RMode Stack doesn't work

[Cjreek]

Hi,

I'm quite confused. My stack only pops garbage -.-
Here's some code:

Initialization

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    mov ax, 0
    mov ss, ax
    mov sp, 0xFFFF

Part of my code

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    mov dx,1
    mov al,1
    call floppy_read
;  ....

floppy_read:
    xor cx,cx
    mov es,cx     
    mov bx,0x7E00

    push dx 
    call lba_to_chs 
    ;....

lba_to_chs:
    pop ax  ; <--- Here I get 0x7C5A instead of 1
;   ...

This is only 1 example. I don't know what I'm doing wrong

Cjreek

[Cjreek]

Ok I got it... 0x7C5A is the return address....

Sorry

[Troy Martin]

You're popping the return IP that's on the stack, not what you want. Use registers for function calls, it works better if you don't understand how to manipulate and work with the stack. And even more so in a bootloader, since it's smaller.

EDIT: Oh, and your mov al,1 before read_sector is probably being trashed by either read_sector or lba_to_chs.

[Cjreek]

Use registers for function calls

Well I did it this way first, but I got problems in following case:

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;prepare params for fun
mov ax,10
mov dl, 2
call calc_third_param
call fun

The problem has been, that changing registers in calc_third_param destroyed my params for "fun". Well you can first call calc_third_param and then set ax and dl, but even with this you can get tricky problems in some cases.

EDIT: Oh, and your mov al,1 before read_sector is probably being trashed by either read_sector or lba_to_chs.

Oh yes I saved al until I thought, that it's not necessary..

[Troy Martin]

The problem has been, that changing registers in calc_third_param destroyed my params for "fun". Well you can first call calc_third_param and then set ax and dl, but even with this you can get tricky problems in some cases.

pusha and popa. Remember that popa pops ALL general-purpose registers off the stack, so you have to save any return values.