Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 245

memory is updated indirectly, such as when a modified data in the cache is cast-out by newer data at a
different address that maps to the same cache block). The default is configured by MD_CTR[WTDEF].
Also, when translation is disabled (real mode), the entire memory space is treated as guarded by default.
The implications of this are:
1. Speculative load/store accesses are stalled until they are no longer speculative.
2. Speculative instruction fetches outside of the current real-mode page are stalled until they are no
longer speculative. The size of real-mode page is determined by MI_CTR[PPM]. If
MI_CTR[PPM] = 0, the real-mode page size is 4 Kbytes; if MI_CTR[PPM] = 1, the real-mode
page size is 1 Kbyte.
This behavior can result in significant performance degradation.
8.3.2 Translation Enabled
Translations are generated on a per-page basis and are stored in tables in memory. Along with the
translation, each table entry holds attributes for that page, for example, whether a location is cacheable.
Recently used translations are kept in translation lookaside buffers (TLBs) in hardware. In the MPC885,
software handles the table lookup and TLB reload with little hardware assistance. This offers a flexible
translation table structure choice, because many systems would not benefit from a full-featured hardware
translation mechanism.
A TLB hit in multiple entries is avoided when a TLB is being reloaded. When TLB logic detects that a
new effective page number (EPN) overlaps one in the TLB (when taking into account pages sizes, subpage
validity flags, user/supervisor state, address pace ID (ASID), and the SH values of the TLB entries), the
new EPN is written and the old one is invalidated.
The MMU supports a multiple virtual address space model. Each translation is associated with an ASID,
which must equal the address space ID (CASID) for a translation to be valid.
Freescale Semiconductor
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Memory Management Unit
8-3