x86_64 - opcode map. Possible mistake?

Question

I am working on the intel opcode map for x86_64, vol2, section B.2.1

I have an issue with the pop instruction.

POP – Pop a Value from the Stack

wordregister
0101 0101 : 0100 000B : 1000 1111 : 11 000 reg16

qwordregister
0100 W00BS : 1000 1111 : 11 000 reg64

wordregister (alternate encoding)
0101 0101 : 0100 000B : 0101 1 reg16

qwordregister (alternate encoding)
0100 W00B : 0101 1 reg64

memory64
0100 W0XBS : 1000 1111 : mod 000 r/m

memory16
0101 0101 : 0100 00XB 1000 1111 : mod 000 r/m

The prefix 0101 0101 apparently used with wordregisters is really annoying me here, because I cannot find any use for it.

Even after submiting the command to objcopy or other disassembler, I never see it appearing.

0:  66 59                   pop    cx
2:  59                      pop    rcx

I can understand the 1000 1111 : 11 000 reg16 and 0100 000B : 0101 1 reg16 parts though.

So, what about that 0x55?

Answer 1

If you set the CPU to x86_64, a disassembler gives you

55    push rbp

And a click on 55 in this reference points to push as well.

The pop instructions have the bit 3 (4^th) to 1, the push have it at 0, like in 0x55.

There is definitely a mistake in the documentation, as the CPU instructions coding logic and other elements available on the Net clearly show 0x55 as being a push operation.