7.2.2. Complexity of Instructions

Machine instructions are generally complex and require multiple clock cycles to complete. In this context, machine instructions are also termed as macro instructions.

Machine instructions, which we refer to as macro instructions, consist of various actions that need to be performed by the CPU. Due to the complexity of these instructions, they often cannot be completed in a single clock cycle. For example, some instructions might involve retrieving data from various memory locations or performing multiple operations sequentially. As a result, these complex instructions require several cycles, thus necessitating a more detailed breakdown into simpler parts known as micro instructions.

Think of macro instructions like a multi-step recipe for making a cake, where each step needs to be done in order. Just as you cannot bake the cake until you've mixed the ingredients, you can't complete a macro instruction without executing each step, which takes time.

Different addressing modes can affect instruction complexity. For example, an indirect machine instruction (e.g., ADD indirect A, 3030) requires going to a location to get another address, whereas an immediate mode instruction (e.g., ADD A, 30) uses the value directly.

Addressing modes determine how the CPU accesses data for operations. An indirect mode involves an additional step where the CPU must first access a memory location to find where the actual data is located. Conversely, with immediate addressing, the data is part of the instruction itself, which makes it faster. This difference in how data is accessed contributes to the overall complexity and execution time of the instruction.

Imagine you are looking for a book in a library. The direct addressing mode is like having the book right in your hands (you have the book’s title). In contrast, the indirect mode is like needing to look up the book’s location in a catalog first before you can find it on the shelf.

Each instruction has to be divided into atomic instructions or micro instructions, which can be implemented in a clock cycle. This division allows for managing the complexity of macro instructions by executing smaller steps sequentially.

Micro instructions are simple commands that are part of a macro instruction. Each micro instruction typically takes one clock cycle to execute. By breaking down complex macro instructions into these smaller, manageable steps, the control unit of the CPU can execute instructions more effectively. For instance, during the execution of an instruction, various micro instructions may include fetching data from memory, processing this data, and writing results back to memory.

Breaking down a task like assembling furniture can illustrate this. Instead of trying to build the entire piece in one go, you follow steps: first find the pieces (fetch), then assemble them (execute), and finally ensure everything is stable (store). Each step represents a micro instruction.

Clock grouping is a method used to optimize the execution of micro instructions. If two micro instructions are non-dependent, they can be executed in the same time unit to save cycles.

Clock grouping allows the CPU to execute multiple non-dependent micro instructions simultaneously, reducing the overall execution time for a macro instruction. This optimization technique means that if one instruction is not waiting on data from another instruction, both can occur during the same clock cycle. This parallel execution results in increased efficiency and performance of the CPU.

Consider multitasking in everyday life, such as cooking dinner. If you can boil vegetables while baking meat in the oven, you save time by performing two tasks at once. In CPU operations, if you can read data from memory at the same time as calculating something, you effectively reduce the time it takes to complete the overall instruction.

The phases involved in executing an instruction include fetch, decode, execute, and sometimes indirect instruction execution. Each phase has its corresponding micro operations.

In the instruction execution process, each instruction undergoes several phases. Initially, in the fetch phase, the instruction is retrieved from memory. Next, during the decode phase, the instruction is interpreted by the CPU to understand what operation is to be executed. The execute phase involves carrying out the operation specified by the instruction. Each of these phases consists of smaller micro operations that complete the larger tasks sequentially. Understanding these phases is crucial for grasping how CPUs manage instruction complexity.

This process can be likened to following a project plan at work. First, you gather all the necessary materials (fetch), then analyze what must be done (decode), and finally, you carry out the tasks to complete the project (execute). The project plan is effectively the macro instruction, while each step corresponds to micro instructions.

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