Avoiding Synonym Problems - Hitachi SH7709S Hardware Manual

3.4.4

Avoiding Synonym Problems

When a 1-kbyte page is recorded in a TLB entry, a synonym problem may arise. If a number of
virtual addresses are mapped onto a single physical address, the same physical address data will be
recorded in a number of cache entries, and it will not be possible to guarantee data congruity. The
reason why this problem only occurs when using a 1-kbyte page is explained below with reference
to figure 3.11.
To achieve high-speed operation of the SH7709S cache, an index number is created using virtual
address bits 11–4. When a 4-kbyte page is used, virtual address bits 11–4 are included in the
offset, and since they are not subject to address translation, they are the same as physical address
bits 11–4. In cache-based address comparison and recording in the address array, since the cache
tag address is a physical address, physical address bits 31–10 are recorded.
When a 1-kbyte page is used, also, a cache index number is created using virtual address bits 11-4.
However, in case of a 1-kbyte page, virtual address bit (11, 10) is subject to address translation
and therefore may not be the same as physical address bit (11, 10). Consequently, the physical
address is recorded in a different entry from that of the index number indicated by the physical
address in the cache address array.
For example, assume that, with 1-kbyte page TLB entries, TLB entries for which the following
translation has been performed are recorded in two TLBs:
Virtual address 1 H'00000000 → physical address H'00000400
Virtual address 2 H'00000400 → physical address H'00000400
Virtual address 1 is recorded in cache entry H'00, and virtual address 2 in cache entry H'C0. Since
two virtual addresses are recorded in different cache entries despite the fact that the physical
addresses are the same, memory inconsistency will occur as soon as a write is performed to either
virtual address. Therefore, when recording a 1-kbyte TLB entry, if the physical address is the same
as a physical address already used in another TLB entry, it should be recorded in such a way that
physical address bit (11, 10) is the same.
Note: In readiness for the future expansion of the SuperH RISC engine family, we recommend
that, when multiple sets of address translation information are mapped onto the same
physical area of memory, you set the VPN numbers so that each VPN [20:10] is equal to
the others. We also recommend that you do not map multiple sets of address-translation
information that include 1- and 4-Kbyte pages to a single physical area.
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