The micro memory of a processor is a
combinatorial circuit sometimes in the form of a
read-only memory, sometimes in the form of an ordinary
combinatorial circuit. At some point, it was fairly common to be
able to alter the contents of the micro memory in order to create
specialized instructions for particular programs. In that case, the
micro memory was similar to an
ordinary read/write memory. Such processors are called micro
The number of address lines required depends on the number of
instructions of the processor and their complexity. In a traditional
CISC machine (Complex Instrution Set Computer) such as the VAX, the
micro memory can be huge, perhaps structured with subroutines and
other control structures otherwise seen on the instruction level.
For simple RISC machines (Reduced Instrution Set Computer), the
micro memory can be so simple that an ordinary combinatorial circuit
will do. In our example computer, the micro memory will have 6
address lines, for a maximum of 64 addresses.
The number of data lines is the same as the number of micro
operations necessary in the machine. In our example computer we
initially need 15 such micro operations, but we will extend it
somewhat in order to accomodate new functionality such as
conditional jumps, stack pointers, etc.