Assembly variables are deleted (16 bit x86 assembly) - variables

Question

I'm still playing with retro programming in turbo C for MS-DOS, and I found some trounble using variables.
If I define some variables at the start of the assembly code (in BSS or DATA), and try to use them inside the assembly function, most of the time these variables are deleted, or end up containing random data.
I learned a bit of assembly for the game boy :) and variables always worked well and never were deleted or modified, I guess x86 asm is different.
Then I tried this using inline assembly and it was a bit better, there is just one variable (width) not working.
void draw_map_column(MAP map, TILE *t){
word *tiledata = &t->data;
int *mapdata = map.data;
int width = map.width<<1;
word tile_offset = 0;
word map_offset = 0;
word screen_offset = 0;
asm{
push ds
push di
push si
mov dx,12 //column
lds bx,[tiledata]
lds si,ds:[bx] //ds:si data address
mov [tile_offset],ds
mov [tile_offset+2],si
les bx,[mapdata]
mov ax,es:[bx]
mov cl,8
shl ax,cl
add si,ax
mov di,screen_offset //es:di screen address
}
loop_tile:
asm{
mov ax,0A000h
mov es,ax
mov ax,16
}
copy_tile:
asm{
mov cx,8
rep movsw
add di,320-16
dec ax
jnz copy_tile
mov ds,[tile_offset]
mov si,[tile_offset+2]
mov ax,map_offset
add ax,[width] //"width" does never contain the value stored at the start
mov map_offset,ax
les bx,[mapdata]
add bx,ax
mov ax,es:[bx]
mov cl,8
shl ax,cl
add si,ax
dec dx
jnz loop_tile
pop si
pop di
pop ds
}
}
Just note the "witdh" variable which is not working at all, if I replace it with a number (40), the code just works as expected (this draws a column of tiles using a map array, and some tiles stored in ram).
I guess it has something to do with the push/pop etc, and something is not set as it should.
Also what happens in pure assembly? none of the variables were working. I defined them as DW and also added:
push bp
mov bp,sp
;function
mov sp,bp
pop bp
Thanks.

Answer 1

Well once again thanks a lot, next time I'll be more patient before asking.
Just in case this is useful for someone, I had defined a variable using the wrong size.
There are other things that can be improved, but that's another question.
Variable "tileoffset" holds a 32 bit address, so it must be a "dword", not a "word". Then the function should be like this:
void draw_map_column(MAP map, TILE *t){
word *tiledata = &t->data;
int *mapdata = map.data;
int width = map.width<<1;
dword tile_offset = 0; //changed to dword to store 32 bit address
word map_offset = 0;
word screen_offset = 0;
asm{
push ds
push di
push si
mov dx,12 //column
lds bx,[tiledata]
lds si,ds:[bx] //ds:si data address
mov word ptr[tile_offset],ds //store a word
mov word ptr[tile_offset+2],si
les bx,[mapdata]
mov ax,es:[bx]
mov cl,8
shl ax,cl
add si,ax
mov di,screen_offset //es:di screen address
}
loop_tile:
asm{
mov ax,0A000h
mov es,ax
mov ax,16
}
copy_tile:
asm{
mov cx,8
rep movsw
add di,320-16
dec ax
jnz copy_tile
mov ds,word ptr[tile_offset] //read a word to the register
mov si,word ptr[tile_offset+2]
mov ax,map_offset
add ax,[width]
mov map_offset,ax
les bx,[mapdata]
add bx,ax
mov ax,es:[bx]
mov cl,8
shl ax,cl
add si,ax
dec dx
jnz loop_tile
pop si
pop di
pop ds
}