06_目标代码

Stack栈区，局部变量 Dynamic data堆区 Text写指令的地方 static data放全局变量

x0：存放0的寄存器

• 伪指令：汇编语言中的语法糖
  • li：load immediate（伪指令）
  • mv是伪指令，实现 add t0, t1,zero
    减法实现：直接加负数

代码演示

add

li t0, 20   # li: load immediate

li t1, 22

add t2, t0, t1

addi

li t0, 20   # li: load immediate

li t1, 22

add t2, t0, t1

ecall

• 函数调用传参
  • a0-a7寄存器
  • a0,a1同时还是存放返回值的寄存器

系统调用

# f = (g + h) - (i + j)
# t6 = (t0 + t1) - (t3 + t4)

li t0, 0
li t1, 10
add t2, t0, t1

li t3, 30
li t4, 40
add t5, t3, t4

sub t6, t2, t5

# 系统调用号一定要放到a7里
li  a7, 1      
mv a0, t6    # add a0, t6, zero   
ecall

data

没有.text就默认文件开始。
.data放全局变量，在内存的data区
.word：告诉汇编器，数据占4Byte
.half
.string：后面有\0
.ascii：后面没有\0

# f = (g + h) - (i + j)
# t6 = (t0 + t1) - (t3 + t4)

.data
g: .word 0
h: .word 10
i: .word 30
j: .word 40

result: .word 0

msg: .string "The result is :"	# .ascii

.text
la t0, g		# la: load address 取地址操作
lw t0, 0(t0)	# lw: load word ;偏移量(基址)

la t1, h		
lw t1, 0(t1)	
add t2, t0, t1

la t3, i		
lw t3, 0(t3)	
la t4, j		
lw t4, 0(t4)
add t5, t3, t4

sub t6, t2, t5

la t0, result
sw t6, 0(t0)	# sw: store word

li  a7, 4    # print 
la a0, msg
ecall

li  a7, 1      
mv a0, t6    # add a0, t6, zero   
ecall

array

.data
numbers: .word -30, 30, -20, 20, -10, 10, 0

.text
la t0, numbers
lw t1, 12(t0) # 偏移12Byte，取第4个数(20)
addi t1, t1, 80
sw t1, 12(t0) # 写回

branch-max

跳转指令

# c = max(a, b)

.data
a: .word 100
b: .word 200
c: .word 0

.text
lw t0, a # risc-v不支持这种写法
lw t1, b

bge t0, t1, greater_equal
mv t2, t1
j end	# j: jump

greater_equal:
mv t2, t0

end:
sw t2, c, t3

array-for

.data
numbers: .word -30, 30, -20, 20, -10, 10, 0
size: .word 7

positive_sum: .word 0
negative_sum: .word 0

.text

la t0, numbers      # t0: the address of the array
lw t1, size         # t1: size = 7
mv t2, zero         # counter, initially 0
li t3, 0                # t3: sum of positive numbers <- 0
li t4, 0                # t4: sum of negative numbers <- 0

loop:
    bge t2, t1, end_loop
    
    # numbers[t2]
    # mul t5, t2, 4  # 下标乘4
    slli t5, t2, 2      # slli: shift left logical immediate 移位操作
    add t5, t0, t5
    lw t5, 0(t5)
    addi t2, t2, 1
    bltz t5, negative               # bltz: branch if less than zero
    add t3, t3, t5
    j loop

negative:
    add t4, t4, t5
    j loop
    
end_loop:
    sw, t3, positive_sum, t5
    sw, t4, negative_sum, t5

proc-max

• 函数调用
  1. 保存返回地址
  2. 转移控制权

# proc-max.asm

.data
max_result: .word 0

.text
.global main

max:
# a0 (argument 0), a1
blt a0, a1, smaller
j end_max

smaller:
mv a0, a1

end_max:

ret
# jr ra	# jr: jump register
# jalr zero 0(ra)	# jalr: jump and link register
# jal: jump and link

############### main ###############
.data
a: .word 100
b: .word 200

.text
main:
lw a0, a
lw a1, b

call max
# jal max
# jal ra, max		# jal: jump and link		ra: return address register
# TODO
sw a0, max_result, t0

proc-fact

.text
.global main

factorial:
beqz a0, base_case

addi sp, sp, -8
sw a0, 4(sp)
sw ra, 0(sp)

# n > 0: n * factorial(n - 1)
addi a0, a0, -1		# a0: n - 1
call factorial		# a0: factorial(n - 1)
mv t0, a0			# t0: factorial(n - 1)

lw a0, 4(sp)			# a0: n
lw ra, 0(sp)
addi sp, sp, 8

mul a0, a0, t0		# a0: n * factorial(n - 1)
j end

base_case:
	li a0, 1

end:
	ret


######### main ########
.data 
n: .word 10

.text
main:
lw a0, n
call factorial

sub-add

# f = (g + h) - (i + j)
# t6 = (t0 + t1) - (t3 + t4)

li t0, 0
li t1, 10
add t2, t0, t1

li t3, 30
li t4, 40
add t5, t3, t4

sub t6, t2, t5