moving bytes

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jacko221

Joined: 12 Nov 2005
Posts: 22

jacko221 27 Jun 2006, 08:16

Hey everyone... i wanted to create a simple application which output the letter of the number at of a string.

Code:

format PE GUI 4.0
entry start

include '%fasminc%\win32ax.inc'

section '.data' data readable writeable
  hexarray db 'ABCDEFGHIJKLMN'
  caption db 2
  message db 'doesnt work'

section '.code' code readable executable

  start:
     mov dl,5   ;5 used as an example
     mov cl,byte [hexarray+dl]  ;i want the 5th byte (E) to be put into cl
     mov byte [caption],cl ;now put ecx into caption
     invoke MessageBox,0,caption,caption,0 ;now display answer



section '.idata' import data readable writeable

  include '%fasminc%\apia\kernel32.inc'
  include '%fasminc%\apia\user32.inc'
  include '%fasminc%\apia\advapi32.inc'

The problem is with this line:
mov cl,byte [hexarray+dl]
Fasm with give me an error saying
Error: Reserved word used as symbol

I would really appreciate some help in solving this problem.
Thanks in advance, Jack

27 Jun 2006, 08:16

zhak

Joined: 12 Apr 2005
Posts: 501
Location: Belarus

zhak 27 Jun 2006, 10:02

you cannot use 8- or 16-bit registers for addressing.
Try

mov cl, byte [hexarray+edx]

but don't forget to XOR EDX, EDX at the beginning

27 Jun 2006, 10:02

jacko221

Joined: 12 Nov 2005
Posts: 22

jacko221 27 Jun 2006, 10:46

thanks... that got rid of the "Error: Reserved word used as symbol" but now it gives me this "memory could not be "read"" error. Sad

oh well... ill try something different.
Thanks again, Jack

27 Jun 2006, 10:46

0x4e71

Joined: 25 Feb 2004
Posts: 50

0x4e71 27 Jun 2006, 17:15

did you zero extend edx

27 Jun 2006, 17:15

jacko221

Joined: 12 Nov 2005
Posts: 22

jacko221 28 Jun 2006, 00:06

Quote:

did you zero extend edx

I do not understand... im still new to asm. So please help me.

28 Jun 2006, 00:06

zhak

Joined: 12 Apr 2005
Posts: 501
Location: Belarus

zhak 28 Jun 2006, 06:29

you should insert

xor edx, edx

instruction at the very beginning of your program. Before

mov dl, 5

EDX register contains non-zero value when your application starts.
You'are using not only DL part of EDX, but the whole register for addressing. That's why higher bytes should be cleared first.
'xor edx, edx' instruction will clear EDX register.[/code]

28 Jun 2006, 06:29

jacko221

Joined: 12 Nov 2005
Posts: 22

jacko221 28 Jun 2006, 08:15

Thanks both for your help... really appreciate it Smile

28 Jun 2006, 08:15

ChrisLeslie

Joined: 04 Jun 2006
Posts: 50
Location: Australia

ChrisLeslie 28 Jun 2006, 09:57

zhak

Quote:

you should insert

xor edx, edx

instruction at the very beginning of your program. Before

mov dl, 5

Should not "mov edx,5" zero the high bits more easily?

Chris

28 Jun 2006, 09:57

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 28 Jun 2006, 10:16

ChrisLeslie wrote:

zhak

Quote:
you should insert
xor edx, edx
instruction at the very beginning of your program. Before
mov dl, 5

Should not "mov edx,5" zero the high bits more easily?
Chris

mov edx,5 can be encoded only as mov r32, imm32 eg. with 32bit immediate. that's 2 bytes for mov r32, imm32 and 4 bytes for value of imm32.

xor edx, edx takes two bytes, mov dl,5 takes 2 bytes, that's 4 bytes. so you save 2 bytes. in reality, it doen't matter very much in handwritten assembly code, this habit just remained to asm programmmers from old times when resources weren't wasted

28 Jun 2006, 10:16

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 28 Jun 2006, 17:11

Actually mov edx, imm is 5 bytes (one byte for the opcode "mov edx, imm").

28 Jun 2006, 17:11

0x4e71

Joined: 25 Feb 2004
Posts: 50

0x4e71 28 Jun 2006, 18:24

Yeah, pity there is no movzx r32, imm8
Again, not that it matters that much anymore.

The 68000 had MOVEQ imm8,r32 (2 bytes)
and SUBQ/ADDQ for values 1-8, these were very very useful, well, back then anyway Rolling Eyes

28 Jun 2006, 18:24

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