在ARM手册中BL指令如下描述(其后跟立即数的描述如下，跟寄存器直接看手册很简单)：

其中比较难理解的就是J1，J2的获取，在手册中有描述，请看如下所示：

I1,I2就是立即数的bit[23:22]两个bit位。

# imm32表示立即数

I2 = imm32 & (0x01 << 22)

I1 = imm32 & (0x01 << 23)

S：表示符号最高为topbit(x)

# offset 带符号的立即数

S = (0x01 << 26) if offset < 0 else 0

则J1，J2的可以按如下计算

J1, J2 = (1 if I1 else 0, 1 if I2 else 0) if S else (0 if I1 else 1, 0 if I2 else 1)

J2 <<= 11

J1 <<= 13

1. 算法1

def signExtend(address, entry):

offset = entry - address - 4

imm32 = offset & 0x7fffffff

# contains the sign bit

S = (0x01 << 26) if offset < 0 else 0

imm11 = (imm32 & 0x00000FFE) >> 1 # [11: 1]

imm10 = (imm32 & 0x003FF000) >> 12 << 16 # [21: 12]

I2 = imm32 & (0x01 << 22)

I1 = imm32 & (0x01 << 23)

J1, J2 = (1 if I1 else 0, 1 if I2 else 0) if S else (0 if I1 else 1, 0 if I2 else 1)

J2 <<= 11

J1 <<= 13

code = 0xF0000000 # [31: 27], BL

instr = code | S | imm10 | 0x0000C000 | J1 | 0x00001000 | J2 | imm11

imm32 = ((instr & 0xffff) << 16) | ((instr >> 16) & 0xffff)

return imm32

2. 算法2

def signExtend2(address, entry):

imm32 = (entry - address - 4) // 2

low = 0xF800 | (imm32 & 0x7FF)

high = 0xF000 | (0x03 << 9) | ((imm32 >> 11) & 0x1FF)

return ((low << 16) | high)

3. 算法3

def signExtend3(address, entry):

offset = ( entry - address - 4 ) & 0x007fffff

high = 0xF000 | offset >> 12

low = 0xF800 | (offset & 0x00000fff) >> 1

imm32 = (low << 16) | high

return imm32

测试示例：

data = signExtend(0x20001580, 0x20000074)

data2 = signExtend2(0x20001580, 0x20000074)

data3 = signExtend3(0x20001580, 0x20000074)

print('0x{:08X}'.format(data))

print('0x{:08X}'.format(data2))

print('0x{:08X}'.format(data3))

输出：

0xFD78F7FE

0xFD78F7FE

0xFD78F7FE

# instr = 0xFD78F7FE

# print(bin(instr))

# 11110 1 1111111110 11 1 1 1 10101111000

# S imm10 J1 J2 imm11