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flat assembler > MenuetOS > align?

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thecf



Joined: 23 Dec 2006
Posts: 23
can anybody tell me what the align macro actually does in the menuet source code? im unsure...

why would you need to align the code? I've never seen it in other tutorials on os development.

macro align value { rb (value-1) - ($ + value-1) mod value }

not sure what the above does. i know it defines a macro with the name align. but dont really understand why you would need to do align 4 before some system functions..

any help will be great Rolling Eyes
Post 24 Feb 2008, 16:47
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bitRAKE



Joined: 21 Jul 2003
Posts: 2732
Location: dank orb
the macro reserves bytes (rb) to insure the current offset is divisible by value. If we look at the case of "align 4":
Code:
$  |  $-3   3-#
0  |   3    0
1  |   0    3
2  |   1    2
3  |   2    1    
All values are MOD 4, to show cases.

Alignment can result in a speed improvement. Here is a macro that does alignment plus an offset:
Code:
macro ALIGN val,off { rb (val+off-($ mod val))mod val }    
...like ALIGN 64,8 would ensure the address is 8 MOD 64.

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Post 24 Feb 2008, 17:53
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edfed



Joined: 20 Feb 2006
Posts: 4180
Location: 2018
the align is cause of hardware
the memory is on 64 bits bus
so, to access a data, the need of aligned addresses on this bus is good.
if you want to access a dword at [5], you should read the first part, three bytes at [5,6,7], and the second part at [8]
so, it consumme one extra memory cycle to read unaligned values.
align is not exclusive to menuet and x86.

it is valid for real documents accesing.

a good example are the dragon ball episodes. to follow the story, you need to watch at least two consecutives episodes. as episodes are unaligned on the story sequence, it is boring...

that's why we try to make aligned DBZ episodes.
Post 24 Feb 2008, 18:15
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