MICROPROGRAMMING

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MICROPROGRAMMING

• by
• Konrad Majka

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WHAT IS MICROPROGRAMMING?

• A microprogram is a highly-specialized
  computer program that allows one computer
  microarchitecture to emulate another, usually
  more-complex architecture. The microprogram is
  usually very small compared to ordinary software,
  hence its name. It is also very carefully
  designed and optimized for the fastest possible
  execution. As with any computer program,
  microprograms consist of series of
  microinstructions. These microinstructions
  usually control the computer's CPU at a very
  fundamental level.

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MICROINSTRUCTIONSWhile a real
instruction might be something like 'add the
contents of registers A and B', a
microinstruction might be something like 'write
out register A to bus Z', or 'read data bus into
register X' - very basic actions that could be
assembled to implement the actual instruction set
of the machine. To simultaneously control of many
features, the microinstruction is often very
wide, for example, 100 bits or more.
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MICROPROGRAMMED CONTROL UNIT (fig. 1)
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HOW IT WORKS?

• The microinstruction is fetched from the ROM and
  placed in the microinstruction register where it
  is decoded. Some of the bits of the
  microinstruction are used for sequencing that
  is, they determine the location of the next
  microinstruction and are able to implement
  conditional branches.
• Some of the bits in the microinstruction register
  are used to control the CPU. The "address mapper"
  in figure 1 converts the bit pattern of the
  machine level microinstruction into the first
  address of the microprogram that interprets it.

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HORIZONTAL VS VERTICAL
MICROCODEA typical micromachine's control
word has a field, a range of bits, to control
each piece of electronics in the CPU. For
example, one simple arrangement might be
register source A register source B
destination register arithmetic and logic unit
operation type of jump jump address The
above is an example of "horizontal" microcode.
This is microcode that sets all the bits of the
CPU's controls on each tick of the clock that
drives the sequencer. Many of the bits in
horizontal microcode contain fields to do nothing.
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Some CPUs use a completely
different design called "vertical" microcode to
reduce cost. It has two fields field select
field value The "field select" selects which
part of the CPU will be controlled by this word
of the control store(the memory in which the
CPUs microcode resides). The "field value"
actually controls that part of the CPU. With this
type of microcode, a designer explicitly chooses
to make a slower CPU to save money by reducing
the unused bits in the control store however,
the reduced complexity may increase the CPU's
clock frequency, which lessens the effect of an
increased number of cycles per instruction.
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WHY MICROPROGRAMMING?

• Without microprogramming, each computer has to be
  constructed from digital circuits that are
  specifically designed to execute the machine code
  instructions. Such a design is specific to a
  particular architecture and organization.
• Microprogramming is a general technique that can
  be applied to any computer. Because the
  microprogrammed control unit is such a regular
  structure, it was possible to develop formal
  methods for the design of microprograms and to
  construct tools to aid the creation, validation,
  and testing of microprograms.

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• Microprogramming made it possible to totally
  divorce architecture from organization. By means
  of a suitable microprogram you could impose any
  architecture on the simplest of CPUs.In
  practical terms, a company could design a
  powerful architecture that, for example,
  supported high-level languages and operating
  systems and then use microprogramming to
  implement this architecture on a range of
  machines.

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ADVANTAGE OF MICROPROGRAMMING

• In the mid 1970s by using a writeable
  microprogram control store users could implement
  their own instructions or dynamically change the
  architecture of the computer. If you changed your
  architecture you could still run your old
  programs on the your new microprogrammed machine
  by emulating the old architecture in microcode.
  Machine emulation was very important at a time
  when computers cost hundreds of thousands of
  dollars and the software base was very small.
• One of the advantages of microprogrammed
  architectures is their flexibility and ease of
  implementation.

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• Another advantage is their efficient use of
  memory. In 1970 main store random access memory
  was both slow and expensive. By implementing
  complex-instruction architectures, the size of
  programs could be reduced to minimize memory
  requirements. Control store was much faster than
  main memory and it paid to implement machine
  level instructions in microcode.
• By the 1980s the cost of memory had dramatically
  reduced and its access time had dropped to 100
  ns. Under such circumstances, the microprogrammed
  architecture lost its appeal.

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CHANGES

• Much has changed, since the first
  microprogramming was introduced ROM, which was
  used to hold the microinstructions is no longer
  faster than RAM, which holds the machine language
  program Instruction set have become much
  simpler leading to reduced complexity in
  control Microprogramming is used today to
  implement some parts of a complex instruction
  set, such as a Pentium.
• 
  

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Thank You !Konrad Majka