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))

0xFD78F7FE
0xFD78F7FE
0xFD78F7FE
# instr = 0xFD78F7FE
# print(bin(instr))
# 11110 1 1111111110 11 1 1 1 10101111000
#       S  imm10       J1  J2   imm11
另外说明一下，对于小系统来说，S, J1, J2，受限于内存，三者在算法1中，都能得到，所以才有2,3的简化算法取值；