While learning about x64, I struggled to understand some notations in the intel manual.

Let’s look at 0xC7 MOV:

| opcode | instruction| Op/Enc| Description|
|:——- |:———-:|:—–:|:—-| —|
|C7 /0 iw | MOV r/m16, imm16 | MI | Move imm16 to r/m16.|
C7 /0 id | MOV r/m32, imm32 | MI | Move imm32 to r/m32. |
REX.W + C7 /0 id |MOV r/m64, imm32 | MI | Move imm32 sign extended to 64-bits to r/m64. |

About the /0../7 it says:

Historically this document only specified the ModR/M.reg field
restrictions with the notation /0 … /7 and did not specify
restrictions on the ModR/M.mod and ModR/M.r/m fields in the encoding
boxes.

My questions are:

• That number after /0 is a restriction code, but which restriction? What does it mean while interpreting an instruction?
• If I’m reading bytes like c7c701020304... How to know if I need to ready imm16(0102 or imm32(01020304)? By testing, I know that in this imm32, but I don’t understand why. I deduce that it is related to rex+modrm.

{ xxd --ps -r | ndisasm -b64 -; } <<<c7c701020304 
00000000  C7C701020304      mov edi,0x4030201

• In other words: Why ndisasm parses that as imm32 instead imm16?