Hardware Reference
In-Depth Information
Figure 3.25. Pentium processor internal architecture.
The Pentium has a 32-bit address bus width, giving it the same 4GB memory-addressing capabilities
as the 386DX and 486 processors. But the Pentium expands the data bus to 64 bits, which means it
can move twice as much data into or out of the CPU, compared to a 486 of the same clock speed. The
64-bit data bus requires that system memory be accessed 64 bits wide, so each bank of memory is 64
bits.
Even though the Pentium has a 64-bit data bus that transfers information 64 bits at a time into and out
of the processor, the Pentium has only 32-bit internal registers. As instructions are being processed
internally, they are broken down into 32-bit instructions and data elements and processed in much the
same way as in the 486. Some people thought that Intel was misleading them by calling the Pentium a
64-bit processor, but 64-bit transfers do indeed take place. Internally, however, the Pentium has 32-
bit registers that are fully compatible with the 486.
The Pentium, like the 486, contains an internal math coprocessor or FPU. The FPU in the Pentium was
rewritten to perform significantly better than the FPU in the 486 yet still be fully compatible with the
486 and 387 math coprocessors. The Pentium FPU is estimated to be two to as much as ten times
faster than the FPU in the 486. In addition, the two standard instruction pipelines in the Pentium
provide two units to handle standard integer math. (The math coprocessor handles only more complex
calculations.) Other processors, such as the 486, have only a single standard execution pipe and one
integer math unit.
To learn more about Pentium processors, including the famous floating-point calculation flaw, see
Chapter 3 of
Upgrading and Repairing PCs,
19
th
Edition, available in its entirety on the disc
packaged with this topic.
