Hardware Reference
In-Depth Information
Computer users have sometimes wondered why a 640KB conventional-memory barrier exists if the
8088 chip can address 1MB of memory. The conventional-memory barrier exists because IBM
reserved 384KB of the upper portion of the 1,024KB (1MB) address space of the 8088 for use by
adapter cards and system BIOS. The lower 640KB is the conventional memory in which DOS and
software applications execute. Windows and other modern operating systems use memory that is
primarily above 1MB.
P2 (286) Processors
In 1982, Intel introduced the Intel 80286 processor, normally abbreviated as 286. The first CPU
behind the original IBM PC AT (Advanced Technology), it did not suffer from the compatibility
problems that damned the 80186 and 80188. Other computer makers manufactured what came to be
known as
IBM clones
, with many of these manufacturers calling their systems AT-compatible or AT-
class computers.
When IBM developed the AT, it selected the 286 as the basis for the new system because the chip
provided compatibility with the 8088 used in the PC and the XT. Therefore, software written for
those chips should run on the 286. The 286 chip is many times faster than the 8088 used in the XT,
and at the time it offered a major performance boost to PCs used in businesses. The processing speed,
or throughput, of the original AT (which ran at 6MHz) is five times greater than that of the PC running
at 4.77MHz. The die for the 286 is shown in
Figure 3.22
.
Figure 3.22. 286 processor die. Photograph used by permission of Intel Corporation.
The 286 was faster than the 8088 for two reasons: It required only 4.5 cycles to perform the average
instruction (versus 12 on the 808x processors), and it handled data in 16-bit chunks. The 286's real
mode imitates an 808x processor, and the protected mode can access memory larger than 1MB. The
286 also supports multitasking.
