454 Part 5 The PMS level Section 3 Computers for multiprocessing and parallel processing
difficult. The cost of smaller units of memory is offset by the lessening of catastrophe in the event of failure of a module.
I/O control module. The I/O control module executes I/O operations defined and initiated by computer module action. In keeping with the system objectives, I/O control modules are not assigned to any particular computer module, but rather are treated in much the same way as memory modules, with automatic resolution of conflicting attempted accesses via the switching interlock function. Once an I/O operation is initiated, it proceeds independently until completion.
I/O action is initiated by the execution of a transmit I/O instruction in one of the computer modules, which delivers an I/O descriptor word from the addressed memory location to an inactive I/O control module. The I/O descriptor is an instruction to the I/O control module that selects the device, determines the direction of data flow, the address of the first word, and the number of words to be transferred.
Interposed between the I/O control modules and the physical external devices is another crossbar switch designated the I/O exchange. This automatic exchange, similar in function to the switching interlock, permits two-way data flow between any I/O control module and any I/O device in the system. It further enhances the flexibility of the system by providing as many possible external data transfer paths as there are I/O control modules.
Equipment complements. A D825 system can be assembled (or expanded) by selection of appropriate modules in any combination of: one to four computer modules, one to 16 memory modules,
Fig. 2. Typical D825 equipment array.
one to ten I/O control modules, one or two I/O exchanges, and one to 64 I/O devices per I/O exchange in any combination selected from: operating (or system status) consoles, magnetic tape transports, magnetic drums, magnetic disc files, card punches and readers, paper tape perforators and readers, supervisory printers, high-speed line printers, selected data converters, special real-time clocks, and intersystem data links.
Figure 2 is a photograph of some of the hardware of a completed D825 system. The equipment complement of this system includes two computer modules, four memory modules (two per cabinet), two I/O control modules (two per cabinet), one status display console, two magnetic tape units, two magnetic drums,