24.3.1. Micro Program and Control Memory

For each instruction of the CPU, there is a corresponding micro program for generating control signals. Each macro instruction corresponds to a sequence of micro instructions, which are stored in a memory composed of zeros and ones. Each position of the control word specifies a particular signal placed in memory, allowing it to be accessed and used efficiently.

This chunk explains the foundational concept of a micro program control memory. Each high-level instruction (macro instruction) that the CPU executes corresponds to a low-level sequence of operations (micro instructions) stored as binary data in memory. The binary data consists of 0s and 1s that determine specific control signals. This organization allows the CPU to manage complex instructions by breaking them down into simpler, manageable tasks, enhancing efficiency.

Consider a recipe book where each recipe (macro instruction) directs you to perform a series of steps (micro instructions) in cooking. The recipe itself can be written out in detail or summarized into simple steps. Similarly, in computers, a macro instruction is like the recipe, while micro instructions are the specific steps needed to carry out the cooking process.

When a macro instruction enters the decoding phase, it first completes a fetch cycle. The Op-Code of the instruction corresponds to a specific micro program, which is loaded into the control memory. Once the fetch is done, the instruction decoder analyzes the Op-Code and directs the micro program counter to the specific address in the micro program control memory that corresponds to that macro instruction.

This chunk describes how a macro instruction is processed in a CPU. Initially, the instruction is fetched from memory. The first step is fetching the instruction, followed by decoding the instruction to understand what operation needs to be performed. The Op-Code guides the CPU in locating the appropriate micro program, which is essential for executing the instruction correctly. The micro program counter (MPC) is crucial here, as it indicates which set of micro instructions to execute based on the identified macro instruction.

Think of a librarian fetching a book from a library. The librarian first checks for the book's title (fetch), reads the categorization tag (decode), and then finds the specific shelf where that book is stored (pointing the MPC). Just like the librarian uses a systematic approach to locate the book, the CPU systematically follows processes to execute instructions effectively.

To reduce memory usage and redundancy, similar micro programs can be combined. For example, both add and subtract instructions can share a common micro program structure, with only minor variations for the subtraction operation. This way, multiple instructions can use the same basic micro program, requiring only branching to handle differences.

This chunk emphasizes how optimization is achieved in micro programs. Instead of writing a completely unique set of micro instructions for each macro instruction, certain common functionalities can be reused. For instance, if adding and subtracting can share the same code structure with only slight adjustments for operations unique to each, this reduces the number of distinct micro programs needed and conserves memory. Using branches to navigate these slight variations allows for efficient code management.

Imagine a school where teachers write lesson plans. Instead of each teacher writing entirely new lessons from scratch, many of them can use a standard template with customizable sections (the branching parts). This saves time and resources, much like how combining micro programs economizes the computer's memory while maintaining functionality.

Branching is crucial within micro programs for efficient control. There are two types of branching: based on conditions set by macro instructions and inherent branching due to optimization, where common operations can be shared between different micro programs. After a macro instruction is executed, newly decoded instructions will direct the MPC back to a fetch cycle for the next operation.

This chunk reviews the importance of branching in micro programs. Branching enables the micro program to divert from a main sequence to perform specific tasks based on conditions. Inherent branching refers to the practice of recycling parts of micro programs that are similar for different macro instructions, enhancing optimization. After completing one instruction, the process seamlessly transitions to fetching a new macro instruction, coordinating the execution flow effectively.

Consider a traffic control system. Traffic lights branch in the flow of cars based on conditions like time of day or traffic volume. Some roads have common routes where the traffic is directed similarly, while certain adaptations occur based on specific circumstances (like turning left or going straight). Similarly, micro programs can branch to execute specific functionalities based on instruction needs, maintaining smooth operations.

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