IBM System/7 Functional Characteristics - All about the IBM 1130 ...

If fewer than 128 words are to be transferred to the disk storage module, a terminatecommand must be used instead of a write data command. The terminate command preventsthe disk storage module from requesting any more data-service interruption requests.Upon receipt of the terminate command, the disk storage module writes data words containingO's in the remainder of the sector, followed by the corresponding check characterfor the entire 128-word data field.CYCLE STEAL FILE OPERATIONSI/O operations for the disk storage module with cycle steal (CS) are performed by readcycle steal, write cycle steal and seek commands. The read/write commands make invalidthe following DPC commands:Read initializeRead dataWrite initializeWrite dataTerminateIssuing those DPC commands to a CS disk storage module causes bit 1 (commandreject) to be set in the DSW. The other DPC disk commands are also used by the CSfeature.One CS I/O command can read or write up to 24 sectors (3,072 words) ofinformation. The I/O command specifies a register that contains the starting addressof the device control block (DCB). The DCB contains the information needed toperform a read cycle steal or write cycle steal operation. (The contents of the DCB aredescribed later in this chapter under "I/O Commands.")Issuing read cycle steal or write cycle steal commands to a processor without the CSfeature causes a program check.Engineering Note. The addition of the cycle steal feature to the 5022 Disk StorageModule reduces the processor loading factor from 33% to approximately 4%(percentages represent processor loading during data transfer operations with thedisk storage module located in an A2, C3 or C6 enclosure). In addition, the cyclesteal feature reduces the load on the channel and on the I/O interface; therefore,system throughput is increased.Figure 15-4 illustrates the reduction in system loading during full-sector 5022 dataoperations.Mode of operationDPCCS (write cycle steal)CS (read cycle steal)Percent of system bandwidth*A2/C3/C6 EnclosureD3/D6 Enclosure**Processor Channel Processor Channel33%4%4%24%17%17%63%4%4%46%30%45%*Percent of bandwidth consumed during full-sector data transfers.Percentages are approximate and vary depending on the locationof the disk storage module within the enclosure and the user'scoding.**Enclosure located at its maximum distance of 200 ft.Figure 15-4. System loading—full-sector operations15-8 GA34-0003