microcode

programming A technique for implementing the instructionset 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 interrupts 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)