基于 risc-v 平台的简单操作系统，旨在学习 risc-v 和实践.zip

# RISC-V 汇编入门 ## 环境搭建 在 Linux 下，直接下载 `riscv` 和 `qemu-riscv` 即可。 如果是 MAC，直接使用`brew`来安装： ```sh brew install qemu-riscv brew install riscv64-unknown-elf-gcc ``` TODO ## 数据加载、存储 ### 加载立即数 ```s # Load Immediate li rd, imm # rd = imm，加载立即数 # Load Upper Immediate lui rd, imm # rd = imm << 12，加载立即数，upper ``` ### 加载内存 ```s # Load Byte (Signed) lb rd, imm(rs1) # rd = [rs1 + imm] # Load Byte (Unsigned) lbu rd, imm(rs1) # unsigned rd = [rs1 + imm] ``` ### 加载地址 ```s # Load Address la x5, label # x5 = label ``` ### 存储内存 ```s # Store Byte sb rs2, imm(rs1) # [rs1 + imm] = rs2 ``` ### 寄存器数据传输 ```s # Move (Register to Register) mv x5, x6 # x5 = x6 ``` ## 运算符操作 ### 加、减法 ```s # Add add x5, x6, x7 # x5 = x6 + x7 # Add Immediate addi x5, x6, 1 # x5 = x6 + 1 # Substract sub x5, x6, x7 # x5 = x6 - x7 # Substract Immediate addi x5, x6, -20 # x5 = x6 - 20 ``` ### 左、右移 ```s # Shift Left Logical Immediate slli x5, x6, 3 # x5 = x6 << 3 # Shift Right Arithmetic Immediate 算术右移，带符号 srai x5, x6, 4 # x5 = x6 >> 4 # Shift Right Logical Immediate 逻辑右移，补0 srli x5, x6, 3 # x5 = x6 >> 3 ``` ### 逻辑运算 ```s # Negate neg x5, x6 # x5 = -x6 # Not not x5, x6 # x5 = ~x6 # xori 异或 xori x5, x6, -1 # x5 = x6 ^ -1 ``` ### nop ```s # Nop nop # do nothing and has no effect on system ``` ## 分支跳转 ### 直接跳转 ```s # Add Upper Immediate to PC auipc x5, 0x12345 # x5 = PC + (0x12345 << 12) auipc x6, 0 # x6 = PC, to obtain the current PC ``` ### 比较跳转 ```s # Branch if Not Equal bne x5, x6, loop # if x5 != x6, goto loop ``` ## 函数调用 ### square 计算平方 ```s # void _start() # { # // calling leaf routine # square(3); # } # # int square(int num) # { # return num * num; # } .text # Define beginning of text section .global _start # Define entry _start _start: la sp, stack_end # prepare stack for calling functions li a0, 3 call square # the time return here, a0 should stores the result stop: j stop # Infinite loop to stop execution # int square(int num) square: # prologue addi sp, sp, -8 sw s0, 0(sp) sw s1, 4(sp) # `mul a0, a0, a0` should be fine, # programing as below just to demo we can contine use the stack mv s0, a0 mul s1, s0, s0 mv a0, s1 # epilogue lw s0, 0(sp) lw s1, 4(sp) addi sp, sp, 8 ret # add nop here just for demo in gdb nop # allocate stack space stack_start: .rept 12 .word 0 .endr stack_end: .end # End of file ``` 函数执行，需要先开辟函数调用栈，即`stack_start`，且内存高位设置为`sp`，指向 栈顶。 `s0`，`s1`是传递参数，返回值为`ra`寄存器。 ### 多级调用 square ```s # void _start() # { # // calling nested routine # aa_bb(3, 4); # } # # int aa_bb(int a, int b) # { # return square(a) + square(b); # } # # int square(int num) # { # return num * num; # } .text # Define beginning of text section .global _start # Define entry _start _start: la sp, stack_end # prepare stack for calling functions # aa_bb(3, 4); li a0, 3 li a1, 4 call aa_bb stop: j stop # Infinite loop to stop execution # int aa_bb(int a, int b) # return a^2 + b^2 aa_bb: # prologue addi sp, sp, -16 sw s0, 0(sp) sw s1, 4(sp) sw s2, 8(sp) sw ra, 12(sp) # cp and store the input params mv s0, a0 mv s1, a1 # sum will be stored in s2 and is initialized as zero li s2, 0 mv a0, s0 jal square add s2, s2, a0 mv a0, s1 jal square add s2, s2, a0 mv a0, s2 # epilogue lw s0, 0(sp) lw s1, 4(sp) lw s2, 8(sp) lw ra, 12(sp) addi sp, sp, 16 ret # int square(int num) square: # prologue addi sp, sp, -8 sw s0, 0(sp) sw s1, 4(sp) # `mul a0, a0, a0` should be fine, # programing as below just to demo we can contine use the stack mv s0, a0 mul s1, s0, s0 mv a0, s1 # epilogue lw s0, 0(sp) lw s1, 4(sp) addi sp, sp, 8 ret # add nop here just for demo in gdb nop # allocate stack space stack_start: .rept 12 .word 0 .endr stack_end: .end # End of file ``` 先调用`aa_bb`，然后再调用两次`square`，参数不同，然后将结果相加返回。 ## 与 C 交互 先定义 C 函数`foo`： ```c int foo(int a, int b) { int c = a + b; return c; } ``` 然后在`test.s`中调用： ```s # ASM call C .text # Define beginning of text section .global _start # Define entry _start .global foo # foo is a C function defined in test.c _start: la sp, stack_end # prepare stack for calling functions # RISC-V uses a0 ~ a7 to transfer parameters li a0, 1 li a1, 2 call foo # RISC-V uses a0 & a1 to transfer return value # check value of a0 stop: j stop # Infinite loop to stop execution nop # just for demo effect stack_start: .rept 12 .word 0 .endr stack_end: .end # End of file ``` ## 运行 ```sh make run M=lb make debug M=lb ``` ```sh $ riscv64-unknown-elf-gcc -nostdlib -fno-builtin -march=rv32ima -mabi=ilp32 -g -Wall -Ttext=0x80000000 -o a.elf ./asm.S $ riscv64-unknown-elf-objcopy -O binary a.elf a.bin $ qemu-system-riscv32 -nographic -smp 1 -machine virt -bios none -kernel a.elf $ qemu-system-riscv32 -nographic -smp 1 -machine virt -bios none -kernel a.elf -s -S & $ riscv64-unknown-elf-gdb -q set disassemble-next-line on b _start target remote : 1234 c ``` ## 参考资料 - https://www.ruanyifeng.com/blog/2015/02/make.html