Using data segment

Question

I'm currently writing a program that writes 20 first fibonacci numbers. What I'm currently asking is if this thing can be possibly done:

MOV DS, 3000H
MOV SI, 4000H
MOV DL, 123
MOV CL, 5
MOV DS:[SI+CL], DL

(This is just a general code that has nothing to do with fibonacci numbers) So it would write '123' into the memory in the address 3000H:4005H. Is that even possible? So I don't need to increase both SI and CL (I use CL to end the program when it reaches 20 or 14h in that matter).

EDIT: This is my actual code, but it's not writing the data properly.

cseg segment
assume cs:cseg
    Start:  mov ax, 3000h
            mov ds, ax
            mov si, 4000h
            mov bx, 0
            mov al, 1
            mov bl, 1
            mov ds:[si], al
            inc bx
            mov ds:[si+bx], bl
            inc bx
    Again:  mov dl, al
            add dl, bl
            mov al, bl
            mov bl, dl
            mov ds:[si+bx], dl
            inc bx
            cmp bx, 20
            jc Again
            int 3h
cseg    ends
end     Start

I just started studying Assembly and that's how my teacher taught me. — HelloWorld
– HelloWorld, Commented Jan 13, 2015 at 13:16

Jester · Accepted Answer · 2015-01-13 13:12:24Z

1

You should re-read the documentation explaining the 16 bit address modes.

In short, you can only use a base register BX or BP, an index register SI or DI, and a constant displacement in effective addresses. As such, you could do:

MOV AX, 3000H
MOV DS, AX
MOV SI, 4000H
MOV DL, 123
MOV BX, 5
MOV [SI+BX], DL

If your displacement is constant 4000H, then you can also write:

MOV AX, 3000H
MOV DS, AX
MOV DL, 123
MOV BX, 5
MOV [4000H+BX], DL

edited Jan 13, 2015 at 13:12

answered Jan 13, 2015 at 13:04

Jester

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7 Comments

HelloWorld Over a year ago

But how will it write it to 3000H:4005H if not using DS:?

Jester Over a year ago

It is using DS by default, you do not need to specify that. You only need an override if what you want doesn't match the default. Obviously I assumed DS has been set to 3000H, I omitted that part from the code. Note that MOV DS, 3000H is illegal, you have to go through a register.

HelloWorld Over a year ago

Can you tell me what's the problem with my code? I edited my original post.

Jester Over a year ago

bl is the low byte of bx, looks like you have conflicting uses.

HelloWorld Over a year ago

No, this is not about that. It worked when I incread CL and SI both without using [SI+BX]. I just replaced my CL to BX and stopped increasing SI, but it doesn't write the data properly and in incorrect addresses.

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