Hardware Reference
In-Depth Information
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 pro-
cessors. The original IBM PC included an 8088 processor that could execute 16-bit in-
structions 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 de-
signed 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 applica-
tions are written using16-bit instructions. These 16-bit OSsandapplications are designed
to run on an original 8088 processor.
See
“
Internal Registers (Internal Data Bus)
,
”
p.
44
(this chapter).
See
“
Address Bus
,”
p.
43
(this chapter).
Later processors such as the 286 could run the same 16-bit instructions as the original
8088,butmuchfaster.Inotherwords,the286wasfullycompatiblewiththeoriginal8088
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 even the OS in memory.
Therefore,ifmorethanoneprogramisrunning,oneofthemcouldbringtheentiresystem
to a crashing halt.
IA-32 Mode (32-Bit)
Thencamethe386,whichwasthePCindustry'sfirst32-bitprocessor.Thischipcouldrun
an entirely new 32-bit instruction set. To take full advantage of the 32-bit instruction set,
a 32-bit OS and a 32-bit application were required. This new 32-bit mode was referred to
