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flat assembler > Main > Allocating an array, several ways

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jorido



Joined: 23 Jan 2017
Posts: 53
Allocating an array, several ways
I'm trying to create an array/buffer where I can read a file into. Are these 3 instructions the same? If now, what's the difference?



Code:

    ; 1 
    bufftimes 1000 db ?
    
    ; 2
    buff rb 1000

    ; 3
    buff db 0 dup 1000





Do they all allocate a space of 1000 elements 1 byte each?
What's the correct and most common way?


When I'm using the #3, only a part of a file is printed out, other parts, beginning and end, are getting lost. Unlike in #1 and #2 -- using them it's printed correctly.


Last edited by jorido on 09 Oct 2017, 02:26; edited 1 time in total
Post 08 Oct 2017, 23:39
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system error



Joined: 01 Sep 2013
Posts: 667
#3
buff db 1000 dup(0)

1000 = size
dup(0) = initializers
Post 09 Oct 2017, 01:45
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jorido



Joined: 23 Jan 2017
Posts: 53

system error wrote:
#3
buff db 1000 dup(0)

1000 = size
dup(0) = initializers




how come, say

"buff db 100" allocates a single byte and puts value "100" in it

and

"buff db 100 dup(0)" allocates 100 bytes?

that is, they have the same part buff db 100 in common but do different things[/b]
Post 09 Oct 2017, 02:34
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15241
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The first option with times gives a slightly different result because there is no size attached to the buff variable.

Code:
buffdb ?
mov [buff],0xff ;<--- error operand size not specified

Compare to this:

Code:
buff db ?
mov [buff],0xff ;<-- okay

Post 09 Oct 2017, 04:17
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6633
Location: Kraków, Poland

jorido wrote:
how come, say

"buff db 100" allocates a single byte and puts value "100" in it

and

"buff db 100 dup(0)" allocates 100 bytes?

that is, they have the same part buff db 100 in common but do different things[/b]

The fasm's language was designed to be mostly LL(1), but there are a few places where it is not, including the DUP feature which was implemented for compatibility with some older assemblers.
Post 09 Oct 2017, 07:07
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jorido



Joined: 23 Jan 2017
Posts: 53
Ok.

"buff db 100" and "buff db 100 dup(0)" indeed do different things: allocating single byte with value 100 and 100 bytes with zeroes, right?
Post 09 Oct 2017, 09:37
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system error



Joined: 01 Sep 2013
Posts: 667

jorido wrote:
Ok.

"buff db 100" and "buff db 100 dup(0)" indeed do different things: allocating single byte with value 100 and 100 bytes with zeroes, right?



Right.

One more little step to make it easier for you is to use a constant

MAXSIZE = 1000

xdata TIMES MAXSIZE db 0
xdata rb MAXSIZE
xdata db MAXSIZE dup(0)

You show some genuine efforts, we give you help some more Very Happy
Post 09 Oct 2017, 19:37
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Furs



Joined: 04 Mar 2016
Posts: 868

jorido wrote:
how come, say

"buff db 100" allocates a single byte and puts value "100" in it

and

"buff db 100 dup(0)" allocates 100 bytes?

that is, they have the same part buff db 100 in common but do different things[/b]

Yeah, it's messed up, unfortunately. It's a syntax quirk FASM got from other assemblers Confused

Sadly, "buf times 100 db 0" is not valid... you should use a macro to get rid of this stupid syntax.
Post 09 Oct 2017, 20:47
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jorido



Joined: 23 Jan 2017
Posts: 53
Are


Code:

    buff rb 1000
    buff db 1000 dup(0)





exactly the same?
Post 11 Oct 2017, 13:42
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15241
Location: 1I/ʻOumuamua

jorido wrote:
Are


Code:

    buff rb 1000
    buff db 1000 dup(0)





exactly the same?

No.

The first is uninitialised. In some file formats the data is not included in the executable, only its size is given.

The second has explicit initialisation of all 0's. The executable will contain every one of those zero bytes.
Post 11 Oct 2017, 13:48
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Furs



Joined: 04 Mar 2016
Posts: 868
Note that "uninitialized" means also zero-ed (by the OS) on load on most platforms (Linux, Windows, etc), so there's no point in using dup(0) at all. It will only waste space on disk for no gain.

If you want something zero-initialized use rb since it saves space on disk and is otherwise "the same" functionally, unless the target OS doesn't zero-initialize data (DOS maybe? idk).
Post 11 Oct 2017, 14:11
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jorido



Joined: 23 Jan 2017
Posts: 53
but when I compile it using first buff rb 1000 and then buff db 1000 dup(0), the sizes, in bytes, of output files are identical
Post 12 Oct 2017, 01:00
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15241
Location: 1I/ʻOumuamua
It depends on the layout of the code. If you are using a PE file for example then if you have initialised values following uninitialised space then the uninitialised will still be included.

Code:
format pe ..
section ...
 uninit rb 1000
 inited db 'a' ;<--- this value will force all preceding values to be inclucded

Compare to:

Code:
format pe ..
section ...
 inited db 'a'
 uninit rb 1000 ;<--- this won't be included in the output file.

Post 12 Oct 2017, 01:40
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jorido



Joined: 23 Jan 2017
Posts: 53
linux, x64

or did Furs mean only if I initialize an array but don't actually use it at all?
Post 12 Oct 2017, 14:16
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Furs



Joined: 04 Mar 2016
Posts: 868
I'm not sure I understand the question, but...

Initialization means that the value of that buffer will be the given value on entry to the program. 'rb' reserves bytes, but those bytes are initialized to zero on Linux/Windows, without taking space on disk. So when at your program's entry point, they'll be zeros.

It's like allocating a block of memory containing all your 'rb' stuff and then zero-ing them in a loop before your program's entry point is even executed. (that's what the OS does)

If you used 'db' instead, it would copy them from the disk to the memory block instead of zeroing it, thus if data is zeros, it would waste it on the disk for no reason, since it ends up being zeros in memory anyway.

Note that, as revolution said, you shouldn't mix db with rb in same section/segment. Just put all 'rb' in their separate section/segment.
Post 12 Oct 2017, 15:37
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15241
Location: 1I/ʻOumuamua

Furs wrote:
Note that, as revolution said, you shouldn't mix db with rb in same section/segment. Just put all 'rb' in their separate section/segment.

I was suggesting that if you put the uninitialised data at the end of a segment then the assembler can mark it with just a length. But you can still put initialised data with it as long as it comes first.
Post 13 Oct 2017, 00:18
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