses need to have an assigned physical device. Each control unit is also marked as High or Low priority on the channel.

Device selection progresses from the channel to each control unit in the order they are physically attached to their channel. At the end of the chain the selection process continues in reverse back towards the channel. If the selection returns to the channel then no control unit accepted the command and SIO returns Condition Code 3. Control units marked as High Priority check the outbound CUU to be within their range. If so, then the I/O wasis processed. If not, then the selection wasis passed to the next outbound CU. Control units marked as Low Priority check for inbound (returning) CUU to be within their range. If so, then the I/O is processed. If not, then the selection is passed to the next inbound CU (or the channel). The connection of three controls unit to a channel might be physically -A-B-C and, if all are marked as High then the priority would be ABC. If all are marked low then the priority would be CBA. If B was marked High and AC low then the order would be BCA. Extending this line of reasoning then the first of N controllers would be priority 1 (High) or 2N-1 (Low), the second priority 2 or 2N-2, the third priority 3 or 2N-3, etc. The last physically attached would always be priority N.

There are three storage fields reserved for I/O; a double word I/O old PSW, a doubleword I/O new PSW and a fullword ''Channel Address Word'' ('''CAW'''). Performing an I/O normally requires the following: