Teknor Industrial Computers PCI-993, Техническое справочное руководство

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System 6-16
6.05 CACHE
There are two separate Caches in the Host subsystem: Internal Cache and External
Cache. The Cache inside the Pentium processor (Internal Cache) is referred to as the
first level Cache (also primary Cache). The External Cache (called System Cache in this
manual) comprises the System Controller’s Cache control circuitry and associated
external memory array; it is referred to as the second level Cache (also secondary
Cache). The second level Cache is unified, which means that both CPU data and
instructions are stored in the Cache.
6.05.1 SYSTEM CACHE
The System Controller integrates a high performance write-back / write-through second
level Cache controller, tag RAM and a full first and second level Cache coherency
mechanism. The System Controller supports a direct-mapped secondary Cache.
System Cache is located at the back of the card at U34, U35, U44, U45, U54, U55, U67
and U68 (#1) on Diagram 6-1; it accepts a total of 256KB standard asynchronous
SRAM.
System Cache copies the most recently accessed data and places it in an area of high
speed memory called SRAM (Static RAM). Cache SRAM is positioned between
System Memory (DRAM) and the CPU. Data is transferred from System Memory to
Cache and then from Cache to the CPU.
Since most program executions are sequential and repetitive, the likelihood is great that
the CPU will find data already stored in either Cache. When the CPU retrieves data
from Cache, a
Cache hit occurs. On the other hand, when the CPU must access data
from System Memory, a Cache miss occurs.
System Cache is enabled and configured in the AMIBIOS Setup program (see Section
15).