Hardware Reference
In-Depth Information
theAppleII.Forthetriviabuffsoutthere,theIBMPClistedfor$1,265andincludedonly
16KB of RAM, whereas a similarly configured Apple II cost $1,355.
It took over two years between the introduction of the 8088 and the release of the IBM
PC. Back then, a significant lag time often occurred between the introduction of a new
processor and systems that incorporated it. That is unlike today, when new processors and
systems using them often are released on the same day.
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
spaceofthe8088forusebyadaptercardsandsystemBIOS.Thelower640KBisthecon-
ventional memory in which DOS and software applications execute. Windows and other
modern operating systems use memory that is primarily above 1MB.
P2 (286) Processors
In1982,IntelintroducedtheIntel80286processor,normallyabbreviatedas286.Thefirst
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 manu-
factured whatcametobeknownas
IBM clones
,withmanyofthesemanufacturers 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, soft-
ware 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 3.18
.
Figure 3.18
286 processor die. Photograph used by permission of Intel Corporation.
