Assembly8086 how to put edit a string(db)

DaSilva

Im new to assembly and wondering how to put stuff into a string at run time. Actually first off am I right in calling it a string? Heres a quick idea of what I mean:

.model small
.stack
.data
strMSG db 'Hello','$'
.code
mov ax,@data
mov ds,ax

[COLOR="Blue"]mov strMSG,'World'[/COLOR]

mov ah,4ch
int 21h
end

The part in blue is what im unsure how to do.
How could I add the word "World" to the string "Hello"?

Thanks

Webmonkey

What you mean here is how do you Concatenate two strings in x86.

I amn't that familer with x86 but this is taken from a Book:
The strcat comes from the UCR library.

The strcat(xx) routines perform string concatenation. On entry, es:di points at the first
string, and for strcat/strcatm dx:si points at the second string. For strcatl and strcatlm the second
string follows the call in the code stream. These routines create a new string by
appending the second string to the end of the first. In the case of strcat and strcatl, the second
string is directly appended to the end of the first string (es:di) in memory. You must
make sure there is sufficient memory at the end of the first string to hold the appended
characters. Strcatm and strcatml create a new string on the heap (using malloc) holding the
concatenated result. Examples:

String1 byte “Hello “,0
byte 16 dup (0) ;Room for concatenation.
String2 byte “world”,0
; The following macro loads ES:DI with the address of the
; specified operand.
lesi macro operand
mov di, seg operand
mov es, di
mov di, offset operand
endm
; The following macro loads DX:SI with the address of the
; specified operand.
ldxi macro operand
mov dx, seg operand
mov si, offset operand
endm
.
.
.
lesi String1
ldxi String2
strcatm ;Create “Hello world”
jc error ;If insufficient memory.
print
byte “strcatm: “,0
puts ;Print “Hello world”
putcr
free ;Deallocate string storage.
.
.
.
lesi String1 ;Create the string
strcatml ; “Hello there”
jc error ;If insufficient memory.
byte “there”,0
print
byte “strcatml: “,0
puts ;Print “Hello there”
putcr
free
.
.
.
lesi String1
ldxi String2
strcat ;Create “Hello world”
printf
byte “strcat: %s\n”,0
.
.
.
; Note: since strcat above has actually modified String1,
; the following call to strcatl appends “there” to the end
; of the string “Hello world”.
lesi String1

strcatl
byte “there”,0
printf
byte “strcatl: %s\n”,0

Martyr

DaSilva wrote:

The part in blue is what im unsure how to do.
How could I add the word "World" to the string "Hello"?

Thanks

.model small
.stack
.data
strMSG db 'Hello','$'
.code
mov ax,@data
mov ds,ax

mov strMSG,'World'

mov ah,4ch
int 21h
end

you could do this, assuming you're using tasm ideal mode.

.model small
.stack

.data
strHello db 'Hello, ',0
strWorld db 'World',0

buffer   db   128   dup  (0)

.code
   mov ax,@data
   mov ds,ax
   
   lea si,[strHello]
   lea di,[buffer]
   call poor_strcpy

   lea si,[strWorld]
   lea di,[buffer]
   call poor_strcat

   mov cx,ax           ; assuming a string was concat, length in ax->cx
   lea dx,[buffer]     ; data to write in dx
   xor bx,bx             ; stdout?
   mov ah,40h         ; write data
   int 21h               
   
   mov ah,4ch
   xor al,al
   int 21h
;================  
poor_strlen:
   push di
   or cx,-1                       ; init to 0ffffh
   xor ax,ax                      ; scan for null byte
   repnz scasb            
   not cx                          ; invert to get length
   pop di                          ; restore di
   ret
;================   
poor_strcat:
   call poor_strlen                ; get length of string at di
   jcxz end_strcat              ; if its zero, exit
   mov ax,cx                       ; save in ax for return
   rep movsb                       ; move bytes from si->di
end_strcat:
   ret
;================
poor_strcpy:
   lodsb                         ; load a byte from si
   stosb                         ; save in buffer at di
   or al,al                       ; check if null byte, end of string
   jnz poor_strcpy            ; keep copying bytes if not zero
   ret

if you're printing dollar terminated strings, you use 09h but output date to stdout handle, then 40h with length in cx & either 1 or 0 in bx..can't remember now.