Hardware Reference
In-Depth Information
on the other hand, holds the data the processor is most likely to need in advance of it actually being
needed. This enables the processor to continue working at either full speed or close to it without
having to wait for the data to be retrieved from slower main memory. Cache memory is usually made
up of static RAM (SRAM) memory integrated into the processor die, although older systems with
cache also used chips installed on the motherboard.
See the
Chapter 6
section, “
Cache Memory: SRAM
,”
p.
329
.
Recent low-cost processor designs typically include two levels of processor/memory cache: Level 1
(L1) and Level 2 (L2). Mid-range and high-end designs also have Level 3 cache. These caches and
their functioning are described in the following sections.
Tip
Use the popular CPU-Z utility discussed earlier in this chapter to determine the types and sizes
of cache memory in your computer's CPUs.
Internal Level 1 Cache
All modern processors starting with the 486 family include an integrated L1 cache and controller. The
integrated L1 cache size varies from processor to processor, starting at 8KB for the original 486DX
and now up to 128KB or more in the latest processors.
Note
Multicore 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 (MBps) bandwidth (throughput).
Both are really time- or frequency-based measurements, and a chart comparing them can be found in
nanosecond cycling, which means you would need 4ns memory to keep pace with a 200MHz CPU.
Also, note that the motherboard of a 233MHz system typically runs at 66MHz, which corresponds to a
speed of 15ns per cycle and requires 15ns memory to keep pace. Finally, note that 60ns main memory
(common on many Pentium-class systems) equates to a clock speed of approximately 16MHz. So, a
typical Pentium 233 system has a processor running at 233MHz (4.3ns per cycle), a motherboard
running at 66MHz (15ns per cycle), and main memory running at 16MHz (60ns 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 how cache 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.
