< Inutitively I know what microcode is. I think of it as hard-coded RO <for the instruction fetch unit of the microprocessor. Since I haven't bee <formally schooled in computer science or microprocessor design, I'd like <understand (in 500 words or less <g>) how microcode works, i.e., how is i <implemented and how does the microprocessor access it. Microcode: In the simplest form it's the control program that runs the CPU. Every computer has it's instructions set, most we know like ADD A,B or MOV A,B. Those are called MACROinstructions. Microcode is the internal coding to actually create the sequence of timing pulses to actually make the ADD happen. The microcode resides in it own storage area and is generally inaccessable (there are exceptions). A typical sequence would be in english: macro instruction: ADD A,B Add contents of register A to register B and place results in A Start: Fetch an instruction (uses program counter register as address or source and destination is the "instruction register). Jump to address @I (use the instruction register as an address of the next microinstruction.) ADD: -signal add instruction to ALU -gate A register to ALU -gate B register to ALU -store ALU to A register (this could al be in one microinstruction or it could be a sequence of several.) jump to START (get next MACROinstuction) NOTE: microcoding is one way to construct a computer control, sequential logic is also possible but as the machines increase in complexity the logic complexity grows to sometimes implementable levels or becomes hard to correct if there is an error. Microcoding makes error correction easier and if new instruction need to be added it's possible if the code space exists. It's also easier to create a machine on paper or as a program that executes microcode to test it. Allison