memory access with dx

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realcr

Joined: 02 Apr 2007
Posts: 39

realcr 23 Jun 2007, 11:30

I wanted to test the fasm for dos assembler , so I wrote this simple code:

Code:


format MZ

      push    cs
  pop     ds

label1:
       mov dx,label1
       mov ax,[dx]

     mov     ax,4C00h
    int     21h

and I get:

Code:

D:\programs\FASM16~1>fasm testf.asm
flat assembler  version 1.67.21  (15360 kilobytes memory)
testf.asm [11]:
        mov ax,[dx]
error: reserved word used as symbol.

I don't understand what the assembler tells me.
When I tried it with si instead of dx , it worked.
Am I doing something wrong?

realcr.

23 Jun 2007, 11:30

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 23 Jun 2007, 11:44

"dx" cannot be used to address memory in 16 bit code.

the error message in this case is very bad, i agree

23 Jun 2007, 11:44

realcr

Joined: 02 Apr 2007
Posts: 39

realcr 23 Jun 2007, 12:55

thanks vid.

Someone told me to try it in masm and see if it works.
I wrote this code:

Code:

.model small
.stack 100
.data
    CLRF    db  13,10,'$'
    MSG     db  'passed the writing test!$'
.code
MAIN   Proc
        mov     AX,@Data
        mov     DS,AX
        
   mov     AH,9
        lea     DX,CLRF
        INT     21H
    label1:
   mov si,label1           ; load the label's location
        mov word ptr [si],1337h ; try to write to memory

        mov ax,1337h            
            cmp [si],ax             ; check the memory write
            jnz exit2

       ; if we get here it means the write to memory
       ; was successful 
        mov     AH,9
        lea     DX,MSG             ; load the message
        INT     21H               ; print the message to screen
        
   mov     AH,9            ; print the crlf thingy
        lea     DX,CLRF
        INT     21H

    exit2:
                             ; if we get here we exit the program

        mov     AH,4CH
        INT     21H

MAIN    Endp
End     MAIN

and I got:

Code:


C:\>test.exe

passed the7<riting test!

and we all know that 0x37 is the char "7" , which proves it is somehow possible.

23 Jun 2007, 12:55

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 23 Jun 2007, 21:27

Nowhere in that MASM example did you actually do a memory access with DX. It worked because you used SI.

23 Jun 2007, 21:27

DOS386

Joined: 08 Dec 2006
Posts: 1903

DOS386 23 Jun 2007, 22:55

Code:

   mov word ptr [si],1337h

Please delete the MA$M code from your post. It's unnecessarily long and not related to the issue at all Confused

Code:

   use16
   org $100

   mov ax,[bx]
;  mov ax,[dx]
   mov ax,[bp]
   mov ax,[si]
   mov ax,[di]
   ud2

Quote:

"dx" cannot be used to address memory in 16 bit code.
the error message in this case is very bad, i agree

Right.

Should be:

Code:

Only BX, BP, SI, and DI can be used for indexing in 16-bit code

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

23 Jun 2007, 22:55

realcr

Joined: 02 Apr 2007
Posts: 39

realcr 26 Jun 2007, 17:03

I'm sorry for sending the wrong masm32 example,
However it does work with masm if you replaec the si with dx.

I tried to see the output in olly , and I fould out that masm actually moves dx into si , and then restores it , just in order to do the memory operation.

So you were right about not being able to use dx.

Thanks for help,
realcr.

26 Jun 2007, 17:03

f0dder

Joined: 19 Feb 2004
Posts: 3175
Location: Denmark

f0dder 26 Jun 2007, 22:30

realcr wrote:

I tried to see the output in olly , and I fould out that masm actually moves dx into si , and then restores it , just in order to do the memory operation.

And that is a thing that an assembler shouldn't be doing, imho - ugly. One thing is if things happen "behind your back" when you use macros, but instructions should never be "magicified" like that.

Oh, and please call the assembler "MASM" and not "masm32" - the former is the name of the assembler, the latter is the name of that particular Australian's hodgepodge package.

_________________

- carpe noctem

26 Jun 2007, 22:30

DOS386

Joined: 08 Dec 2006
Posts: 1903

DOS386 28 Jun 2007, 00:59

Quote:

that masm actually moves dx into si

NOT amused Sad

28 Jun 2007, 00:59

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