xerox 19771107 Adding New Devices To The Channel Interface free download

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Inter-Office Memorandum DRAFT - DRAFT - DRAFT - DRAFT To Pilot interest Date Novem ber 7, 1977 From Paul McJones Location Palo Alto Subject Adding new devices to the Channel interface Organization SO~/SO XEROX XEROX SDD ARCR1VES _ I have .roaCt and undo1"::;tood. Filed on: [lfs]Channel.memo rag es __,_.---'fo - , - - - - - 'f'I _ _ __ 1h~e Reviewer_----- # of Pages _ __ Ed. -t1SDQ-.:2 feJ4 The problem The Channel interface in the second draft of the Pilot Functional Specification has the property that adding a new device requires editing and recompiling the Channel DEFINITIONS module, and thus all the modules which include it. This is in conflict with the desire not to recompile the system except for the semiannual releases (assuming new devices are to be added between releases). Thus we propose a revised Channel interface, which, it is claimed, solves the recompilation problem. The problem with the current interface stems from its dependence on the enumerated type Channel.DeviceType, which has a value for each implementation of the interface. Adding a new implementation requires adding a new value to DeviceType, and also adding corresponding variants to the definitions of the DeviceType-tagged variant record types DeviceSelection, PhysicalRecordStatus, DeviceCommand. DeviceStatus, and "Rep" (the private record to which a Handle points). (It is worth noting that these variant records constitute a nuisance for both client and implementor. Most values crossing the interface must be wrapped in a variant record constructor, and then "unwrapped" with a one-armed WITH statement essentially doing at runtime what Mesa is meant to do at compile time.) A solution Half of the operations in the current interface do not depend on DeviceType. and so are not a problem: Delete Suspend Restart GetPhysicalRecord PutPhysicalRecord Those which do depend on DeviceType are: Create (accepts DeviceSelection) WaitPhysicalRecord (retu rns PhysicalRecordStatus) Comm