Silly or BUG: INC [EBX] vs INC [EBP] ???

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DOS386

Joined: 08 Dec 2006
Posts: 1905

DOS386 12 Jun 2010, 23:20

Code:

INC DWORD [EBX]

2 Byte's

Code:

INC DWORD [EBP]

3 Byte's

No way to encode [EBP] with just 2 Byte's ???

12 Jun 2010, 23:20

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 12 Jun 2010, 23:56

DOS386 wrote:

Code:
INC DWORD [EBX]
    
2 Byte's
Code:
INC DWORD [EBP]
    
3 Byte's

No way to encode [EBP] with just 2 Byte's ???

Refer to "Table 2-2. 32-Bit Addressing Forms with the ModR/M Byte" in Intel's manual vol2A. As you'll see, "INC DWORD [EBP+$00]" is the shortest.

12 Jun 2010, 23:56

score_under

Joined: 27 Aug 2009
Posts: 27

score_under 14 Jun 2010, 19:55

MOV EAX,[ESP+4] = 4 bytes.
MOV EAX,[EBP+4] = 3 bytes.

It's just one of those cases where there actually *is* a difference between registers on x86 Wink

14 Jun 2010, 19:55

bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 14 Jun 2010, 23:50

Also note: DOS386's code assumes use32

14 Jun 2010, 23:50

score_under

Joined: 27 Aug 2009
Posts: 27

score_under 20 Jun 2010, 19:37

bitshifter wrote:

Also note: DOS386's code assumes use32

Uhh... real men use64 or something? Razz

20 Jun 2010, 19:37

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 20 Jun 2010, 20:57

score_under,

16/64-bit inc [ebx] requires address-size prefix and can't be 2 bytes long. 32-bit can.

20 Jun 2010, 20:57

DOS386

Joined: 08 Dec 2006
Posts: 1905

DOS386 27 Jun 2010, 11:00

Thanks.

> assumes use32

Right.

27 Jun 2010, 11:00

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