20.3. Micro-programmed Control Units

In this unit, we focus on generating control signals using micro-programmed control, which is more flexible and can be modified, unlike a hardwired control unit that is fixed and fast.

Micro-programmed control units allow the generation of control signals through a programming approach, making them adaptable to different requirements. This contrasts with a hardwired control unit, which uses fixed circuits to generate signals and cannot be easily changed once implemented.

Consider a hardwired control unit as a traditional manual car, which has fixed gears. You must follow a specific sequence to change gears without flexibility. In contrast, a micro-programmed control unit acts like an automatic car that adjusts itself based on driving conditions, allowing for smoother transitions and adjustments.

A micro program is a sequence of instructions that correspond to micro operations, where each instruction maps to specific control signals in a control memory, called micro-program memory.

Every micro program consists of instructions stored in a special memory called micro-program memory. Each instruction is made up of control signals that dictate the state of various components in a processor. This organization makes it easier to manage and implement complex control signals as they are defined in a clear and structured manner.

Imagine a cookbook filled with recipes (micro programs) where each recipe consists of steps (micro instructions) that guide you on how to prepare a dish. Each step may require specific ingredients or cooking methods (control signals) that correspond to the task at hand. Just as following the recipe allows for a variety of dishes, the micro-program structure enables flexibility in executing different operations.

Generating control signals in a micro-programmed control unit is straightforward. The control signals are fetched directly from the micro-program memory, eliminating complex circuit designs.

In a micro-programmed system, fetching control signals is a matter of accessing the appropriate memory location, thus simplifying the generation of these signals. This process contrasts with hardwired control units, where generating signals involves navigating through a finite state machine with complex circuitry.

Think of a remote control for a TV. When you press a button (fetching a memory location), the remote directly sends a signal to the TV to perform an action (generate a control signal). In a hardwired setup, it would be as if you had to manually switch on each electrical pathway for every button press, which is complicated and time-consuming.

Sequencing in micro-programmed control units can be tricky. Unlike hardwired systems that follow a defined flow of states, micro-programs use a counter to sequentially access memory locations, but can also jump based on conditions.

In micro-programmed control units, sequencing the access of instructions is like stepping through a list. You generally move from one instruction to the next in order, but sometimes you might need to jump back or forward based on conditions, which requires additional logic. This makes the sequencing aspect slightly more complex than in hardwired designs where states are fixed.

Consider a game of hopscotch. You typically progress from one square to the next. However, if you land a special square, you may have to jump back to a previous square or advance further than the next one, much like jumping in micro-programmed sequences depending on certain flag conditions.

The architecture of a micro-programmed control unit is comparable to that of a traditional program. It involves fetching instructions, processing them, and jumping when necessary, but uses a micro-program counter instead.

Although the structure of the micro-programmed control unit resembles that of traditional instruction execution, there is a key difference in how control signals are handled. Instead of decoding an opcode, the system directly utilizes pre-defined signals stored in memory, which streamlines the operation of generating control signals.

This can be likened to following a map versus finding an address in a GPS. When using a map (traditional methods), you interpret symbols and directions (opcodes and operands). However, with GPS (micro-programming), you simply enter a destination, and the system guides you, using pre-programmed routes (control signals) for efficiency.

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