Abstract: A dirty memory for a computer system is configured hierarchically. This provides for more rapid identification of pages of memory that have been dirtied and require attention. For example for the reintegration of an equivalent memory state to the memories of respective processing sets in a fault tolerant computer following a lockstep error. The dirty memory includes at least two levels. A lower level includes groups of dirty indicators, each dirty indicator being settable to a given state indicative that a page of memory associated therewith has been dirtied. At least one higher level includes dirty group indicators settable to a predetermined state indicative that a group of the lower level associated therewith has at least one dirty indicator in a state indicative that a page of memory associated therewith has been dirtied. There can be more that two layers. Logic controls the operation of the hierarchical dirty memory.

Abstract: A dirty memory that includes dirty indicators settable to indicate dirtied pages of memory is provided with control logic operable automatically to interrogate the dirty memory to identify dirty indicators that are set. Implementing the control of the dirty RAM in hardware or firmware enables interrogation of the dirty RAM to identify set dirty indicators in a rapid and reliable manner. The control logic can advantageously be operable to interrogate the dirty memory word-by-word to determine words including a set bit. A comparator can be provided for comparing bits of a word to a predetermined value to determine where a dirty indicator is set. The comparison could be performed serially for bits within a word, but it is advantageously done in parallel for the bits of the word. For example, by using associative memory, the interrogation of the dirty memory could be effected associatively in parallel to determine words including a word with a set bit.

Abstract: A computer system comprising at least two processing sets. Each processing set includes main memory. A bridge connects the processing sets. At least a first processing set further including a dirty memory having dirty indicators for indicating dirtied blocks of the main memory of the first processing set. The bridge includes a direct memory access controller that is operable to copy blocks of the first processing set indicated in the dirty memory to the main memory of another processing set. The processors do not, therefore, need to carry out the copying, whereby the processor overhead associated therewith can be avoided, increasing the efficiency of memory reintegration. The direct memory access controller can be arranged to search the dirty memory for dirty indicators indicative of dirtied blocks. Alternatively, the dirty memory can include control logic operable to search the dirty memory for dirty indicators indicative of dirtied blocks.

Abstract: A memory controller controls access to one or more memory units. The memory controller includes access control logic operable to receive a memory access request that references at least one memory address. It further includes a fake response record operable to record a fake response indication for an address for which a response is to be faked. The access control logic is operable on receipt of a memory access request to access the fake response record and to fake a response where a fake response indication for an address indicates that a response is to be faked. By providing such a faked response, an embodiment of the invention is able avoid multiple exceptions for the same memory location in a CPU. Also, by providing such a faked response, multiple bus errors for a memory location can also be avoided where a bus supports Direct Memory Access (DMA). The memory controller can be implemented in an integrated circuit.

Abstract: Resource access control is provided in a manner that avoids unnecessary resource accesses where a resource is already known to be faulty. The resource can be a memory location, a peripheral or any other addressable system component. A resource access mechanism in a processor controls access to resources. The resource access mechanism includes an address control mechanism having a plurality of address control entries, each address control entry providing fake response identification indicating whether or not a response for the corresponding address is to be faked. The resource access mechanism also includes a fake response generator for selectively generating a faked response for an address in response to the fake response identification of the corresponding address control entry indicating that a response is to be faked.

Abstract: Resource access control is provided in a manner that avoids unnecessary resource accesses where a resource is already known to be faulty. A resource access controller controls access to resources addressed by at least one central processing unit. The resource access controller includes an address translation mechanism providing fake response identification as to whether or not a response is to be faked. The resource access controller also includes a fake response generator for selectively generating a faked response where the fake response identification of the corresponding translation entry indicates that a response is to be faked. The resource access controller is able to associate fake response indications with a resource and to generate a fake response when an attempt is made to access a resource labeled such that a faked response should be returned.

Abstract: To provide efficient resource access control in a computer system, a trap handler for handling a trap in the event of a faulty resource access being detected is arranged to define a diversion for subsequent access attempts to the same resource. An address translation mechanism is responsive to indication of a diversion for a resource access to modify an address mapping, whereby subsequent attempts to access the resource are diverted in accordance with the diversion. The trap handler can be arranged in a conventional manner to process an exception of the first faulty access to the resource. However, by defining the diversion, which can be used to map further attempts to access the same resource, unnecessary exception processing can be avoided.