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RISC-V

为了这个语言专门开了 cs61c 这个坑。

Basic Introduction

What is ISA

  • Instruction Set Architecture, an abstract model that generally defines how software controls the CPU in a computer or a family of computers.
  • In general, an ISA defines the supported instructions, data types, registers; the hardware support for managing main memory, fundamental features (such as the memory consistency, addressing modes, virtual memory), and the input/output model of implementations of the ISA.
  • The level between software & hardware.

CISC & RISC

  • CISC: Complex Instruction Set Computing
    • difficult to learn
    • less work for the compiler
    • complicated hardware runs more slowly
  • RISC(Reduced): Simpler/smaller instruction set

What is RISC-V?

  • Open-source Instruction Set specification

  • Appropriate for all levels of computing

    • 32 / 64 / 128-bits
    • microcontrollers / supercomputers

  • RISC-V / x86 / ARM 之间的对比

  • Little Endian: 小端序，一个多位数，低位存小地址，高位存大地址（和正常阅读顺序相反）

How to Use Green Card?

  • 本节课使用 32 位机，greencard 中的以 w 为后缀的是为了 >32 位机准备，使得操作统一在 32 位。
  • FMT 是指操作系统把汇编语言转化为机器语言的方式，具体可见 greencard 中的右下角。

Registers

  • The more registers you have, the slower it is to access any of them.
  • number: 32, x0~x31.

  • Registers have no type.
  • Register zero(x0) always has the value 0 and cannot be changed.

Basic Instructions

Arithmetic Instructions

  • op dst, src1, src2：dst = src1 <op> src2.
  • opi dst, src, imm：
    • addi t1, s1, 5：a = b + 5.
    • 没有 subi：因为完全可以通过加负数来实现（但是 sub 是有的！！！）
  • mul dst, src1, src2; mulh dst, src1, src2：src1*src2，第 32 位用 mul 计算，高 32 位用 mulh 计算.
  • div dst, src1, src2; rem dst, src1, src2.

Compare Instructions

  • slt dst, reg1, reg2：dst = (reg1 < reg2)
  • slti dst, reg1, imm.

Data Transfer

memop reg, off(bAddr)

  • access memory at bAddr+off.
  • Load Word(lw): Takes data at address bAddr+off from the memory and place it into reg.
  • Store Word(sw): store the reg to the address.
  • word: 32-bits, 4 bytes.（通用于 RISC-V）

  • 有关小端序和 sign extention 的例子

  • Half-Word Instructions：

Control Flow Instructions

  • beq reg1, reg2, label: If reg1 == reg2 goto label, otherwise go to the next instruction.
  • bne reg1, reg2, label: not equal.
  • j label: jump.
  • blt reg1, reg2, label: reg1 < reg2.
  • bge reg1, reg2, label: reg1 >= reg2.

Program Counter

A special register that keeps track of the memory address of the next instruction to be executed in a program.

Loops

Environmental Call

  • ecall is a way for an application to interact with the operating system.
  • The value in register a0 is given to the OS which performs special functions.（不同操作系统所用的寄存器可能不相同）

Function in RISC-V

  • a0-a7: eight argument registers to pass parameters.(more arguments go to the stack)
  • a0-a1: two argument registers also used to return values.
  • sp: "stack pointer", holds the current memory address of the "bottom" of the stack.

Jumping and Returning

  • jal ra sum(jump and link): 设置 ra 为该指令的下一个地址(link)，然后去执行 sum 函数，执行结束以后再执行 jr ra，返回 ra 继续执行。

  • 在实际的 risc-v 中，其实没有 j label 这个语句，它是通过 jal x0 label 来实现的，因为对 x0 的写入是无效的。

    （jal x0 ra 是错误的，因为第二个参数必须是一个 label）

  • jr ra 可用 jalr x0, ra, 0 来实现，其中 0 是程序跳到 ra 的后续偏移量。

Local Storage for Variables

  • 主要靠移动栈指针 sp 来实现。

  • 核心问题出在函数嵌套上面——ra is overwritten?

  • s0-s11 作为 saved registers，当 callee 被调用时，要先把 caller 用的 s 全部存到栈里面。

    • s0: frame pointer（参考 x86 笔记—— stack frame）

  • 对于 ra 寄存器，在 caller 调用 callee 时，会提前把它进行存储。

    其他要存储的寄存器：t0-t6(tempporary registers), a0-a7(return address and arguments)

（注意！栈的保存是从高地址到低地址）

  • It's callee's duty to store the registers it wants to use to the stack before overrites it.

Instruction Formats

原则：尽量 32 位中相同的位置在不同的 Formats 中能有相同的作用。

R-Format

有 dest 三寄存器运算(rd: register destination)。

I-Format

有 dest 二寄存器一立即数运算；如读取数据操作。

S-Format

无 dest 二寄存器一立即数运算；如保存数据操作。

SB-Format

用于 branch 操作。

  • Extensions to RISC-V base ISA support 16-bit compressed instructions.
  • 16-bit = half-word.
  • To enable this, RISC-V scales the branch offset to be half-words even there are no 16-bit instructions.
  • PC-Relative Addressing: 以 PC 为中心定位代码地址（所以一个完整的代码无论存在哪里相对偏移量都是不变的）

U-Format

Dealing With Large Immediates.

  • lui writes the upper 20 bits of the destination with the immediate value.
  • Together with addi to set low 12 bits.
  • The key to PC-Relative Addressing: auipc x10 0.

UJ-Format

  • For branches, we assumed that we won't branch too far, but for general jumps(jal), we may jump anywhere in code memory.
  • notice: jalr is I-Format.(rd = PC+4; PC = rs1 + offset )