Abstract: Disclosed is a hardware lock unit for limiting concurrent use of shared resources, such as segments of a memory, by a plurality of devices, such as processors, in a program controlled system. In such a system, devices wishing to use a shared resource make a use request to the lock unit by means of a memory READ command accompanied by an address that is associated in the lock unit with that resource, which each requesting device sends to the lock unit. The requesting devices then wait for an answer, as from a slow memory. The hardware lock unit determines whether the resource is free for access by a device, or whether the resource is busy being accessed by a device. When the resource is busy, the requesting devices are caused to wait. When the resource becomes free, the lock unit hardware selects one of the requesting devices and responds to its request to grant it exclusive use of the resource.