with the processor bus. The VESA Local Bus operates
at the speed of the video processor.
Several other bus systems have been developed,
many of which have not found widespread acceptance
in the PC world. Each of these has introduced some
technology that is common in the modem bus systems.
(MCA). The MicroChannel Architecture (MCA) bus
was developed by IBM in 1987 and increased the bus
speed to 10 MHZ. The MCA Bus also introduced the
ability to configure the boards IRQ and DMA channels
through a software configuration program. MCA was
the first system to use bus mastering. Bus mastering is
a system that allows an intelligent controller board to
take control of the bus system for a specified period of
time. This allows operations to be completed quickly.
Bus mastering differs from DMA in that DMA allows
for direct transfer from a peripheral controller to RAM,
Bus mastering allows for direct transfers between
controllers. An example of bus mastering is the ability
of a hard drive to transfer graphics directly to the
graphics driver, bypassing the CPU and RAM.
The major disadvantage of MCA was that it is not
compatible with the old ISA standard. Therefore, if you
have an MCA machine, the old ISA controller boards
will not work.
EXTENDED INDUSTRY STANDARD
ARCHITECTURE. To compete with MCA, The
Extended Industry Standard Architecture was (EISA)
developed. The EISA Bus included the following
32-bit data path
64K of I/O address
Capability to address up to 4 gigs-bytes of
Software configuration of boards
Unfortunately, the EISA Bus still operates with an 8
MHZ clock, and did not add any additional DMA or
PERIPHERAL COMPONENT INTERCON-
NECT (PCI). The Peripheral Component
Interconnect (PCI) system was designed to increase I/O
bus speeds while still maintaining compatibility with
previous ISA and EISA boards. A PCI computer has
two separate banks of expansion slots, one bank for PCI
boards and one bank for the older ISA/EISA boards.
The PCI bus uses a bridge circuit to isolate the
processor bus from the main I/O bus. This bridge
circuit is designed so that I/O functions can run
independently from the CPU.
The PCI bus is a 64-bit data bus, but can also
support 32-bit computers. This makes the PCI bus
useful in both Pentium and 486 systems. The PCI bus
can operate a speed up 33 MHZ and also supports bus
Finally, the PCI bus supports the
Plug-n-Play standard for software configuration of
UNITS AND BUSES
This chapter has introduced you to central
processing units (CPUs) and buses. The following
information summarizes important points you should
CENTRAL PROCESSING UNITS All the
computational operations (logical and arithmetic) and
operational decisions are made in the CPU. The CPU
controls all computer operations. The CPU has a
control section and an arithmetic logic unit (ALU).
CONTROL SECTION The control section
directs the sequence of CPU operations, interprets the
instructions, and provides the timing and control signals
to carry out the instructions.
TIMING Timing in a computer regulates the
flow of signals that control the operation of the
computer. Computer operations rely on both
synchronous and asynchronous operations. Timing
circuits are used throughout the computer.
INSTRUCTION AND CONTROL The
instruction execution and control portion of the control
section includes the combinational and sequential
circuits that make up the decision-making and the
memory-type functions. The general process of
execution of a machine instruction is fetch the
instruction, update the program counter or equivalent,
translate the instruction, and execute the instruction.
INTERRUPTS Interrupts are a method of
diverting the attention of the computer from whatever
process or program it is performing to handle the special
condition or event that caused the interrupt signal.
Interrupts allow the computer to respond to high
priority demands and still be able to perform normal or
lower priority processing. An interrupt is defined as a
break in the normal flow of operation of a computer
caused by an interrupt signal. The break occurs in