Instructions: Language of the

2.5 Representing Instructions in the Computer

顾名思义，这一节是讲如何将RISC-V指令在电脑中实现。

先看2个概念：

instruction format A form of representation of an instruction composed of fields of binary numbers.

machine language Binary representation used for communication within a computer system.

这是一层一层的翻译了 —— 指令集有其格式，而表示指令集格式的语言是机器语言，告诉机器如何操作的语言。

第一个例子，讲一下基本逻辑：

针对以上的例子，有如下的定义：

opcode: Basic operation of the instruction, and this abbreviation is its traditional name.

rd: The register destination operand. It gets the result of the operation.

funct3: An additional opcode field.

rs1: The first register source operand.

rs2: The second register source operand.

funct7: An additional opcode field.

除了R型（R-type (for register)），进一步扩展，还有其他两种类型（I和S）至今用到的：

可以看到，指令长度是相同的：

The compromise chosen by the RISC-V designers is to keep all instructions the same length, thereby requiring distinct instruction formats for different kinds of instructions.

这背后是第三个原则：

Design Principle 3: Good design demands good compromises.

第二个示例：

参照指令格式的表格，答案是：

其他几个示例：

以上这种将指令也作为数字的处理方式，背后是一个重要的概念：

Today’s computers are built on two key principles:

1. Instructions are represented as numbers.

2. Programs are stored in memory to be read or written, just like data.

These principles lead to the stored-program concept; its invention let the computing genie out of its bottle.

一切都是数字：

这带来了技术和商业上的集中化 —— 根源于其背后的是，数字与代码的零复制成本：

One consequence of instructions as numbers is that programs are often shipped as files of binary numbers. The commercial implication is that computers can inherit ready-made software provided they are compatible with an existing instruction set. Such “binary compatibility” often leads industry to align around a small number of instruction set architectures.