IBM 5280 Distributed Data System - Index of

The main microprocessor checks the partition lOB pointer D until it finds apointer that indicates that a program is loaded in the partition. If there are noactive attention lines pending, the main microprocessor goes to the address indicatedin the partition lOB pointer fJ. The first 256 bytes of the partition holdsthe partition lOB II .The partition lOB contains such information as the partition size and the addressof the object code instruction to execute next. When the main microprocessorenters the partition, it sets a timer. This timer controls how long the main microprocessoris to remain within the partition. The main microprocessor then goes tothe object code instruction address in the partition storage area II. It executesinstructions in the storage area until the time limit is up or until it encounters anonoverlapped I/O instruction. If the timer times out, the main microprocessorcompletes the execution of the instruction it is currently working on, returns to thepartition lOB and stores the address of the next instruction to execute when itreturns to this partition, and goes back to the system control block. If no activeattention lines are pending, it continues checking the partition lOB pointers; whenit finds a partition lOB pointer that indicates that a program is loaded in the partition,it goes to that partition and performs the same steps as described above.If the main microprocessor encounters an I/O instruction before the timer timesout, it uses the data set number specified in the instruction as an index into thelogical I/O table II. It goes to the appropriate entry in the logical I/O table tofind the address II of the device lOB that describes the I/O operation. The mainmicroprocessor then goes to the device lOB II, loads the instruction into thelOB, and activates the device attention line to the appropriate I/O device. If theI/O instruction specified overlapped I/O, the main microprocessor continues executinginstructions within the partition while the I/O device is performing theI/O. If the instruction specified nonoverlapped I/O, the main microprocessor exitsthe partition. The instruction following the I/O instruction is not executed untilthe I/O instruction is completed by the device.When a device microprocessor senses an active device attention line, it checks thedevice lOB pointers II in the system control block until it finds a pointer thatcontains an lOB address. It then goes to the address 0 and performs the workdescribed in the lOB. The lOB contains the instruction op code and parameters,the address of the I/O buffer or buffers, and other information such as formataddresses and data set type. When the device microprocessor encounters a conditionthat it cannot handle, it clears the first two bits of the status byte and sets theexternal status bit in the status byte of the device lOB, and activates an attentionline to the main microprocessor. If the device microprocessor finishes the I/Owork in a normal way, it clears the first 2 bits of the status byte in the device lOB.The device microprocessor then checks the device lOB to determine the address ofthe next lOB on the lOB chain for this device 1m. It processes the lOBs on thechain until it encounters an lOB that is marked as the first on the chain. Exceptfor the printer microprocessor, which has only one lOB pointer, the device microprocessorthen returns to the system control block and checks the next devicelOB pointer. If it finds another device lOB pointer that contains an lOB address,it goes to the lOB and uses the I/O device associated with the lOB pointer toprocess the lOB chain as described above.5280 Functions and Storage Overview 25