Abstract: A disk drive of a mass storage subsystem includes areas on a disk surface wherein a vendor-generated defective sector log, a software-generated defective sector log and an alternate sector log are stored. A random access memory (RAM) stores a copy of the defective sector logs. During a seek operation, firmware tests the defective sector logs in RAM to generate the alternate sector log for that cylinder number. During the read or write operation, the alternate sector log is checked before processing the sector to determine if it is a defective sector. If the sector is defective, the head is positioned to another cylinder at a head and sector address read from the alternate sector log.

Abstract: A track on a disk surface of a disk drive is formatted in sectors, each sector having an address portion and a data portion. The disk drive generates a byte clock signal which increments a counter. The counter output signals address a read only memory which generates signals to control the address comparison in the address portion and the reading or writing of data bytes in the data portion of the sector.

Abstract: A track of a disk device is formatted on a single revolution of the disk by using a read only memory (ROM) to store control codes and a random access memory (RAM) to store address field and data field bytes. A DMA controller simultaneously addresses ROM and RAM. Control codes are read into a control first in-first out memory and data codes are read into a data first in-first out memory. The control codes are applied to a decoder whose output signals control cyclic redundancy check and error detection and correction logic as well as the data first in-first out memory. The serial output from both the data first in-first out memory and the cyclic redundancy check logic are written on disk track.

Abstract: An error detection system is disclosed for not only indicating but eliminating certain errors which may occur during the transfer of information between communication busses in a data processing system wherein plural communication busses each provide a common information path to plural data processing units including memory units, peripheral control units, central processing units and ISL units, and wherein each of the plural communication busses are in electrical communication with an ISL unit, and ISL units are electrically connected in pairs. The error detection system requires no special supporting software or firmware on the part of any data processing unit on any of the communication busses.

Abstract: A logic system is provided for accommodating the exchange of information between two or more communication busses of a data processing system, wherein plural central processing units and plural memory units on independent communication busses may have same logic addresses. Memory and CPU addresses are translated at the bus rate through a multiplicity of flexible address translation ranges to enable a data processing unit on one communication bus to access an apparent contiguous range of addresses encompassing all data processing units on all communication busses.

Abstract: An intersystem communication control system in an intersystem link (ISL) unit is provided to accommodate the simultaneous bidirectional transfer of binary coded information between communication busses in a data processing system, wherein the plural communication busses are electrically interconnected by ISL unit twins, and information may be transferred between plural communication busses asynchronously.

Abstract: A logic control system in an intersystem link (ISL) unit accommodating the transfer of binary coded information between communication busses in a data processing system is disclosed, wherein information may be transferred between plural communication busses while further information flow continues on each communication bus at the bus rate, and additional information transfers between the communication busses continue to be handled by the ISL unit.

Abstract: A logic control system in an intersystem link (ISL) unit accommodating the transfer of binary coded information between communication busses in a data processing system is disclosed, wherein an ISL unit may be reconfigured to reallocate communication bus resources without incurring excessive software overhead time losses.

Abstract: A logic control system is disclosed for verifying the operability of memory and non-memory data and control paths in both local and remote intersystem link (ISL) units electrically interconnecting a local and remote communication bus in a data processing system. The data processing system may include two or more communication busses each pair of which are electrically interconnected by twin ISL units. The control logic architecture accommodates the receipt of a test mode command from a CPU on a local bus to initiate a test mode operation wherein the memory and non-memory data and control paths of both the local and the remote ISL units are excerised while on-line, and binary coded information received from the local bus is passed through the ISL units, onto the remote bus, and returned to a local bus memory resource for verification. No remote bus resources are used or affected, and the remote ISL unit shall ignore any communications received from any other data processing unit on the remote bus.

Abstract: A logic system in an intersystem link (ISL) unit accommodating the transfer of binary coded information between communication busses in a data processing system is disclosed, wherein dedicated locations in a file register are selected at the bus rate in response to binary coded information received from a local communication bus. ISL transactions to be initiated in response to bus cycle requests thereby are identified. ISL transactions are handled in parallel, and memory transfers are segregated from non-memory transfers to avoid unnecessary delays in memory transfers.

Abstract: A logic system in an intersystem link (ISL) unit is provided for avoiding deadlock conditions which may occur in a data processing system wherein multiple CPUs on one communication bus attempt to communicate with resources on remote communication busses.The data processing system has plural communication busses, each providing a common information path to plural data processing units including memory units, peripheral control units, central processing units (CPUs) and ISL units, and each of the plural communication busses are in electrical communication with an ISL unit, and ISL units are electrically connected in pairs.