SH7708 datasheet(86/633 Pages) RENESAS

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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 3.10.

To achieve high-speed operation of the SH7708 Series cache, an index number is created using

virtual address bits 10–4. When a 4-kbyte page is used, virtual address bits 10–4 are included in

the offset, and since they are not subject to address translation, they are the same as physical

address bits 10–4. 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 10-4.

However, in the case of a 1-kbyte page, virtual address bit 10 is subject to address translation and

therefore may not be the same as physical address bit 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'40. Since

the 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 10 is the same.

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