Motorola MC68030 User Manual page 311

The page size (PS) field of the TC register specifies the page size for the
system. The number of pages in the system is equal to the logical address
space divided by the page size. The maximum number of pages that can be
defined by a translation tree is greater than 16 million (2
32
/2
8 ).
The minimum
number is 4 (2
17
/2
15 ).
The function code can also be used in the table lookup,
defining as many as seven regions of the above size (FC
=
0-6). The entire
range of the logical address space(s) can be defined by translation tables of
many sizes. The MC68030 provides flexibility that simplifies the implemen-
tation of large translation tables.
The use of a tree structure with as many as five levels of tables provides
granularity in translation table design. The LIMIT field of the root pointer can
limit the value of the first index and limits the actual number of descriptors
required. Optionally, the top level of the structure can be indexed by function
code bits. In this case, the pointer table at this level contains eight descriptors.
The next level of the structure (or the top level when the FCL bit of the TC
register is set to zero) is indexed by the most significant bits of the logical
address (disregarding the number of bits specified by the IS field). The num-
ber of logical address bits used for this index is specified by the TIA field of
the TC register. If, for example, the TIA field contains the value 5, the index
for this level contains five bits, and the pointer table at this level contains at
most 32 descriptors.
Similarly, the TIB, TIC, and TID fields of the TC register define the indexes
for lower levels of the translation table tree. When one of these fields contains
zero, the remaining Tlx fields are ignored; the last nonzero Tlx field defines
the index into the lowest level of the tree structure. The tables selected by
the index at this level are page tables; every descriptor in these tables is (or
represents) a page descriptor. Figure 9-6 shows how the Tlx fields of the TC
register apply to a function code and logical address.
LOGICAL AOORESS
FC
IS
_I
'TlA
_I
'TlB
_I
>TIC
_I
>TID
+
PS
~ I~
____
~
___
A __
~
____
~
____
~
____
~
_______ OF_FSE_T ____
~
31
Figure 9-6. Derivation of Table Index Fields
MOTOROLA MC68030 USER'S MANUAL 9-9