Hardware Reference
In-Depth Information
features were limiting, they became external units on the PC, although the PC keyboard
layout and electrical designs were copied from the DataMaster.
Several other parts of the IBM PC system also were copied from the DataMaster, includ-
ing the expansion bus (or input/output slots), which included not only the same physic-
al 62-pin connector, but also almost identical pin specifications. This copying of the bus
design was possible because the PC used the same interrupt controller as the DataMaster
and a similar direct memory access (DMA) controller. Also, expansion cards already de-
signed for the DataMaster could easily be redesigned to function in the PC.
The DataMaster used an Intel 8085 CPU, which had a 64KB address limit and an 8-bit
internal and external data bus. This arrangement prompted the PC design team to use the
Intel8088CPU,whichofferedamuchlarger(1MB)memoryaddresslimitandaninternal
16-bitdatabus,butonlyan8-bitexternal databus.The8-bitexternal databusandsimilar
instructionsetenabledthe8088tobeeasilyinterfacedintotheearlierDataMasterdesigns.
IBM brought its system from idea to delivery of functioning systems in one year by using
existing designs and purchasing as many components as possible from outside vendors.
The Entry Systems Division was granted autonomy from IBM's other divisions and could
tapresourcesoutsidethecompany,ratherthangothroughthebureaucraticproceduresthat
required exclusive use of IBM resources. IBM contracted out the PC's languages and OS
to a small company named Microsoft. That decision was the major factor in establishing
Microsoft as the dominant force in PC software.
Note
ItisinterestingtonotethatIBMhadoriginallycontactedDigitalResearch(thecompanythat
created CP/M, then the most popular personal computer OS) to have it develop an OS for
the new IBM PC. However, Digital was leery of working with IBM and especially balked at
thenondisclosureagreementIBMwantedDigitaltosign.Microsoftjumpedontheopportun-
ity left open by Digital Research and, consequently, became the largest software company in
the world. IBM's use of outside vendors in developing the PC was an open invitation for the
after-market to jump in and support the system—and it did.
On August 12, 1981, a new standard was established in the microcomputer industry with
thedebutoftheIBMPC.Sincethen,hundredsofmillionsofPC-compatiblesystemshave
been sold, as the original PC has grown into an enormous family of computers and peri-
pherals. More software has been written for this computer family than for any other sys-
tem on the market.
