simple question

[elscorcho]

ok so i was playing around with asm and did this simple program:
.model small
.stack 100h
.data

sum dw ?

.code
extrn Writeintroc, Crlfroc

main proc
mov ax,@data
mov ds,ax
mov dx,1
mov bx,10
mov ax,0

L1:
add ax,dx
add dx,dx
mov sum,ax
call Writeint
call Crlf
loop L1

mov ax,4c00h
int 21h

main endp
end main

....it adds 1+2+4+8+16+32.........+1024

im just wondering how would i change this code to do

-1 + 2 -3 +4 -5 + 6 .......+ (-1 )^n N ?

 

[straiph]

if I understood correctly:

make dx=1 and subtract from ax then add 2 to dx then add to ax.

pseudo code-ish:

ax=0h ;start value=0h
dx=01h ;start adjuster=01h
cx=01h ;number of results
loop: ;start of loop
sub ax,dx ;perform first calculation in loop
call printax ;print result
dec cx ;decrease number of results required
cmp cx,0h ;if more results=0
je loopend ;then jump to end
add dx,02h ;adjust dx
add ax,dx ;perform second calculation in loop
call printax ;print result
dec cx ;decrease number of results required
cmp cx,0h ;if more results=0
je loopend ;then jump to end
add dx,02h ;adjust dx
jmp loop ;jump to start of loop
loopend: ;no more results required, exit loop

i think something like this should produce then results you require. notice on each calculation 2 is added to dx and alternately a sub then an add is performed on ax.
i have used cx as the number of results required.
although the dec operation sets the zero field in flags i have put a compare operation for clarity.

i hope this helps. "People who have nothing to say, say it too loud and have little knowledge. It's the quiet ones you need to worry about!"