Abstract: Self-test techniques for checking driver circuits connected to a bus are described that particularly involve the detection and isolation of failures in off-chip-drivers and connections.

Abstract: Data transfers between input/output (I/O) devices and a central processing unit (CPU) take place under instruction or base cycle steal control a byte at a time where the I/O device attachments connect to ports and the ports connect to the CPU. Data transfer can be synchronous or asynchronous. The port involved in the data transfer sends out a device address and command information simultaneously on port data bus out and command bus out, respectively, to the I/O attachments. The addressed I/O device can respond any time within a predetermined time interval. If an I/O device does not respond within the time interval, a blast condition generated by the port causes the I/O attachments to clear the busses between it and the port. During execution of an I/O instruction, the CPU clock is first held in a particular time state while phase clocks and port clocks continue to run and synchronization between the port and I/O attachment is taking place.

Abstract: The first time period of the instruction fetch cycle is eliminated when fetching the branch to instruction in a computer system operating in a non-overlap mode. Whenever a branch instruction is decoded, the storage address register (SAR) is directly loaded during execution of the branch instruction with certain bits from a storage data register (SDR) concatenated with certain bits from an operand register to form the branch to address in SAR. The instruction counter is incremented in the usual manner but the incremented address is not loaded into SAR. The clock is advanced to the second rather than the first time state of the next instruction fetch cycle. Thereafter, the branch to address which is residing in the operand register, is incremented and loaded into the instruction counter.

Abstract: Control circuitry in a computer system is responsive to an allow cycle steal signal from an I/O attachment operating in a burst or dedicated data transfer mode and generates control signals whereby the next data storage cycle is made available to an I/O device which is also capable of operating in a cycle steal mode. Upon completion of the next storage cycle, the operation reverts to burst mode and the I/O attachment operating in the burst mode is granted ensuing data storage cycles until it relinquishes a storage cycle to an I/O device capable of using and having a need for it.