第5章アルゴリズムのハードウェアによる実現：コンピューターシステム設計

The instruction set architecture (ISA) is the set of instructions recognized by each processor family. For example, both Intel and AMD processors use the same ISA, called x86-64, which is different from the ISA recognized by ARM or IBM. This makes us revisit the concept of compilers. A compiler takes as input a program written in an HLL, and we have a compiler for each HLL. The output of the compiler is an assembly program in a specific ISA, and we have a compiler for each different ISA (e.g., ISA 1 and ISA 2). So, if we have programs written in three HLLs and we need to generate assembly for processors of two different families, then we need six compilers as shown in Table 5.1.

As we saw earlier, the output of the compiler is the input to the assembler so we need assemblers for each ISA in existence to generate a machine language file (e.g., .exe file) that the processor can execute.

As we cross the layer of ISA in Figure 5.5, we cross the boundary between software and hardware. Before we discuss hardware, we need to understand two words: translator and interpreter. Both words mean “translating from language 1 to language 2” regardless of what those languages are. The main difference is the process by which translation is done. A translator takes a whole program in language 1 and generates another program in language 2. For example, the compiler takes an HLL as input (language 1) and generates the corresponding assembly language program (language 2). The interpreter takes one line (or command) in the program in language 1 and generates one (or more) instructions in language 2. Python is a popular example of an interpreted language.

Understanding the hardware level allows us to see how computers execute programs. The main part of the computer hardware that does the execution is called the processor. The processor takes one instruction from the machine language file, executes it, and writes the results back in a designated place. Then, it fetches the following instruction and does the same. It keeps doing this until the program ends or an error occurs. This is an oversimplification, but it conveys the general idea. As you can see, it takes one instruction at a time, which is why the vertical arrow coming out of the machine language box in Figure 5.5 shows the word interpreter. But how does the processor do its job of fetching an instruction and executing it? To answer this question, we need to look at the main components of a processor.

The transistor is the building block of the hardware of any computer. A transistor is merely an on/off switch; when it is on, it lets the electrical current pass. When it is off, it blocks the electrical current. This is very similar to the light switch you find in your room. But with these transistors, we can do more: with transistors, we build computers. Read this article to learn more about transistors (https://openstax.org/r/ 76Transistors) and how they work.

The architecture (i.e., design) of the microprocessor (i.e., the processor) is called microarchitecture (Figure 5.9). Its main job is to design the different components of the processor and decide how to connect them so that the processor can do its job. For example, one important piece inside the processor is the part that fetches the next instruction of the program. Another crucial part, called the control unit, is responsible for decoding, or understanding, what this instruction wants to do and then tells the other components of the processor to execute this instruction. So, the control unit’s job is to take an instruction as input and generate signals to control the rest of the processor to make it execute the instructions. Other parts of the processor include the execution units that do the actual computations such as divide, multiply, add, and subtract.

To summarize, inside the processor you find those different pieces that fetch instructions, decode them, and execute them. But what is inside each one of these black boxes?