7.2.1. Instruction Cycle and Micro Operations

In this case, we will see an instruction cycle, which are always saying fetch, decode, execute, store and sometimes there may be an interrupt and what are the micro instructions involved for each of the instruction that is what is the first unit on.

The instruction cycle is a series of steps that a computer takes to execute a single instruction. This cycle consists of four main stages: fetch, decode, execute, and store. In some cases, there is also an interrupt involved. In the fetch stage, the computer retrieves the instruction from memory. Next, during the decode phase, the instruction is analyzed to determine what action is required. Then, the execute stage carries out the action defined by the instruction, and finally, the store stage saves the result back to memory. The discussion will also encompass the micro instructions that represent smaller, atomic operations that occur within these phases.

Think of the instruction cycle like a cooking recipe. First, you gather all your ingredients (fetch), then you read the instructions to understand how to combine them (decode), next you actually cook (execute), and finally, you serve or store the food (store). Each recipe might also have special steps or adjustments (interrupts) based on what's available in your pantry.

Machine instructions are generally complex and require multiple clock cycles to complete. For example, an indirect machine instruction is more complex than an immediate machine instruction.

Machine instructions, which are also called macro instructions, can vary significantly in complexity. For instance, an indirect machine instruction may involve multiple steps, as it requires accessing an address within a variable that itself must be fetched from another location in memory before the actual operation can be performed. In contrast, immediate addressing allows the CPU to access memory directly, which is quicker. This variance in complexity results in different execution times, often requiring multiple clock cycles for more complex instructions.

Consider sending someone to retrieve a book from a library (indirect instruction). They first need to find out where the book is located, walk to the spot, and then get the book. This is much more complex than simply asking someone for a book you have in your own bag (immediate instruction).

Micro instructions basically are the atomic instructions, which execute in a single clock unit and which totally comprise a generic instruction.

Micro instructions represent the smallest set of operations that the control unit of a CPU can perform. Each macro instruction, which can be thought of as a 'big' instruction that tells the computer what to do, is made up of these smaller micro instructions. These micro instructions are designed to be executed within a single clock cycle, making them essential for the smooth and fast execution of larger instructions.

Think of macro instructions as sentences and micro instructions as the individual words in those sentences. Just as a sentence is made up of multiple words that combine to communicate an idea, a macro instruction is composed of multiple micro instructions that together execute a task.

The operations involved in the 4 cycles can be carried out using 1 or 4 micro operations in some predefined frequency.

Each phase of the instruction cycle can employ different types of micro operations. For example, data transfer operations involve moving data between CPU registers and memory. Arithmetic operations include calculations that are performed on data held in the CPU. Each type of operation may consist of several individual micro operations, all of which must be executed correctly to ensure that the CPU functions as intended.

Imagine going to a post office (CPU) to send a letter (data). The actions you take can be compared to micro operations: getting in line to fill out a form (data transfer), placing the letter in the mailbox (arithmetic operation), and finally making sure it gets sent at the right time (execution). Each step is important and contributes to the overall task of mailing a letter.

We call it clock grouping; if you give directly separate time units to all this then you require more number of micro instructions to operate a macro instruction.

Clock grouping is a technique used to optimize the execution of instructions by allowing certain micro instructions that can be executed simultaneously to be processed in the same clock cycle. This reduces the overall execution time required for macro instructions, as it minimizes the number of cycles needed to perform the necessary operations.

Think of clock grouping like a team of workers at a construction site. If one worker is responsible for pouring concrete (micro operation A) and another for placing bricks (micro operation B), and these tasks can happen at the same time without getting in the way of each other, then completing both tasks at once saves time. If the tasks are dependent, however, they need to be completed in separate time units, thus increasing the total time needed.

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