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Index > Windows > what does LocalAlloc,LMEM_FIXED do with a register

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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
Hi, i have use the LocalAlloc to set up a register to be like a reserved byte, but how do i do this with interrupts? or just even mov/add instructions? and can it be done

ps this is how i did

invoke LocalAlloc,LMEM_FIXED,100
mov edx,eax


just for getting edx with 100 maxmimum symbols

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Post 09 Feb 2005, 12:55
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beppe85



Joined: 23 Oct 2004
Posts: 181
beppe85
LocalAlloc will return you a pointer to an area you can use. It will return a different results each time

If you only need one such area, you can do a "rd":

; this needs a static size, like 100 is
my_stuff: rd 100

If you need a dynamic ize, you can grab some from stack
sub esp, size
; your area is pointed by esp

Or plz explain it a bit more.
Post 09 Feb 2005, 18:17
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
hmm, thinks that is what i need, but just donno how the full code should be, should it be like this?

sub esp,100
mov edi,esp


would that reserve some space i can use in edi(100) ?? or should i do like this

sub esp,100
lea edi,esp

would it be almost the same as

invoke LocalAlloc,LMEM_FIXED,100
mov edi,eax

??
Post 10 Feb 2005, 17:14
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beppe85



Joined: 23 Oct 2004
Posts: 181
beppe85
There's no definitive answer...LocalAlloc is more general and should fit everywhere. You already using this correctly. Rememberto free this memory when you no longer need. Ex:

invoke LocalFree, edi

The stack approch is faster, and simple to dealloc. But you hardly could resize the block, because you free a block on stack all in one place. I recommend you to use ebp. Ex:

push ebp
mov ebp, esp
sub esp,100
mov edi,esp
...
; use block pointed by edi
...
; this will automatically free all memory
mov esp, ebp
pop ebp
Post 10 Feb 2005, 18:04
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
do i need to do that, can i not just free it with

mov edi,0x0

or

xor edi,edi

:p ? i know im weird :-s
Post 10 Feb 2005, 18:18
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beppe85



Joined: 23 Oct 2004
Posts: 181
beppe85
No, you will not free with that...but you still can put it after LocalFree, so you can check for a null. If you leave the register unchanged, you could dereference a no more valid pointer. This is a phenomenon known as pointer dangling.

PS: when alloc'ing on stack, you just put "xor reg, reg", all blocks will be freed by the "mov esp, ebp"
Post 10 Feb 2005, 18:54
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
what about this, why wont that work

Code:
org 0x100
sub sp,128
mov dx,sp
mov ah,0x1a
mov ax,dx
int 0x21
mov ah,0x4e
mov cx,0x3f
mov dx,place
int 0x21
mov si,ax+30
mov ah,0xe
reply:
lodsb
int 0x10
loop reply
RET
place db 'c:\winnt\*.*',0



    
still only a test
Post 10 Feb 2005, 20:43
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beppe85



Joined: 23 Oct 2004
Posts: 181
beppe85
If you mean won't compile, there is no such instruction for "mov si, ax+30". Try lea si, [ax+30]. But I don't know 16bit very well, so you must test.

PS: can I ask a favour to you? please give detailed information about errors and what you intended to do, you'll get fast answers.
Post 11 Feb 2005, 00:00
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
shall not be a favour, MY BAD, heres what its says error(ps i've have tryed so many things, like mov si,word[ax+30] and such things, but doesn't work)

mov si,ax+30 = says extra charactor line
lea si,[ax+30] = says reserved word used as symbol


hope this helps you a bit

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Post 11 Feb 2005, 08:22
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
anybody?
Post 11 Feb 2005, 15:34
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beppe85



Joined: 23 Oct 2004
Posts: 181
beppe85
Oh yeah ax cannot be used as base.

mov si, ax
add si, 30

or instead of ax use other:
lea si, [bx+30]
Post 11 Feb 2005, 17:41
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
i can't understand why this wont work.

Code:
org 0x100
sub sp,128
mov dx,sp
mov ah,0x1a
mov bx,dx
int 0x21
mov ah,0x4e
mov cx,0x3f
mov dx,place
int 0x21
call print
mov ah,0x4f
mov dx,place
mov ah,4fh
loopy:
int 0x21
jnc loopy
RET
place db 'c:\winnt\*.*',0
imhere db 'im here$',0
print:
lea si,[bx+30]
mov ah,0xe
reply:
lodsb
int 0x10
loop reply
mov ah,0x9
mov al,13
int 0x10
mov al,10
int 0x10
mov ah,0x0
int 0x16
RET       

ps i was reading a tutorial and wrote down my own pieces of code, but doesn't work.

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Post 11 Feb 2005, 20:15
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gumletis



Joined: 18 Dec 2004
Posts: 128
gumletis
anyone?
Post 11 Feb 2005, 22:07
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