Opcode format

   opcode  hexadecimal digit(s)

   /digit (digit is between 0 and 7)
           Indicates that the ModR/M byte of the instruction uses only the
           r/m (register or memory) operand. The reg field contains the
           digit that provides an extension to the instruction's opcode.

   /r      Indicates that the ModR/M byte of the instruction contains both
           a register operand and an r/m operand.

   cb, cw, cd, cp
           A 1-byte (cb), 2-byte (cw), 4-byte (cd), or 6-byte (cp) value
           following the opcode that is used to specify a code offset and
           possibly a new value for the code segment register.

   ib, iw, id
           A 1-byte (ib), 2-byte (iw), or 4-byte (id) immediate operand to
           the instruction that follows the opcode, ModR/M bytes, or
           scale-indexing bytes. The opcode determines if the operand is a
           signed value. All words and doublewords are given with the
           low-order byte first.

   +rb, +rw, +rd
           A register code, from 0 through 7, added to the hexadecimal
           byte given at the left of the plus sign to form a single opcode
           byte. The codes are:

           rb          rw          rd
           al = 0      ax = 0      eax = 0
           cl = 1      cx = 1      ecx = 1
           dl = 2      dx = 2      edx = 2
           bl = 3      bx = 3      ebx = 3
           ah = 4      sp = 4      esp = 4
           ch = 5      bp = 5      ebp = 5
           dh = 6      si = 6      esi = 6
           bh = 7      di = 7      edi = 7