Note: char in ARM processor is unsigned

  On the C standard, char can be signed or unsigned, but the specific platform is different. In most architectures, char is signed, but in the ARM system, char is unsigned and its value range is 0-255.

Such as the following code:

                         Figure one

On the X86 platform:

                                      Figure II

On the ARM-based HI3518EV200 platform:

① Compile

                                              Figure three

② Operation

                             Figure Four