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Home > Forums > Programmers > Languages > Assembly Forum

register indirect mode in Assembly

thread272-1240215
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posterise (TechnicalUser)
8 Jun 06 9:35
Is it allowed to do the following?:

mov CX, [EAX]
mov [EAX], CX

???

mov W1, [EAX]
mov [EAX],W1
where W1 is not doubleword (word or byte)

In plain english, what is the real difference between direct and indirect access mode ?
I can't find examples of invalid descriptions and the reasons why.
TessaBonting (TechnicalUser)
9 Jun 06 3:04
All of the above are correct.

Indirect addressing, as it says, lets you get the memory
value that is pointed by the given register.

So if EAX = 100

    mov ebx,[eax] gets the 100th dword relativ to DS into
EBX.

    mov [eax],ebx puts the contents of EBX into the double
word at address DS:100

Succes, Tessa
posterise (TechnicalUser)
9 Jun 06 12:38
Thank you TessaBonting, but I don't understand what exactly happens in case I try to do this kind of thing:
mov W1, [EAX]     W1 - type of the W1: word or byte
or
mov BX, [EAX]

if the number, that [EAX] points to, is a doubleword, is this number going to be just truncated when trying to put into BX or W1(which is in the memory), because BX and W1 can contain only word meaning ? Or because of the different types is it going to display an error, just like in case of move BX, EAX ?

(I just started learning Assembly recently, and don't get some of that abstract stuff)
denc4 (Programmer)
10 Jun 06 11:06

Quote (TessaBonting):


So if EAX = 100

    mov ebx,[eax] gets the 100th dword relativ to DS into
EBX.
shouldn't that be the 25th dword??

posterize

most assemblers will fetch the byte/word (depends on the size of W1) from [EAX] and handle this correctly.

if W1 is a word and you want to be sure the assembler will
treat it that way you can explicitely tell it with what it
is dealing with:

  mov W1,word ptr [EAX]

or in case of a byte sized variable:

  mov W1,byte ptr [EAX]

i'm a 16bit programmer myself, so i'm not 100% convinced
if it will work for you. just try it and you'll see.
posterise (TechnicalUser)
10 Jun 06 13:16
Thank you denc4

I've tried it in a program practically, and actually this is what is going on:
it accepts when I use mov BX, [EAX]
but it doesn't when I use
mov W1, [EAX]   where W1 - word

Actually it gives the same number of the error which appears in case I put for expample add W1, W2
so, I guess that declaration is incorrect because parameters are affiliated only with the memory, which is incorrect
denc4 (Programmer)
10 Jun 06 15:30
exactly.
to add a value in memory to another value in memory you have to use an intermediate step.

for example:
to add word at [eax] to W1 use:

  mov ax,[eax]
  add W1,ax

to add W2 to W1 use:

  mov ax,W2
  add W1,ax
posterise (TechnicalUser)
10 Jun 06 16:27
yeah, after I tried that experimentally, practically, and also compared numbers of mistakes, I finally realized what is the difference between [EAX] and EAX - indirect and direct register access, - which I could not understand before, that's why in the first post I asked what it means in plain english.

Anyway, everybody, thanks for participation in the thread! :)
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