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

Sector Identifier FormatEach identification area recorded on a sector contains six 8-bit bytes—three bytes of flagsand address information and three bytes of check information:Flags I AddressCheck charactersF C S CC CC BCAF (flag byte): Contains flagging information in bits 6 and 7 to indicate the condition ofthe track surface. All other bits in this byte should be 0. Flagging information is coveredin more detail under "Flagging," later in this chapter.C (cylinder byte): Contains the binary number corresponding to the physical location ofthe track on the disk.S (sector byte): Contains the number of the sector. Sectors on the top surface of thedisk have sector numbers from 0 through 23. Sectors on the lower surface of the diskhave sector numbers from 32 through 55.CC (cyclic check): Contains an automatically generated check character used by the filecircuits to verify the data field.BCA (bit count appendage): Contains an automatically generated check character usedby the file circuits to verify the entire read or write operation.The address of any individual sector is contained in the first three bytes (F, C, and S)of the sector identifier. This address applies only to the disk on which these bytes arelocated. Sectors occupying the same physical location on all disks have identical binarynumbers in the cylinder and sector bytes. The sectors on the upper surface of the disk arenumbered 0 through 23, starting with the index mark, and the sectors on the lower surfaceare numbered 32 through 55. A specific sector, then, is addressed by specifying theupper or lower disk, a cylinder number, and the sector number.OPERATING AND PROGRAMMING RESTRICTIONSThe disk storage module operates only when (1) the drawer containing the disk file driveis closed, and (2) a disk cartridge is properly installed on the drive (models 1 and 2 only).The drawer cannot be opened unless System/7 power is on and the file ready indicator isturned off. The drawer should be kept closed at all times unless a disk cartridge is beinginserted or removed. A disk cartridge must be stored in the operating environment for atleast two hours before the cartridge is used for processing.On a disk storage module without CS, a data overrun condition occurs if an interruptionrequest for data (called a data-service interruption request) is not servicedwithin 40 microseconds. Although the disk storage module can be prepared to requestinterruptions on any priority level, care should be exercised in assigning a priorityinterruption level to the disk. It is advisable to assign the disk storage module to thehighest priority level in order to avoid a data overrun condition. (If a data overruncondition occurs, data is not lost, but the data transfer rate is reduced due to therotational delay of the disk before the I/O operation can be started again.)On a disk storage module with CS, an overrun occurs when the channel fails torespond in time to a request for service from the module. In a typical operatingenvironment, occurrence of an overrun should be extremely rare. It can occur only whenother modules cause a high level of activity on the channel during a read orwrite operation on the disk storage module. An overrun condition results in terminatingthe operation and presenting an interruption. Overrun (bit 2), any error (bit 3),channel end (bit 11), and device end (bit 13) will be present in the ISW.15-6 GA34-0003