Processor Types and Specifications - Upgrading and Repairing PCs - page 38

time and how it moves data around internally within the chip. This is sometimes also referred to as
the internal data bus . A register is a holding cell within the processor; for example, the processor
can add numbers in two different registers, storing the result in a third register. The register size
determines the size of data on which the processor can operate. The register size also describes the
type of software or commands and instructions a chip can run. That is, processors with 32-bit internal
registers can run 32-bit instructions that are processing 32-bit chunks of data, but processors with 16-
bit registers can't. Processors from the 386 to the Pentium 4 use 32-bit internal registers and can run
essentially the same 32-bit OSs and software. The Core 2, Athlon 64, and newer processors have
both 32-bit and 64-bit internal registers, which can run existing 32-bit OSs and applications as well
as newer 64-bit versions.
Processor Modes
All Intel and Intel-compatible processors from the 386 on up can run in several modes. Processor
modes refer to the various operating environments and affect the instructions and capabilities of the
chip. The processor mode controls how the processor sees and manages the system memory and the
tasks that use it.
Table 3.4 summarizes the processor modes and submodes.
Table 3.4. Processor Modes
Real Mode
Real mode is sometimes called 8086 mode because it is based on the 8086 and 8088 processors. The
original IBM PC included an 8088 processor that could execute 16-bit instructions using 16-bit
internal registers and could address only 1MB of memory using 20 address lines. All original PC
software was created to work with this chip and was designed around the 16-bit instruction set and
1MB memory model. For example, DOS and all DOS software, Windows 1.x through 3.x, and all
Windows 1.x through 3.x applications are written using 16-bit instructions. These 16-bit OSs and
applications are designed to run on an original 8088 processor.
See the section, “ Internal Registers (Internal Data Bus) , ” p. 44 .
See the section, “ Address Bus ,” p. 42 .
Later processors such as the 286 could run the same 16-bit instructions as the original 8088, but much
faster. In other words, the 286 was fully compatible with the original 8088 and could run all 16-bit
software just the same as an 8088, but, of course, that software would run faster. The 16-bit
instruction mode of the 8088 and 286 processors has become known as real mode . All software
running in real mode must use only 16-bit instructions and live within the 20-bit (1MB) memory
architecture it supports. Software of this type is usually single-tasking—that is, only one program can
run at a time. No built-in protection exists to keep one program from overwriting another program or