Request forwarding system

Abstract

Storage access requests are forwarded from plural input/output channels to shared main storage. An address word in each request designates the identity of the source channel (CHID) and 'destination' address (of a doubleword space in storage relative to which one, two or four 'data' words shall be transferred). EOT tag signal provides demarcation of requests and also uniquely identifies '1-wide' input (Store) requests. Quadword (QW) tag, presented with '4-wide' requests, enables the storage access system to use a single address in the request to locate two contiguous doubleword spaces in storage. Data tags (D1, D2), presented on a selective basis enable the access system to selectively steer (reorder the positions of) data words in an input request relative to word halves of the addressed space. Consequently the data repositioning function associated with Read Backward operations does not require reordering of data words in the channel buffers (saving handling time and expense of 'steering' circuits in the individual channels). In association with a '1-wide' input request a single data tag, D1 or D2, is used to steer the single data word of the request into either half of the addressed space on a selective basis.