addiu instruction encoding (MIPS,GCC)

Here is addiu instruction opcode (16-bit instructions, GCC option -mmicromips):

 full instruction: addiu sp,sp,-280
 opcode, hexa:     4F75         
 opcode, binary:   1001(instruction) 11101(sp is $29) 110101

My purpose is to detect all instruction of this kind (addiu sp,sp,) and then to decode the immediate, in the above case (-280) (to follow the sp). What I don't understand is the encoding of (-280). Linked to: How to get a call stack backtrace?(GCC,MIPS,no frame pointer)

Copyright Notice：Content Author:「David」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/45941568/addiu-instruction-encoding-mips-gcc

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