RDTSCP instruction missing (and 64 bit problems)

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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 05 Sep 2005, 03:27

Code:

        use64
        rdtscp                  ;<--_Error: illegal instruction (Encoding: 0F 01 F9)
        mov     rax,cr8         ;Ok
        mov     rax,cr9         ;<--_Error: undefined symbol
        mov     rax,cr10        ;<--_Error: undefined symbol
        mov     rax,cr11        ;<--_Error: undefined symbol
        mov     rax,cr12        ;<--_Error: undefined symbol
        mov     rax,cr13        ;<--_Error: undefined symbol
        mov     rax,cr14        ;<--_Error: undefined symbol
        mov     rax,cr15        ;<--_Error: undefined symbol
        mov     rax,dr8         ;Ok
        mov     rax,dr9         ;Ok
        mov     rax,dr10        ;Ok
        mov     rax,dr11        ;Ok
        mov     rax,dr12        ;Ok
        mov     rax,dr13        ;Ok
        mov     rax,dr14        ;Ok
        mov     rax,dr15        ;Ok
        mov     rax,[cs:rbx]    ;<--_generates unnecessary CS: override 2Eh
        mov     rax,[ds:rbx]    ;Ok
        mov     rax,[es:rbx]    ;<--_generates unnecessary ES: override 26h
        mov     rax,[ss:rbx]    ;<--_generates unnecessary SS: override 36h

05 Sep 2005, 03:27

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 05 Sep 2005, 09:16

I haven't included CR9-CR15, because I haven't found their existence documented, but I will include them for completness, OK. RDTSCP is perhaps something from the latest documentation updates? My old printed manuals don't mention it. As for the segment prefixes optimizations, I considered them while designing x86-64 support, however I decided they shouldn't be there - making assembler remove the ES: setting for you might make you think that it is the same as DS:, while the ES prefix is simply ignored, no matter what descriptor it points to. So I decided to leave it to programmer. The other solution might be to make usage of those prefixes illegal in long mode, but in such case I shouldn't also add the CR9-CR15 registers for example, since those opcodes are also illegal.

05 Sep 2005, 09:16

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 05 Sep 2005, 09:36

The AMD manual "AMD64 Architecture Programmer’s Manual Volume 3: General-Purpose and System Instructions" 24594 Rev. 3.10 February 2005 page 406 Table A-14 show all the CRx and DRx listed completely up to number 15.

Quote:

making assembler remove the ES: setting for you might make you think that it is the same as DS:

As for the prefixes: ES *IS* the same as DS (and CS, SS) in use64 mode. Sometimes a macro or EQUate will have a segment register that can be optimised out of the instruction stream at compile time in 64 bit mode and inserted in 32 bit mode. It just seems a little wastful to keep a redundant prefix in the code. Making the prefixes illegal is not necessary.

05 Sep 2005, 09:36

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 05 Sep 2005, 09:57

ES prefix is ignored so you end up using standard addressing space. But the selectors and descriptors pointed to by DS and ES can be different (even though ignored), thus I felt it's better to leave it to programmer to realize what is he doing - he just shouldn't use any such segment overrides in long mode.

As for the manuals - yes, there have been some updates to the printed ones I have, including RDTSCP instruction, I had to download the new PDF version.

05 Sep 2005, 09:57

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20298
Location: In your JS exploiting you and your system

revolution 06 Sep 2005, 02:26

I downloaded the latest version of 1.64 (5 September 2005). I'm having a problem with programming CR8. The REX prefix is missing from the generated code.

Code:

 use64
 mov cr8,rax  ; <-- Missing REX prefix
 mov cr9,rax  ; <-- Missing REX prefix
 mov cr10,rax ; <-- Missing REX prefix
 mov cr11,rax ; <-- Missing REX prefix
 mov cr12,rax ; <-- Missing REX prefix
 mov cr13,rax ; <-- Missing REX prefix
 mov cr14,rax ; <-- Missing REX prefix
 mov cr15,rax ; <-- Missing REX prefix
 mov dr8,rax  ; <-- Missing REX prefix
 mov dr9,rax  ; <-- Missing REX prefix
 mov dr10,rax ; <-- Missing REX prefix
 mov dr11,rax ; <-- Missing REX prefix
 mov dr12,rax ; <-- Missing REX prefix
 mov dr13,rax ; <-- Missing REX prefix
 mov dr14,rax ; <-- Missing REX prefix
 mov dr15,rax ; <-- Missing REX prefix

Quote:

he just shouldn't use any such segment overrides in long mode

Okay, fair enough, then you might want to also include the DS override if it is specified. By not including DS but including the other ES, CS and SS makes the generated code little bit inconsistent. All four DS, ES, CS and SS have no special meaning on long mode but DS (and I am guessing SS when RBP is used) *IS* being treated specially.

06 Sep 2005, 02:26

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8349
Location: Kraków, Poland

Tomasz Grysztar 06 Sep 2005, 08:47

Sorry, I have not much time for assembly now and the updates were a bit in a hurry.

The code generation just retains the consistency of the compatibility modes (classic ones, note the SS:BP etc.), to keep it as much compatible and predictable as possible. The ignoring of some of the segment prefixes in long mode I treat more like an execution specifics of long mode rather than instruction encoding specifics - like the division by zero or something like that.

06 Sep 2005, 08:47

tom tobias

Joined: 09 Sep 2003
Posts: 1320
Location: usa

tom tobias 06 Sep 2005, 18:00

Tomasz Grysztar wrote:

Sorry, I have not much time for assembly now ....

06 Sep 2005, 18:00

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