Browse Prior Art Database

I/O Sharing Mechanism in a Dual CPU System

IP.com Disclosure Number: IPCOM000089266D
Original Publication Date: 1977-Oct-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Pauporte, A: AUTHOR [+2]

Abstract

In conventional systems with two processors sharing the same I/Os (inputs/outputs), each I/O adapter is generally connected to two busses (one per processor). This may be a drawback whenever the I/O adapters are to be constructed by using LSI (large-scale integration) technologies, since each adapter then requires twice as many output pins as required for connecting it to a single bus.

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I/O Sharing Mechanism in a Dual CPU System

In conventional systems with two processors sharing the same I/Os (inputs/outputs), each I/O adapter is generally connected to two busses (one per processor). This may be a drawback whenever the I/O adapters are to be constructed by using LSI (large-scale integration) technologies, since each adapter then requires twice as many output pins as required for connecting it to a single bus.

In the present arrangement, the number of pins is halved by connecting the two CPUs (central processing units) (A and B) to the I/O adapters through the same bus and by providing means for protecting each I/O against any access from other CPUs while it is operating with one of the CPUs. Each I/O may either be owned by CPU (A) or by CPU (B) or be free. Each CPU stores an OCW (ownership control work) with one-bit position per I/O adapter. This bit position is raised to one whenever the adapter is owned by the CPU.

Then, whenever a CPU (e.g., CPU (A)) wants access to an I/O, a special procedure is initiated. CPU (A) starts by testing its own OCW to check that it does not already own the requested I/O. In case of a positive answer, CPU (A) requests attention from CPU (B), and then executes a read memory instruction performing a memory (B)-to-memory (A) transfer of OCW (B). Then OCW (B) is tested to check whether CPU (B) owns the requested I/O. In case of positive test, CPU (A) gets a negative answer to its I/O access request. But if OCW (B) in...