sub rdx,$2B6109100 out of range, why?

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Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 25 Jul 2009, 04:12

This compiles and runs

Code:

          sub     rdx,qword[v2B6109100]
retf
v2B6109100     dq      $2B6109100

This gives out of range error

Code:

              sub     rdx,$2B6109100
retf

Why? I am sure I entered 64bit mode successfully or the first one wouldn't be working.. but then shouldn't the second one work? Confused

25 Jul 2009, 04:12

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 25 Jul 2009, 04:30

There is no such instruction (sub r64, imm64). You may want to read this: http://x86asm.net/articles/x86-64-tour-of-intel-manuals/index.html

25 Jul 2009, 04:30

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 25 Jul 2009, 04:48

http://x86asm.net/articles/x86-64-tour-of-intel-manuals/index.html wrote:

Code:


Table 3-4. Effective Operand- and Address-Size Attributes in 64-Bit Mode
L Flag in Code Segment Descriptor      1       1       1       1       1       1       1       1
REX.W Prefix       0       0       0       0       1       1       1       1
Operand-Size Prefix 66H    N       N       Y       Y       N       N       Y       Y
Address-Size Prefix 67H    N       Y       N       Y       N       Y       N       Y
Effective Operand Size     32      32      16      16 ---->64<----   64 ---->64<----   64
Effective Address Size    64      32      64      32 ---->64<----   32 ---->64<----   32

It looks like I should be able to do it with a prefix or something?

25 Jul 2009, 04:48

bitRAKE

Joined: 21 Jul 2003
Posts: 4072
Location: vpcmpistri

bitRAKE 25 Jul 2009, 05:33

10 bytes...

Code:

push -$56C21220
pop rax
lea rdx,[rdx+rax*8]

...nah, has to be memory or a register source for 64bit.

25 Jul 2009, 05:33

bitRAKE

Joined: 21 Jul 2003
Posts: 4072
Location: vpcmpistri

bitRAKE 25 Jul 2009, 07:51

Not without merging more functionality, imho.

Code:

use64

bitRAKE:push -$56C21220
 pop rax
     lea rdx,[rdx+rax*8]

Azu:     mov     rax,(1 shl 64)-$2B6109100
   add     rax,rdx

display $30 - Azu - Azu + $ + bitRAKE

...looks like yours is three bytes longer, but one less instruction.

25 Jul 2009, 07:51

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 25 Jul 2009, 07:54

aaaa where'd my post go Sad

25 Jul 2009, 07:54

asmcoder

Joined: 02 Jun 2008
Posts: 784

asmcoder 25 Jul 2009, 13:58

[content deleted]

Last edited by asmcoder on 14 Aug 2009, 14:48; edited 5 times in total

25 Jul 2009, 13:58

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 25 Jul 2009, 14:52

he was talking about sub, not mov

25 Jul 2009, 14:52

asmcoder

Joined: 02 Jun 2008
Posts: 784

asmcoder 25 Jul 2009, 14:59

[content deleted]

Last edited by asmcoder on 14 Aug 2009, 14:48; edited 1 time in total

25 Jul 2009, 14:59

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 25 Jul 2009, 20:14

asmcoder wrote:

Quote:
add rax,0x555555555555

LOL?
is that a joke? why it doesnt work?
so i do have to load another GPR to make add/sub? it suck

I think the specification is broken, someone needs to fix it or something Sad

25 Jul 2009, 20:14

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 25 Jul 2009, 21:19

No, there simply is no instruction encoding for it.

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Previously known as The_Grey_Beast

25 Jul 2009, 21:19

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 25 Jul 2009, 23:24

The instruction encodings aren't part of the specification? Confused

25 Jul 2009, 23:24

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