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flat assembler > Main > Strange behaviour when using structures.

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The_Unknown_Member



Joined: 28 Aug 2017
Posts: 17
Hi. I wonder what is wrong with this code:
Code:
; This is an Educational Program format PE console use32 ; x86_32 entry start include 'win32a.inc' struct PNT x dd ? y dd ? z dd ? q dd ? ends ; This is the data section: ; ======================================================= section '.data' data readable writeable my_PNT PNT 2,3,4,5 ;mem dd 1 dup(0) ; ======================================================= section '.text' code readable executable start: ; Your program begins here mov eax, [my_PNT] ; Output -> 2 mov eax, [my_PNT + 1] ; Output -> 3000000 mov eax, [my_PNT + 2] ; Output -> 30000 mov eax, [my_PNT + 3] ; Output -> 300 mov eax, [my_PNT + 4] ; Output -> 3 ;call print_eax ; Printing the number in hexa format in the console. This comes from the fine training.inc ; Exit the process: push 0 call [ExitProcess] include 'C:\Users\MrNoLife\Desktop\ASM resources\x86-asm-foundations\#asm_prog_ex-master\include\training.inc'

Look at the code that begins after the start label. In the first line I move the thing number that is in address my_PNT in eax (the number is my_PNT.x which is 2) now after that I move from the base address of my_PNT to the next byte I get this strange result "3000000". Why is this happening ? Can someone explain me ?
Post 13 Sep 2017, 11:31
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15872
Location: 162173 Ryugu
Your values are defined as dd so they take four bytes each. You will need to adjust the offsets:
Code:
mov eax, [my_PNT + 4*0] ;first value mov eax, [my_PNT + 4*1] ;second value mov eax, [my_PNT + 4*2] ;third value mov eax, [my_PNT + 4*3] ;fourth value mov eax, [my_PNT + 4*4] ;fifth value
But actually since you use a structure you can access the value by the symbol name:
Code:
mov eax, [my_PNT.x] mov eax, [my_PNT.y] mov eax, [my_PNT.z] mov eax, [my_PNT.q]
Post 13 Sep 2017, 11:48
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