x64 instruction encoding and the ModRM byte

Question

The encoding of

call qword ptr [rax]
call qword ptr [rcx]

is

FF 10
FF 11

I can see where the last digit (

Answer 1

The ModR/M byte has 3 fields:

bit 7 & bit 6 = mod
bit 5 through bit 3 = reg = /digit
bit 2 through bit 0 = r/m

This is depicted in Figure 2-1. Intel 64 and IA-32 Architectures Instruction Format of Vol. 2A of Intel® 64 and IA-32 Architectures Software Developer’s Manual.

So, there:

  0x10 = 00.010.000 (mod=0, reg/digit=2, r/m=0)

and

  0x11 = 00.010.001 (mod=0, reg/digit=2, r/m=1).