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Big Mac MMUSpeclflcation Introduction: The Big Mac MMU is a memory management unit intended for use with the Motorola MC68020. The MMU features a 64 entry, fully associative cache that maps 4096 byte pages from the logical address bus of the MC68020 to the physical address bus of main memory. This allows a logical (virtual) memory space of 4 Gbytes; a maximum physical memory space of 16 Mbytes and a 256 Kbyte user working set. The MMU operates on the upper 20 bits of the logical address yielding the upper 12 bits of the phyical address. The lower 12 bits of the logical address bus are passed straight through to the physical address bus. 68020 CPU BERR 32 bit logical address 12 bit page offset 20 bit logical page address ...- -...- - - - - - - -... 12 bit physical. page address BERR 24 bit physical address Main Membry 1 Big Mac MMU Specification May 15,1985 If the upper 20 bits of the logical address are contained in any of the 64 entries in the MMU, the upper 12 bits of the physical address are provided within 65ns. This permits the MMU to function without wait states. If the page Is resident In the MMU. the access control bits associated with that page are also checked to see H write protection and or execute only protection is in effect. If the upper 20 bits of the logical address are DQ1 contained in any of the 64 entries in the MMU, a Bus Error (BERR) signal will be asserted. This will suspend the execution of the instruction that caused the "miss", and system software can then load the MMU with the correct physical address of the page that caused the miss. The suspended instruction can then be restarted. If the logical address is contained in the MMU and a write protection or execute only protection violation is detected, a Bus Error is also generated. Ten 16 bit status registers are implemented within the MMU to allow access to the following information: Page Accessed (status registers 0 through 3 for use in generating LRU info), Page Dirty (status registers 4 through 7), Exception Cause and MMU mode (status register 8) and Logical Address Latch (status register 9). Operation: Supervisor and user states are defined by the function code 2 bit (FC2) of the CPU (with FC2-=1 --> supervisor and FC2-=O _ > user). .. The CPU and MMU combination operate with Supervisor state addresses always unmapped. User state can operate either mapped (by clearing the MODE bit in status register 8), or unmapped. The unmapped state is entered by setting the MOD