exp4b

data segment
string db "hello world$"
pal db 01h
data ends

extra segment
rev db ?
extra ends

code segment
assume cs:code,ds:data,es:extra
start:
mov ax,data
mov ds,ax
lea si,string
up:mov al,[si]
cmp al,'$'
jz down
inc cx
inc si
jmp up

down:mov bx,cx
dec si
lea di,rev
up1:std
lodsb
cld
stosb
dec bx
jnz up1

lea si,string
lea di,rev
repe cmpsb
cmp cx,0000h
jz exit
mov pal,00h

exit:int 03h
code ends
end start