IE36475L - Controlling digital data processing systems - Google Patents

Abstract

1389066 Data processing DIGITAL EQUIPMENT CORP 15 March 1972 [15 March 1971] 12183/72 Heading G4C A digital data processing system including a random access memory 12 and one (or more) serial access memory units 26, 28, 32, 36 (e.g. magnetic drums, discs or tapes), queues requests for data transfer between them and selects for execution the request requiring the least delay. in positioning the transducers of the serial access unit. The system comprises a computer having an arithmetic unit 14, an 1/0 controller 16, controller units, 20, 34, 38 and channel units 22, 24. The computer executes a running program and, whenever a . request arises for data transfer between memory 12 and one of the serial access units, an executive routine is entered. The request specifies whether the transfer is from or to memory 12, which serial memory unit is involved and the desired tracks and segments thereof. If the read/write heads have to be repositioned and the controller unit is transferring data, the request is moved to an awaiting positioning queue (in memory 12) for that memory unit. If the controller unit is avaiable the read/write head is positioned. If the head is positioned correctly and the controller unit or data channel unit is transferring data the request is passed to an awaiting transfer queue for the data channel unit. If the head is positioned correctly and the controller and channel units are free, then the request is executed. Every time a head positioning or data transfer operation occurs, the running program is interrupted and the executive routine transfers requests for which the head is correctly positioned to the awaiting transfer queue. In the positioning queues the transfer with the smallest positioning time is selected first while in the other queue the transfer with the smallest latency time (i.e. time to find the desired segment), is selected but, in both cases, transfers are sometimes dealt with on a first come first served basis, or some transfers have priority. An awaiting seeking queue is also described and the system has time sharing facilities. [GB1389066A]