Instruction Decoding Process: A Breakdown of CPU Operations

When a computer retrieves instructions from memory, the process of decoding these instructions is as intricate as it is essential. This article delves into how the instruction decoding process unfolds, how CPUs fetch instruction bits, and the different methods used for instruction decoding units. We will also explore the complex decoding design and provide insights through a historically inspired example. Understanding these foundational concepts is vital for anyone studying computer engineering or pursuing a university degree in the field.

Instruction Decoding Mechanisms

When the Central Processing Unit (CPU) retrieves instructions from memory, it does not fetch them bit by bit but in their entirety. At least, each instruction is fetched as a complete unit. This mechanism (often referred to as 'fetching a whole instruction at a time') allows for efficiency and speed in processing instructions. The fetched instruction is then placed in the Instruction Register (IR), marking the beginning of the decoding process.

Differential Decoding Methods

In the technical world of CPU design, there are multiple approaches to instruction decoding, each tailored to specific goals like efficiency, speed, and resource availability. Some instructions can be vastly different from others, leading to distinct decoding strategies.

Single Approach: Microinstruction Sequencer

In one method, the instruction is fetched in its entirety and is subsequently decoded by a decoder that retrieves microinstructions and inputs them into a microinstruction sequencer. This process might sometimes be facilitated through a simple shift register. While this is a common practice, it is essential to note that there are other methods and variations in implementation.

Alternative Approaches

The exact mechanism may vary based on the computer architecture and design. Some CPUs might use more sophisticated decoders or even pipelines, akin to what is found in modern CPU architectures. Regardless of the method, the goal is to efficiently convert a byte into a functional instruction for the CPU to execute.

Example of Instruction Decoding

Let's delve into a historical example. Imagine a small computer designed and built by a team in the mid-1970s. This computer featured an 8-bit data bus and a 12-bit address bus, providing an address range of 4096 bytes.

The first 2048 addresses were reserved for RAM, the next 1024 for input/output purposes, and the remaining 1024 addresses were used for bootstrap ROM, with the last 768 addresses left unused. The first 16 bytes of RAM served as registers, with the first register acting as an accumulator, and subsequent registers defining conditional jumps and addressing rules.

Instruction Decoding Breakdown

The two most significant bits (bit 2 and bit 3) define the function of the instruction. The next two bits (bit 2 and bit 3) define the address mode. The remaining four bits are used to create data addresses.

Address Mode Decoding

0 - Direct Addressing Mode: The four address bits directly address one of the 16 registers, using their contents directly. 1 - Indirect Addressing Mode: The four address bits point to a pair of registers. A 12-bit address is created from these two registers, providing an indirect address to any address in the total 4096 address range. 2 - Relative Addressing Mode: Four address bits replace the least significant four bits of the instruction address, allowing data access within a 16-byte block, keeping the top 8 bits of the instruction address. 3 - Absolute Addressing Mode: The four address bits become the most significant four bits of a 12-bit address, with the next instruction byte providing the least significant eight bits of the 12-bit address.

Depending on the address mode, the instruction address registers are adjusted to access the next instruction from one or two bytes beyond the current instruction's location. Once the address mode and function of the instruction are decoded, the next instruction's address is established.

Instruction Function Decoding

The instruction function is then decoded based on the two most significant bits (bit 0 and bit 1) of the instruction byte. Based on these bits, the CPU performs different operations such as addition, NAND, storage, and conditional jumps.

Example Functions

If bit 0 and bit 1 are 0, the ADD function adds the addressed byte into the accumulator (the very first byte of RAM). If bit 0 and bit 1 are 1, the NAND function NANDs the addressed byte into the accumulator. If bit 0 and bit 1 are 2, the STORE function stores the accumulator in the addressed byte, then clears the accumulator. If bit 0 and bit 1 are 3, the conditional jump function conditionally jumps to the instruction in the addressed byte based on the rule set up in the second byte of RAM.

While this was a very basic computer, it illustrates how complex the decoding process can be. From this example, you can see the intricate and multifaceted nature of CPU operations.