need help in adding two 64bit numbers

Hello Yeahdude,

There are two problems in your code.

1. You need to increase BX after each
mov [bx],ax
Otherwise first byte of GAMMA will have an incorrect value and the other three bytes will keep the value they had before adding 64-bit numbers. You're doing well with INC SI and INC DI, just include INC BX.

2. When loop is finished, being CX equal to zero, you should clean the stack, which at that point keeps the flags (see PUSHF opcode before DEC CL). This is particularly important if your routine is called by another part of program; not doing so will most likely result in crash. Therefore, just add POPF after "JNZ GO".

Note: It is suggested to inspect CF (Carry Flag) after addition to verify that there's no overflow or process it accordingly.

Good luck.

Iaki Viggers

Well, you may need to test it, but I don't think that parts like "alpha[si]" are correct; that's OK in C syntax, not in assembler.

One point was missing in previous postings: Since we're processing two bytes at a time (ADD AX,DX instead of ADD AL,DL), increase of DI, SI and BX must be done twice. Therefore code ADD SI|DI|BX,02h instead of INC SI|INC DI|INC BX.

Assembled with tasm:

.model small
.stack 20h
.data
alpha db 1h,2h,3h,4h
beta db 5h,6h,7h,8h
gamma db 0,0,0,0
.code
start:
mov ax,@data
mov ds,ax
mov es,ax
;;;;;;;;;;;;; Here begins your code
;; needed changes are in uppercase
lea si,alpha
lea di,beta
lea bx,gamma

mov ax,[si]
mov dx,[di]
add ax,dx
mov [bx],ax
pushf

mov cl,03h
go:
ADD SI,02h ;;instead of inc si, since we're processing two bytes at a time
ADD DI,02h ;; see above
ADD BX,02h ;; see above; we don't want to override first byte of buffer
mov ax,[si]
mov dx,[di]
popf
adc ax,dx
mov [bx],ax
pushf
dec cl
jnz go
;;;;;;;;; Here ends your code

popf ;; clean stack

;; no screen display, but you can debug and verify correct addition
lea si,gamma
cld
lodsw ;; get the two least-significant bytes of result
lodsw ;; get the two most-significant bytes of result
mov ah,4ch
int 21h
end start

See you.

Iaki

hi iviggers,
can you help me to display the input and output? please, i really need it..
sorry for asking too much, you're my only hope.. =(

thanks in advance

Hello Yeahdude,

I'm glad you got it. Good luck.