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

There has been a debate among companies who are designing hardware as to whether CISC or RISC is better. Read this article chronicling this debate (https://openstax.org/r/76DebCISCvsRISC) from MicrocontrollerTips.

In our discussion of Figure 4.18 we saw assembly (output of the compiler) and object code, binary code, and machine code, which all designate output of the assembler. Machine code is the binary presentation of the assembly code. In some cases, there are assembly instructions that do not have a counterpart in the machine code called pseudo-assembly. For example, there are instructions in assembly that execute a go to if a number is less than another number. The only conditions known in machine code are equal and not equal, but not less than, less or equal, and so on. We can see this in an instruction set like MIPS.

The assembler’s job is to ensure that the machine code file only contains instructions that are native to the processor; that is, part of the ISA. So, we can think of the machine code as being a subset of the assembly code. You never find an instruction in the machine code file that is not part of the ISA of the processor for which you want to generate the binary. The reason there are pseudo-assembly instructions is to give the compiler a bit more freedom to generate efficient code. Let us assume that you write a program in C and you think about functions calling each other. If you write a program in C++ or Java, you think in terms of objects, methods, inheritance, and so on (refer to Chapter 4 Linguistic Realization of Algorithms: Low-Level Programming Languages). We call this the programmer view of the language. What if you write (or read) assembly code? What do you see? This view is summarized in Figure 5.17.

Figure 5.17 shows the following:

• There is a processor (CPU) and memory.

• The CPU and the memory are connected by a bus, which is a data pathway. When you access the memory, you may want to get data and must provide an address. Or you may want to write data to memory, so you must provide both the data and the address to which this data will be written. In both cases, the CPU must provide an address, therefore, the address bus is single directional. But the data bus is bidirectional because you can send data to memory or get data from memory.

• Data is not the only thing you need to bring from the memory to the CPU. The main job of the CPU is to execute instructions on data. For example, adding two numbers involves data (the two numbers) and the command for addition (instruction), which is why there is a single directional bus from memory to CPU for getting instructions from the memory.

• The memory holds several things: data, the instructions of the programs (shown as “Text” in Figure 5.17), some data needed by the OS to manage your program, and the resources it needs. The stack and heap are places in the memory to store data depending on the program at hand. The stack is used to store local variables (and some other stuff that we will discuss later), the heap stores dynamically allocated data, an Data in Figure 5.17 is another area in the memory to store global variables.

• Inside the CPU, there are registers which carry hardware parts that store data, instructions, addresses, and so on. Each register stores one item. An x86 programmer has access to 16 registers, as we will see shortly. Because the CPU is executing a program, which is a series of instructions, the CPU must keep track of what the next instruction to be executed is.

• Keeping track is the job of a specific register, shown separately from the other registers, called the program counter (PC). The PC is updated after executing each instruction to point to the next instruction to be executed. Also, it is useful to keep some information about the result of the instructions executed, such as whether the result generated by the current instruction is positive, negative, or zero, which is the job of the flags. A flag tells a program if a condition has been met.