Assembly

Question regarding output

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RayvenHawk RayvenHawk is offline

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Question regarding output

Oct 22nd, 2009

I have an assignment due for my Assembly class and I have a quick couple of questions regarding it.

The requirement is initialize 3 variables as 16bit words with values (10, -60, 30) add those values as the following equation y = var1 + var2 + var3, use no more than 3 lines of code to produce results. Show results in EAX register.

Question #1: The professor gave us nothing to go off of to ensure we have our code done correctly, so can someone give me the display of the EAX register as it should display if I accomplish the above correctly? (no code, just a screen shot of the output).

Here is the code I have so far:

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
.data
arrayD WORD 10, -60, 30
 
.code
mov esi, OFFSET arrayD
add eax, [esi]
add eax, [esi]
add eax, [esi]
 
call DumpRegs
call WaitMsg
 
exit
.data
arrayD WORD 10, -60, 30

.code
mov esi, OFFSET arrayD
add eax, [esi]
add eax, [esi]
add eax, [esi]

call DumpRegs
call WaitMsg

exit

As you can see I have 4 lines of code to produce the results (at least I'm assuming the output is correct). What am I missing that would allow me to drop 1 line of code?

Thanks.

Oh and I realize there is lines missing from the above code, I left them out intentionally since they weren't relevant to my questions.

Last edited by RayvenHawk; Oct 22nd, 2009 at 1:34 pm.

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Join Date: Aug 2008

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RayvenHawk RayvenHawk is offline

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Oct 22nd, 2009

Ok I modified the code as follows, is there any problem with it?

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
.data
      arrayD WORD 10,-60,30
 
.code
       mov esi,OFFSET arrayD
       add eax,[esi+esi+esi]
 
       call DumpReg
       call WaitMsg
 
       exit
.data
      arrayD WORD 10,-60,30

.code
       mov esi,OFFSET arrayD
       add eax,[esi+esi+esi]

       call DumpReg
       call WaitMsg

       exit

Old saying: "If it ain't broke don't fix it"
Microsofts saying: "If it ain't broke, update it so it will be."

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NotNull

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Oct 22nd, 2009

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
      .data
      arrayD WORD 10, -60, 30
      .code
      mov esi, OFFSET arrayD
      mov ax, [esi+0] ; AX=10 initialize AX to value of
                                ; first word
      add ax, [esi+2] ; AX=AX + -60 = -50
      add ax, [esi+4] ; AX=AX + 30 = -20
      call DumpRegs
      call WaitMsg
      exit
      .data
      arrayD WORD 10, -60, 30
      .code
      mov esi, OFFSET arrayD
      mov ax, [esi+0] ; AX=10 initialize AX to value of
                                ; first word
      add ax, [esi+2] ; AX=AX + -60 = -50
      add ax, [esi+4] ; AX=AX + 30 = -20
      call DumpRegs
      call WaitMsg
      exit

In your original code you used the Extended 32-bit EAX
register for adding these 16-bit words, this would
cause it to treat two words at a time as a single value.
I believe MOVZX is the only MOVe which can use
two different datatypes.
MOV AX, [ESI] ; moves a word into AX
MOV EAX, [ESI] ; moves a dword into EAX
MOVZX EAX, BYTE PTR [EBX] ; move byte zero-extended into EAX
Every instruction needs complementary datatypes.
1111111111101100
AX=FFEC = -20

Good day.

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