Why the risc-v instruction "addi sp,sp,-32" is converted to binary code "11 01"?

I'm learning risc-v instruction set. I know that "addi sp,sp,-40" will be converted to "fd810113". But I don't know why "addi sp,sp,-32" is converted to binary code "11 01".

enter image description here

Solution

That is using the compressed instruction format:

C.ADDI[2:0] nzimm[5] dest[4:0] nzimm[4:0] C1[1:0]

With C.ADDI being 000, SP being register #2 and C1 being 01 it gives:

000 1 00010 00000 01 = 0001 0001 0000 0001 = 0x1101

Note that the compressed instruction only takes immediate from -32 to +31 due to having 6 bits of space which is why the other instruction with -40 is using the 32 bit format.