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 (0/1) comes from (the register number), but I'm trying to figure out where the second last digit (1) comes from. According to AMD64 Architecture Programmer’s Manual Volume 3: General-Purpose and System Instructions page 56,

"/digit - Indicates that the ModRM byte specifies only one register or memory (r/m) operand. The digit is specified by the ModRM reg field and is used as an instruction-opcode extension. Valid digit values range from 0 to 7."

The equivalent Intel document says something similar, and call via a register is specified to be encoded as

FF /2

and... I have no idea what that means, or how the 2 in the specification connects to the high 1 digit in the end result. Is there a differently worded explanation available anywhere?

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).

Answer 2

I think you want to check table 2-2 in Intel® 64 and IA-32 Architectures Developer's Manual: Combined Volumes, Volume 2: Instruction Reference Set, Chapter 2: Instruction Format, 2.1.5 Addressing-Mode Encoding of ModR/M and SIB Bytes:

Table 2-2. 32-Bit Addressing Forms with the ModR/M Byte

r8(/r)                       AL   CL   DL   BL   AH   CH   DH   BH
r16(/r)                      AX   CX   DX   BX   SP   BP   SI   DI
r32(/r)                      EAX  ECX  EDX  EBX  ESP  EBP  ESI  EDI
mm(/r)                       MM0  MM1  MM2  MM3  MM4  MM5  MM6  MM7
xmm(/r)                      XMM0 XMM1 XMM2 XMM3 XMM4 XMM5 XMM6 XMM7
(In decimal) /digit (Opcode) 0    1    2    3    4    5    6    7
(In binary) REG =            000  001  010  011  100  101  110  111
Effective Address Mod R/M    Value of ModR/M Byte (in Hexadecimal)
[EAX]              00 000    00   08   10   18   20   28   30   38
[ECX]                 001    01   09   11   19   21   29   31   39
[EDX]                 010    02   0A   12   1A   22   2A   32   3A
[EBX]                 011    03   0B   13   1B   23   2B   33   3B
[--][--] *1           100    04   0C   14   1C   24   2C   34   3C
disp32 *2             101    05   0D   15   1D   25   2D   35   3D
[ESI]                 110    06   0E   16   1E   26   2E   36   3E
[EDI]                 111    07   0F   17   1F   27   2F   37   3F
[EAX]+disp8 *3     01 000    40   48   50   58   60   68   70   78
[ECX]+disp8           001    41   49   51   59   61   69   71   79
[EDX]+disp8           010    42   4A   52   5A   62   6A   72   7A
[EBX]+disp8           011    43   4B   53   5B   63   6B   73   7B
[--][--]+disp8        100    44   4C   54   5C   64   6C   74   7C
[EBP]+disp8           101    45   4D   55   5D   65   6D   75   7D
[ESI]+disp8           110    46   4E   56   5E   66   6E   76   7E
[EDI]+disp8           111    47   4F   57   5F   67   6F   77   7F
[EAX]+disp32       10 000    80   88   90   98   A0   A8   B0   B8
[ECX]+disp32          001    81   89   91   99   A1   A9   B1   B9
[EDX]+disp32          010    82   8A   92   9A   A2   AA   B2   BA
[EBX]+disp32          011    83   8B   93   9B   A3   AB   B3   BB
[--][--]+disp32       100    84   8C   94   9C   A4   AC   B4   BC
[EBP]+disp32          101    85   8D   95   9D   A5   AD   B5   BD
[ESI]+disp32          110    86   8E   96   9E   A6   AE   B6   BE
[EDI]+disp32          111    87   8F   97   9F   A7   AF   B7   BF
EAX/AX/AL/MM0/XMM0 11 000    C0   C8   D0   D8   E0   E8   F0   F8
ECX/CX/CL/MM/XMM1     001    C1   C9   D1   D9   E1   E9   F1   F9
EDX/DX/DL/MM2/XMM2    010    C2   CA   D2   DA   E2   EA   F2   FA
EBX/BX/BL/MM3/XMM3    011    C3   CB   D3   DB   E3   EB   F3   FB
ESP/SP/AH/MM4/XMM4    100    C4   CC   D4   DC   E4   EC   F4   FC
EBP/BP/CH/MM5/XMM5    101    C5   CD   D5   DD   E5   ED   F5   FD
ESI/SI/DH/MM6/XMM6    110    C6   CE   D6   DE   E6   EE   F6   FE
EDI/DI/BH/MM7/XMM7    111    C7   CF   D7   DF   E7   EF   F7   FF
NOTES:
1. The [--][--] nomenclature means a SIB follows the ModR/M byte.
2. The disp32 nomenclature denotes a 32-bit displacement that follows the ModR/M byte (or the SIB
byte if one is present) and that is added to the index.
3. The disp8 nomenclature denotes an 8-bit displacement that follows the ModR/M byte (or the SIB
byte if one is present) and that is sign-extended and added to the index.