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

Device Control Block (DCB)The DCB is an eight word area in storage set up by the user's program and contains theinformation needed to perform a read cycle steal or write cycle steal operation. The formatof the DCB is as follows:0 15Word 0 ID AWord 1. Reserved A + 1Word 2 Reserved A + 2Word 3 Reserved A + 3Word 4 Reserved A + 4Word 5 XI Priority option (bit 0) A +tWord 6 Word count A + 6Word 7 Starting data address A + 7A—the starting address in storage of the DCB. This address is the contents of the index register (R), orthe accumulator if R=000, specified in the read cycle steal or write cycle steal command.IDThe ID word has the following format:0 2 9 11 15F C T S1 1 1 1 1 1 1All fields are the same as for write initialize or read initialize.Priority OptionIf word 5 bit 0 is on at DCB fetch time, a higher priority will be assigned to the device atthe interface multiplexer.Word CountThe word count is the number of words to be transferred during the operation. If theword count is 0 when the DCB is fetched, a normal end interruption occurs immediatelyand the operation is suppressed. The word count is fetched by the subchannel inthe 5022 attachment and is decremented by 1 for each word transferred until aninterruption condition is met. If the data transfer continues successfully until the countequals 0, an interruption occurs indicating the successful end of the operation. If theword count goes to 0 before the end of a sector:Read cycle steal—The remainder of the sector is read (but no data is transferred) and thecheck character is verified prior to the end interruption.Write cycle steal—The remainder of the sector is set to all O's and the appropriate checkcharacter is accumulated and written at the end of the sector prior to the end interruption.Data AddressThe data address is the address of the first word of data to be transferred. This parameteris fetched by the 5022 subchannel which holds it in a register, updates it, and presentsit to the channel during data transfer. On a read operation, the first word from thesector specified in the DCB is stored at this address, the next word from that sector isstored at the next sequential address and so on until the read operation ends. On a writeoperation, the first word in the data table (pointed to by the data address field) is thefirst word written onto the sector specified in the DCB until the operation ends.The read or write operation continues, crossing over sector boundaries, until a terminationcondition is met. As each new sector is detected during the operation, its ID ischecked to ensure that it is, indeed, the next sequential sector.IBM 5022 Disk Storage Module 15-19