browse words by letter
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
microcode |
1 definition found From The Free On-line Dictionary of Computing (13 Mar 01) [foldoc]: microcodeA technique for implementing the {instruction set} of a processor as a sequence of microcode instructions ("microinstructions"), each of which typically consists of a large number of bit fields and the address of the next microinstruction to execute. Each bit field controls some specific part of the processor's operation, such as a gate which allows some {functional unit} to drive a value onto the {bus} or the operation to be performed by the {ALU}. Several microinstructions will usually be required to fetch, decode and execute each {machine code} instruction ("{macroinstruction}"). The microcode may also be responsible for {polling} for hardware {interrupt}s between each macroinstruction Writing microcode is known as "microprogramming". Microcode may be classified as "horizontally encoded" or "vertically encoded". Horizontal microcode is as described above where there is a fairly direct correspondence between the bit fields in a microinstruction and the control signals sent to the various parts of the CPU. Not all combinations of bits will be valid (e.g. two units driving the bus at once). Vertical microcode is closer to {machine code} because a bit field value may pass through some intermediate combinatory logic which generates the actual control signals. This allows a few bits of a microinstruction to determine several control signals and ensure that only valid combinations of those signals are generated (e.g. a field may be decoded to determine which unit drives the bus). The disadvantage with vertical encoding is that the encoding is usually fixed and takes extra time compared with horizontal encoding which allows any combination of signals to be generated and takes no time to decode. The alternative to a microcoded processor is a {hard-wired} one where the control signals are generated directly from the bits of the {machine code} instruction. This is more common in modern {RISC} architectures because it is faster. Microcode is usually stored in {ROM} chips though some processors (e.g. the {Orion}) use fast RAM, making them dynamically microprogrammable (1996-11-26)
This work is licensed under a
