计算机组成

Chapter2 Instructions

2.1 Introduction

To command a computer’s hardware, you must speak its language. The words of a computer’s language are called instructions, and its vocabulary is called an instruction set. In this chapter, you will see the instruction set of a real computer, both in the form written by people and in the form read by the computer. We introduce instructions in a top-down fashion. ...

2.2 Operation of the Computer Hardware

一条指令的格式一般是 [operation] [operand] [operand] [operand]

Design Principle Simplicity favors regularity

2.3 Operands of the Computer Hardware

arithmetic operands

一条算术指令的 operand 只能来自于 register

Note

RISC-V architecture 提供 32 个数据寄存器，分别命名为 x0 ~ x31 ，每个寄存器的大小是 64 位。在 RISC-V architecture 中，一个 word 为 32 位，一个 doubleword 为 64 位。这些寄存器中的一部分有专门的用途，我们稍后对其进行讨论。

为什么只有32个？ Design Principle: Smaller is faster.

memory operands

As explained above, arithmetic operations occur only on registers in RISC-V instructions; thus, RISC-V must include instructions that transfer data between memory and registers. Such instructions are called data transfer instructions.
To access a word or doubleword in memory, the instruction must supply the memory address. Memory is just a large, single-dimensional array, with the address acting as the index to that array, starting at 0.

Note

The data transfer instruction that copies data from memory to a register is traditionally called load. The format of the load instruction is the name of the operation followed by the register to be loaded, then register and a constant used to access memory. The sum of the constant portion of the instruction and the contents of the second register forms the memory address. The real RISC-V name for this instruction is ld, standing for load doubleword

The instruction complementary to load is traditionally called store; it copies data from a register to memory. The format of a store is similar to that of a load: the name of the operation, followed by the register to be stored, then the base register, and finally the offset to select the array element. Once again, the RISC-V address is specified in part by a constant and in part by the contents of a register. The actual RISC-V name is sd, standing for store doubleword

Constant or Immediate Operands

2.4 Representing Numbers in the computer

2.5 Representing Instructions in the computer

• R type (for register)

• I type (with constant) 用于 使用立即数的算术运算、ld类

PS：

• S type (for sd类)

计算机会根据最低位的 opcode 分辨当前指令的类型

2.6 Logical operation

Instructions for making decisions

Branch: If-else & Loop

two decision-making instructions: (branch if eual and branch if not equal) beq rs1, rs2, L1 and bne rs1, rs2, L1 等

Bounds Check Shortcut

bgeu x20, x11, IndexOutOfBound // if x20 >= x11 or x20 < 0, goto IndexOutOfBounds 因为负数使用补码表示的特殊性，可以用无符号数的比较，对一个数组的 index 进行 out-of-bounds check

branch指令经常和 slt 指令合用

Case/Switch

Procedures

What is and what is not preserved across a procedure call? and how?

Memory Layout

注意堆栈和堆都属于 Dynamic Data区

Tips

leaf procedure(没有调用其他函数的块)，可以任意使用 temporary register

Branch Addressing & Jump Addressing

指令中存放的只有 11 ([12:1]) 位, 实际跳转地址时还要将所加立即数 乘 2

Disassembly