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Index > OS Construction > UEFI data structures alignment

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zhak



Joined: 12 Apr 2005
Posts: 490
Location: Belarus
zhak
Hi guys,

UEFI spec says:
Quote:
Unless otherwise specified all data types are naturally aligned. Structures are aligned on boundaries equal to the largest internal datum of the structure and internal data are implicitly padded to achieve natural alignment.

Does it mean that I should manually align all structures?
Example:
There's BOOLEAN type defined as follows:
Quote:
Logical Boolean. 1-byte value containing a 0 for FALSE or a 1 for TRUE. Other values are undefined.

There's also UINT32, which is 4-byte unsigned value, and a structure:
Code:
//*******************************************************
// SIMPLE_TEXT_OUTPUT_MODE
//*******************************************************
typedef struct {
  INT32 MaxMode;
// current settings
  INT32 Mode;
  INT32 Attribute;
  INT32 CursorColumn;
  INT32 CursorRow;
  BOOLEAN CursorVisible;
} SIMPLE_TEXT_OUTPUT_MODE;    

Does it mean that I should explicitely add 3-byte padding to BOOLEAN when defining the structure for fasm?
Code:
struct SIMPLE_TEXT_OUTPUT_MODE
  MaxMode               rd 1
  Mode                  rd 1
  Attribute             rd 1
  CursorColumn          rd 1
  CursorRow             rd 1
  CursorVisible         rb 1
                        rb 3    ;needed?
ends    
Post 03 May 2014, 19:09
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sid123



Joined: 30 Jul 2013
Posts: 340
Location: Asia, Singapore
sid123
Any sane compiler should always align the structures; so yes I think you should stick with alignment.
P S.: I'm not sure if FASM really aligns the structures or not
Post 04 May 2014, 07:49
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17639
Location: In your JS exploiting you and your system
revolution
sid123 wrote:
P S.: I'm not sure if FASM really aligns the structures or not
It won't align anything unless you tell it to.
Post 04 May 2014, 09:23
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cod3b453



Joined: 25 Aug 2004
Posts: 619
cod3b453
[Nothing to see here Rolling Eyes ]


Last edited by cod3b453 on 06 May 2014, 16:26; edited 1 time in total
Post 05 May 2014, 20:52
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17639
Location: In your JS exploiting you and your system
revolution
cod3b453 wrote:
In the example, no padding is needed because CursorVisible is aligned to its size (1).
The overall size of the structure should also be respected with proper padding since there is no guarantee that the API will not write to unused padding bytes. If you have some other data that follows the structure in the memory space of the padding bytes than it might be at risk of being corrupted.
Post 05 May 2014, 23:31
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cod3b453



Joined: 25 Aug 2004
Posts: 619
cod3b453
revolution wrote:
cod3b453 wrote:
In the example, no padding is needed because CursorVisible is aligned to its size (1).
The overall size of the structure should also be respected with proper padding since there is no guarantee that the API will not write to unused padding bytes. If you have some other data that follows the structure in the memory space of the padding bytes than it might be at risk of being corrupted.
Yeah, I'd misread the second line in that spec so ignore me Embarassed
Post 06 May 2014, 16:25
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