Micro memory

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 programmable.

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.