PC Components, Features, and System Design - Upgrading and Repairing PCs - page 30

newer chips it is a serialized point-to point link, transferring fewer bits at a time but at a
much higher rate. Older designs often had several components sharing the bus, whereas
the newer point-to-point links are exclusively between the processor and the chipset.
See ““ Data I/O Bus , ” p. 42 ( Chapter 3 , “ Processor Types and Specifications ”).
Table 2.2 lists all the Intel and AMD x86 processors, their data bus widths, and their in-
ternal register sizes.
Table 2.2 Intel and Intel-Compatible Processors and Their Data Bus/Register Widths
A common confusion arises in discussions of processor “widths.” Some people take the
width to refer to how many bits of data can be read or written at a time, whereas others
refer to the size of the internal registers, which control how much data can be operated on
at a time. Although many processors have had matching data bus widths and internal re-
gistersizes,theyarenotalwaysthesame,whichcanleadtomoreconfusion.Forexample,
mostPentiumprocessorshave64-bitdatabuswidthsandyetincludeinternalregistersthat
areonly32bitswide.ThenewerAMDandIntelprocessorswithx86-64architecturehave
64-bit internal registers and can run in both 32-bit and 64-bit modes. Thus, from a soft-
ware point of view, there are PC processors capable of running 16-bit, 32-bit, and 64-bit
instructions. For backward compatibility, those having 64-bit registers can also run 32-bit
and 16-bit instructions, and those with 32-bit registers can run 16-bit instructions. In any
case, remember that bus widths and register sizes are completely unrelated. Also note that
most newer processors use serial buses which are narrow, but also very fast.