Chapter 43 The structure of SYSTEM/360 601
I/O operation, or by operator intervention at the I/O device, enable the CPU to provide appropriate programmed response to conditions as they occur in I/O devices or channels. Conditions responsible for I/O interruption requests are preserved in the I/O devices or channels until recognized by the CPU.
During execution of START I/O, a command can be rejected by a busy condition, program check, etc. Rejection is indicated in the condition code of the PSW, and additional detail on the conditions that precluded initiation of the I/O operation is provided in a CSW.
The need for manual control is minimal because of the design of the system and supervisory program. A control panel provides the ability to reset the system; store and display information in main storage, in registers, and in the PSW; and load initial program information. After an input device is selected with the load unit switches, depressing a load key causes a read from the selected input device. The six words of information that are read into main storage provide the PSW and the CCW's required for subsequent operation.
The SYSTEM/360 instructions, classified by format and function, are displayed in Table 2. Operation codes and mnemonic abbreviations are also shown. With the previously described formats in mind, much of the generality provided by the system is apparent in this listing.