Page 87 - A Practical Guide from Design Planning to Manufacturing

Computer Components 63

By providing system initialization and the first level of hardware abstrac-
tion, the BIOS forms a key link between the hardware and software.

Memory Hierarchy
Microprocessors perform calculations at tremendous speeds, but this is
only useful if the needed data for those calculations is available at sim-
ilar speeds. If the processor is the engine of your computer, then data
would be its fuel, and the faster the processor runs, the more quickly it
must be supplied with new data to keep performing useful work. As
processor performance has improved, the total capacity of data they are
asked to handle has increased. Modern computers can store the text of
thousands of books, but it is also critical to provide the processor with
the right piece of data at the right time. Without low latency to access the
data the processor is like a speed-reader in a vast library, wandering for
hours trying to find the right page of a particular book.
Ideally, the data store of a processor should have extremely large
capacity and extremely small latency, so that any piece of a vast amount
of data could be very quickly accessed for calculation. In reality, this isn’t
practical because the low latency means of storage are also the most
expensive. To provide the illusion of a large-capacity, low-latency
memory store, modern computers use a memory hierarchy (Fig. 2-6).
This uses progressively larger but longer latency memory stores to hold
all the data, which may eventually be needed while providing quick
access to the portion of the data currently being used.
The top of the memory hierarchy, the register file, typically contains
between 64 and 256 values that are the only numbers on which the
processor performs calculations. Before any two numbers are added,
multiplied, compared, or used in any calculation, they will first be loaded

Capacity Latency
0.5–1 KB Register file 0.5–2 ns
Processor
8–64 KB Level 1 cache 1–4 ns

256 KB–2 MB Level 2 cache 5–20 ns

128–256 MB Main memory 35–50 ns
Figure 2-6 Memory hierarchy.
10–50 GB Hard drive 5–10 ms