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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
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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 MODE bit in register 8. In the unmapped
state the MMU presents logical address bits LA23 through LA 12 directly to
the physical address bits PA23 through PA12.
Independent of the mapping of User state, the registers of the MMU
are accessible for reading or writing ~ from Supervisor state.
The registers LARO through LAR63 (located at byte offsets $00000
through $3FOOO) contain the logical addresses of the 64 pages to be
mapped into the physical pages. The registers PARO through PAR63
(located at offsets $40000 through $7FOOO) contain the corresponding
physical addresses and access protection bits. Setting WP -= 1 will cause a
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Big Mac MMU Specification May 15,1985
. violation If a write is pE; normed to the particular page. Setting E P -1 will
cause a violation if an y'thing other than a fetch (i.e. a read or write) is
performed to the particular page.
Status registers 0 through 3 (located at byte offsets $80000
through $83000) contain the bits corresponding to page accesses made to
the 64 physical pages defined by the physical address registers 0 through
64. Note the page addressed by PARO corresponds to bit 0 of status
register 0 and the page addressed by PAR63 corresponds to bit 15 of
status register 3. Any access to a physical memory page (while in mapped
User state) will set the appropriate bit in the status registers 0 through 3.
These status registers are intended for use by a Least Recently Used page
replacement algorithm. (These registers can be periodically read by the
CPU, any set bit increments its associated usage counter in main memory.)
Writes to byte offset $80000 through $83000 can set and clear the bits in
these registers individually. while a write operation to byte offset
$84000 will clear all bits in status registers 0 through 3.
Status registers 4 through 7 (located at byte offsets $85000
through $88000) contain the bits corresponding to page writes made to the
64 physical pages defined by the physical address registers 0 through 63.
Note the page addressed by PARO corresponds to bit 0 of status register 4
and the page addressed by PAR63 corresponds to bit 15 of status register
7. Any write to a physical memory page (while in mapped user state) will
set the appropiate bit in these status registers 4 through 7. These status
registers are intended for use in implementing a page replacement policy
i.e. these registers reflect whether or not a page intended for replacement
has been written to (ie. dirtied) since it was brought into mapped memory.
Writes to byte offset $85000 through $88000 can set and clear the bits in
these registers individually, while a write operation to byte offset
$89000 will clear all bits in status registers 4 through 7.
Status register 8 (located at byte offset $8AOOO) contains the
information necessary to determine the cause of an MMU exception; WP
being set for violation of write protection and EP being set for violation of
execute only protection (i.e. a write or a read that is not a fetch). It also
contains the MMU mode bit and the index of the register that caused the
access violation. The bits in the register are ordered to optimize the
search for the exception cause by the use of the MC68020 BFFFO (bit field
find first one) instruction. In the case of a translation miss, the cache
address field will hold an undefined value. When the MMU mode bit is set
to a 1, the MMU will be forced into the unmapped state. with the logical
address bits LA23 through LA 12 passed directly to the physical address
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Big Mac MMU Specification May 15, 1985
4 bIts PA23 through PA12.
Status register 9 (located at byte offset $8BOOO) Is the logical
address latch which holds the logical address of the access violation.
When a physical address register Is written, the corresponding logical
address register will be updated with the contents of the logical address
latch.
Signal Summary:
Type Description
LA12-L.A31 Input Logical address used by the MMU for translation
t> physical ackjresses.
PA12-PA23 Translated ackiresses
012-031 110 Oatabus
lAS Input Logical address strobe.
PAS Physical address strobe.
SlZEO.8lZE1 Input VlOred by MMU.
DSACKD,DSACK1 Tristate MMU drives both lines low when a status
register is aocessed.
Input Iglored by MMU.
Input When asserted the MMU win force an physical
address and data ou1puts k> tristate.
Input VX>red by MMU.
Input When asserted, forces the MMU Into the
tJlI1laRl9d ~.
Input Inclcates val"d data on data bus.
Input Iglored by MMU.
Input System clock.
Tristate Driven low by the MMU when an exception
occurs.
ANI Input Indicates read or write.
FCO,FC1,FC2 Input CPU function codes (see MC68020 manual).
FC3 Input VX>red by MMU.
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Big Mac MMU Specification May 15,1985
BI6 HAC HMU PROGRAttMER-S HODEL
IDJ1 DIO 1>11 1>12
LARO [LUt I LUO I