A string of bits that encodes a task for the Processor, specified in the Instruction Set Architecture
The fastest Microarchitecture is one with fixed format. The beginning of the instruction is the Opcode The remaining bits depend on the instruction, but in the case of ADD, for example it would follow with: destination register, source register, 0s, source register 2 Machine Instructions are stored contiguously in memory. The Program Counter keeps track fo the instruction we want to execute.