3. Principles of Microprogrammed Control

This section details the core theory of the microprogrammed control unit, the flexible alternative to the hardwired FSM. This approach fundamentally changes the design from a complex, fixed logic circuit to a simple, programmable one.

• 3.1 Core Concept: The "Computer-within-a-Computer"

  • The most effective analogy for a microprogrammed control unit is that it is a "computer-within-a-computer." The main CPU (which has assembly instructions like ADD, LDA, JMP) is the "outer" computer. The Control Unit itself is a tiny, hidden, "inner" computer.
  • This inner computer has its own simple program, called a microprogram. The microprogram is composed of a sequence of instructions called micro-instructions.
  • The key relationship is this: One assembly instruction (like LDA) is interpreted by the CU as a command to run a specific micro-routine (a "subroutine" of micro-instructions). The micro-routine is the step-by-step recipe that defines how to execute that single assembly instructior.

• 3.2 Architectural Components

  • To function as a simple computer, the microprogrammed CU has its own set of internal hardware components, separate from the main datapath.

  • 3.2.1 Control Store (or Control Memory):

    • This is a small, high-speed Read-Only Memory (ROM) that is located inside the Control Unit.
    • Its sole purpose is to store the entire microprogram—that is, all the micro-routines for every assembly instruction in the CPU's instruction set (e.g., the routines for LDA, STA, ADD, JMP, etc.).

  • 3.2.2 Micro-Sequencer (Next Address Logic):

    • This is the "Program Counter" for the Control Unit (often called a uPC or "micro-Program Counter").
    • Its job is to determine the address of the next micro-instruction to be fetched from the Control Store.
    • It decides this address based on several inputs: the main instruction's opcode (for the initial "decode" jump), CPU status flags (for conditional branches like JEQ), and sequencing fields from the current micro-instruction (e.g., SEQ_NEXT, SEQ_FETCH).

  • 3.2.3 Micro-instruction Register (uIR):

    • This is a register that holds the current micro-instruction that was just fetched from the Control Store.
    • This is the most critical component for execution: the output bits of this register are the actual control signals that are sent to the datapath.
    • For example, a 16-bit uIR might directly output 16 control signals (like Register_Enable, ALU_Subtract, Memory_Write, etc.) to the rest of the CPU.