Editing Microcode (section)

===Instruction sets===
At the hardware level, processors contain a number of separate areas of circuitry, or "units", that perform different tasks. Commonly found units include the [[arithmetic logic unit]] (ALU) which performs instructions such as addition or comparing two numbers, circuits for reading and writing data to external memory, and small areas of onboard memory to store these values while they are being processed. In most designs, additional high-performance memory, the [[register file]], is used to store temporary values, not just those needed by the current instruction.<ref name=CPU>{{cite web |url=https://www.redhat.com/sysadmin/cpu-components-functionality |title=The central processing unit (CPU): Its components and functionality |website=Red Hat |first=David |last=Both |date=23 July 2020}}</ref>

To properly perform an instruction, the various circuits have to be activated in order. For instance, it is not possible to add two numbers if they have not yet been loaded from memory. In [[RISC]] designs, the proper ordering of these instructions is largely up to the programmer, or at least to the [[compiler]] of the [[programming language]] they are using. So to add two numbers, for instance, the compiler may output instructions to load one of the values into one register, the second into another, call the addition function in the ALU, and then write the result back out to memory.<ref name=CPU/>

As the sequence of instructions needed to complete this higher-level concept, "add these two numbers in memory", may require multiple instructions, this can represent a performance bottleneck if those instructions are stored in [[main memory]]. Reading those instructions one by one is taking up time that could be used to read and write the actual data. For this reason, it is common for non-RISC designs to have many different instructions that differ largely on where they store data. For instance, the [[MOS 6502]] has eight variations of the addition instruction, {{code|ADC}}, which differ only in where they look to find the two operands.<ref>{{cite web |url=http://www.6502.org/tutorials/6502opcodes.html |title= NMOS 6502 Opcodes |first= John |last=Pickens |website=6502.org}}</ref>

Using the variation of the instruction, or "[[opcode]]", that most closely matches the ultimate operation can reduce the number of instructions to one, saving memory used by the program code and improving performance by leaving the [[data bus]] open for other operations. Internally, however, these instructions are not separate operations, but sequences of the operations the units actually perform. Converting a single instruction read from memory into the sequence of internal actions is the duty of the [[control unit]], another unit within the processor.<ref name=microcode>{{cite web |url=http://www.righto.com/2022/11/how-8086-processors-microcode-engine.html#:~:text=In%201951%2C%20Maurice%20Wilkes%20came,memory%20called%20a%20control%20store. |title=How the 8086 processor's microcode engine works |website=Ken Shirriff's blog |first=Ken |last=Shirriff}}</ref>