assembly - Understanding Instruction Encoding?

Question

I used a website to encode this:

movw $8, 4(%r8d,%esi,4)

and got:

encoding (hex): 67 66 41 C7 44 B0 04 08 00

Thanks to you I nearly understand everything except 2 small points:

1. Here we are moving 2 bytes immediate to 4 bytes address. They used C7 opcode which according to the table I have means one of the following:

• mov imm16 to r/m16
• mov imm32 to r/m32
• mov imm32 (sign extended) to r/m64

Why there is no match?

2. Why immediate is 2 bytes? according to what?

Answer 1

Waitting for answers