Instructions: Language of the

2.3 Operands of the Computer Hardware

两个概念：

word A natural unit of access in a computer, usually a group of 32 bits.

doubleword Another natural unit of access in a computer, usually a group of 64 bits; corresponds to the size of a register in the RISC-V architecture.

这两个是用来定义RISC-V的寄存器（register）的：

The size of a register in the RISC-V architecture is 64 bits; groups of 64 bits occur so frequently that they are given the name doubleword in the RISC-V architecture. (Another popular size is a group of 32 bits, called a word in the RISC-V architecture.)

而寄存器的数量是有限的，通常是32个：

One major difference between the variables of a programming language and registers is the limited number of registers, typically 32 on current computers, like RISC-V.

也就是说，寄存器能提供32个64位的存储空间，这符合了：

Design Principle 2: Smaller is faster.

寄存器的命名方法，是加上x：

Although we could simply write instructions using numbers for registers, from 0 to 31, the RISC-V convention is x followed by the number of the register, except for a few register names that we will cover later.

而在存储（memory）的空间中：

Since 8-bit bytes are useful in many programs, virtually all architectures today address individual bytes. Therefore, the address of a doubleword matches the address of one of the 8 bytes within the doubleword, and addresses of sequential doublewords differ by 8.

如下图所示：

示例：

而从常数操作上，两种方法都可以：