International Computers Limited

Geoffrey Poskitt

Terence M Cole

David P Crane

A data processing unit accesses data in a cache using a virtual address. If the data is not in the cache, the virtual address is translated by a memory management unit (MMU) into a physical address for accessing a main memory. The MMU can also access the cache, using the physical address, to retrieve page table entries held in the cache. This avoids the need for a main memory access to retrieve the page table entries, and hence speeds up the address translation operation. The physically addressed entries in the cache are tagged with a reserved context number to distinguish them from the virtually addressed data.

Data processing system with cache memory addressable by virtual and

A data processing system consists of a number of processing modules and memory modules interconnected by a common bus. If a memory module is not free to accept addresses or data from the bus, it asserts an address wait (AW) signal or a data wait (DW) signal, as the case may be. When a processing module sends an address or data over the bus, it normally holds it there for one clock cycle only. However, if the relevant wait signal AW or DW is asserted, the address or data is held on the bus until this wait signal is removed. This arrangement avoids the need for acknowledgement on the bus, and hence speeds up the transaction of information.

Data processing system with information transfer bus and wait signal

A multi-processor data processing system, comprises a plurality of data processing modules. A set of interrupt lines are connected in parallel to all the processing modules. Each processing module has a priority level, and the lowest priority module is selected to act as an interrupt handler, with responsibility for handling all balanced interrupts. Whenever a processing module changes it priority, it clocks to see whether the interrupt handler is still the lowest priority module and, if not, causes the role of interrupt handler to be transferred.

Interrupt handling in a multi-processor data processing system

A multi-cache data storage system has a number of cache units and a main memory. The caches are addressed by a virtual address. When data is updated in one of the caches, the virtual address is translated into a physical address and sent to the main memory over a bus, along with the updated data value. Each cache continuously monitors the bus for updates from other caches and checks whether it holds a data item corresponding to the physical address. If so, the data item is updated or invalidated, so as to ensure cache coherency.

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Camberley, United Kingdom

Geoffrey Poskitt