The AMD64 specification talks about /0 with regards to instruction encoding but I don't have a clue what is meant by that. For example, in Volume 3 one variant of the ADD instruction has three forms:
ADD reg/mem16, imm16 81 /0 iw Add imm16 to reg/mem16
ADD reg/mem32, imm32 81 /0 id Add imm32 to reg/mem32.
ADD reg/mem64, imm32 81 /0 id Add sign-extended imm32 to reg/mem64.
These all use the opcode 81 followed by /0 which I presume distinguishes between the three followed by the immediate value iw or id. But what does /0 mean?