Specifications

Computer Architecture and Maintenance (G-Scheme-2014)
Note : Multi-core processors include separate L1 caches for each processor core. Also,
L1 cache is divided into equal amounts for instructions and data.
To understand the importance of cache, you need to know the relative speeds of
processors and memory. The problem with this is that processor speed usually is
expressed in MHz or GHz (millions or billions of cycles per second), whereas memory
speeds are often expressed in nanoseconds (billionths of a second per cycle). Most
newer types of memory express the speed in either MHz or in megabyte per second
(MB/s) bandwidth (throughput).
Both are really time- or frequency-based measurements. You will note that a 233
MHz processor equates to 4.3-nanosecond cycling, which means you would need 4 ns
memory to keep pace with a 200 MHz CPU. Also, note that the motherboard of a 233
MHz system typically runs at 66 MHz, which corresponds to a speed of 15 ns per cycle
and requires 15 ns memory to keep pace. Finally, note that 60 ns main memory
(common on many Pentium-class systems) equates to a clock speed of approximately
16 MHz. So, a typical Pentium 233 system has a processor running at 233 MHz (4.3 ns
per cycle), a motherboard running at 66 MHz (15 ns per cycle), and main memory
running at 16 MHz (60 ns per cycle). This might seem like a rather dated example, but
in a moment, you will see that the figures listed here make it easy for me to explain
howcache memory works.
Because L1 cache is always built into the processor die, it runs at the full-core
speed of the processor internally. By full-core speed, I mean this cache runs at the
higher clock multiplied internal processor speed rather than the
external motherboard speed. This cache basically is an area of fast memory built into
the processor that holds some of the current working set of code and data. Cache
memory can be accessed with no wait states because it is running at the same speed as
the processor core.
Cache is even more important in modern processors because it is often the only
memory in the entire system that can truly keep up with the chip. Most modern
processors are clock multiplied, which means they are running at a speed that is really
a multiple of themotherboard into which they are plugged. The only types of memory
matching the full speed of the processor are the L1, L2, and L3 caches built into the
processor core.
If the data that the processor wants is already in L1 cache, the CPU does not have
to wait. If the data is not in the cache, the CPU must fetch it from the Level 2 or Level 3
cache or (in less sophisticated system designs) from the system bus—meaning main
memory directly.
According to Intel, the L1 cache in most of its processors has approximately a
90% hit ratio. (Some processors, such as the Pentium 4, are slightly higher.) This means
Prepared By – Prof. Manoj.kavedia (9860174297 – 9324258878 ) (www.kavediasir.yolasite.com)
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