Intel iapx 432 manual Overiay

00 meaning for windows 2-4, which suWOrt random transfers to 432 system memory only; the random transfer mode is described in section 3-2. Attempting to set the transfer mode of windows 2-4 will cause a fault.

OVERIAY

Sane Peripheral Subsystems (e.g., those based on processors with limited address spaces) may not be able to dedicate a block of memory space for exclusive use as IP window subranges. Such systems may elect to co-Iocate all or part of the IP' s range with real PS memory. If a window is then opened with the over lay attr ibute, the IP will inhibit the co-located memory fran responding to memory references in the subrange. Closing a window that overlaid menory re-enables the memory to respond to subsequent address references in that subrange. Thus, when the IP and PS merrory both occupy the same addresses, memory will respond to all references except those that fall in the subrange of a window open with the overlay attribute.

Figure 3-1 illustrates a hypothetical configuration in which a bank of memory and an Interface Processor both occupy a 64K byte block of addresses in the Peripheral Subsystem memory space. A window with a subrange base address of 32K and a subrange size of 4K has been opened with the overlay attribute set. Any address reference falling in the subrange will cause the IP to respond rather than the co-Iocated memory. Any address reference outside the subrange will select the memory rather than the IP.

The overlay facility is ~lemented by an inhibit signal that the IP asserts when it recognizes an address reference that falls in an

overlaid subrange. (See the iAPX 43203 Interface Processor Data Sheet, Order No. 171874, for a description of this signal). Use of the overlay facility slows IP response time somewhat.

Note that opening a window with the overlay attribute set when there is 00 co-located memory is safe, but it slows IP response

unnecessarily. On the other hand, opening a window without specifying overlay when there is co-Iocated memory will produce an undefined result when both components attempt to respond to a subsequent address reference that falls in the overlaid subrange.

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