I have provided 2 different implementations of switching segments when calculating the string length. I don't understand why we are switching the extra segment and data segment or more precisely copying data segment (register DS) contents into extra segment (register ES). If I remove the lines push ds and pop es the result does not change, but when I replace les di, [bp+4] by mov di, [bp+4] the message is displayed on the screen but with a bunch of other different characters.
Why is it happening? I am new in assembly.
org 0x0100
jmp start
message: db 'Hello world!', 0 ; First string to be concatenated
; Subroutine to calculate the length of a string
; Takes the segment and offset of a string as parameters
strlen:
push bp
mov bp, sp
push es
push cx
push di
les di, [bp+4] ; Point es:di to string
mov cx, 0xffff ; Load maximum number in cx
xor al, al ; Load a zero in al
repne scasb ; Find zero in the string
mov ax, 0xffff ; Load maximum number in ax
sub ax, cx ; Find change in cx
dec ax ; Exclude null from length
pop di
pop cx
pop es
pop bp
ret 4
; Subroutine to print a string
; Takes the x position, y position, attribute, and address of a null-terminated string as parameters
printstr:
push bp
mov bp, sp
pusha
push di
push ds ; Push segment of string
mov ax, [bp+4]
push ax ; Push offset of string
call strlen ; Calculate string length
cmp ax, 0 ; Is the string empty?
jz exit ; No printing if string is empty
mov cx, ax ; Save length in cx
mov ax, 0xb800 ; Video memory base address
mov es, ax ; Point es to video base
mov al, 80 ; Load al with columns per row
mul byte [bp+8] ; Multiply with y position
add ax, [bp+10] ; Add x position
shl ax, 1 ; Turn into byte offset
mov di, ax ; Point di to required location
mov si, [bp+4] ; Point si to string
mov ah, [bp+6] ; Load attribute in ah
cld ; Clear direction flag for auto-increment mode
nextchar:
lodsb ; Load next char in al
stosw ; Print char/attribute pair
loop nextchar ; Repeat for the whole string
exit:
pop ds
pop di
popa
pop bp
ret 8
start:
mov ax, 30
push ax ; Push x position
mov ax, 20
push ax ; Push y position
mov ax, 0x7 ; Blue on white attribute
push ax ; Push attribute
mov ax, message
push ax ; Push address of message
call printstr ; Call the printstr subroutine
mov ax, 0x4c00 ; Terminate program
int 0x21
Alternative version:
push bp
mov bp, sp
pusha
push ds
pop es ; load ds in es
mov di, [bp+4] ; point di to string
mov cx, 0xffff ; load maximum number in cx
xor al, al
repne scasb
mov ax, 0xffff
sub ax, cx
dec ax
jz done
mov cx, ax
...
when i chnage LES with
mov di, [bp+4]the message is still displayed on the screen
Key is here that the segment registers in your program are all equal to each other from the start. That's because you are creating a .COM program (ORG 0x100 does that). All that changing of DS or ES does not really count, nor is it necessary in this case.
I dont understand why are we sitcing the extra segment and data segment or more precisely copying data segemnt contents into extra segment
Setting up the ES segment register is required for using the scasb instruction that always depends on ES:DI.
Now, there is an important mismatch in the way you return from the strlen subroutine (ret 4) and the way you exit from the printstr subroutine (exit: pop ds).
ret 4 (that discards both the offset and the segment parts of the far pointer) then you need to remove that pop ds.pop ds then you need to write ret 2 (that only discards the offset part of the far pointer).Tip: printstr does change ES different to DS (setting it to 0xB800). There you would normally preserve ES on the stack.