x86 optable reference
---------------------
(incomplete)

    P   - modrm (reg mmx)
    PR  - modrm (rm mmx, mod must be 11b)
    Q   - modrm (rm mmx if mod=11b else mem)

    V   - modrm (reg - xmm)
    VR  - modrm (rm xmm, mod must be 11b)
    W   - modrm (rm xmm if mod=11b else mem)

    G   - modrm (reg - gpr)
    S   - modrm (reg - seg)
    VR  - modrm (rm gpr, mod must be 11b)
    E   - modrm (rm gpr if mod=11b else mem)
    M   - modrm (mem), mod!=11b

    I   - immediate
    J   - relative immediate
    O   - memory offset
    
    C   - control reg
    D   - debug reg


    opc <>

        /n       - modrm reg field extends opcode
        /Mnn     - disassembly mode extends opcode
        /Onn     - operand mode extends opcode
        /mod=!11 - modrm mod field extends opcode