I am having trouble with a homework in assembly. The goal of this program is to reverse each letter so CIS 335/535 is a great course to esruoc taerg a si 535/533 SIC which my program does but then it is also supposed to convert that to course great a is 335/535 CIS which my program does not hence why I have the final output commented. My code spews out something like cis 335/535 si a great course in that last block.
TITLE MASM Assignment 3 reverse a string word by word (main.asm)
; Description: Reverse a string character by character in-place then reverse word for word using nested loops
;
; Revision date:
INCLUDE Irvine32.inc
.data
source BYTE "CIS 335/535 is a great course",0
ecxbkp DWORD ? ;save ecx if necessary
.code
main PROC
mov edx,OFFSET source
call WriteString
call Crlf ; print\r\n
mov ecx, LENGTHOF source ;initialize loop counter
mov esi, OFFSET source ;esi starting address of string
mov edi, OFFSET source
dec edi
END_STRING:
inc edi
mov al,[edi]
cmp al,0 ;find zero byte
jnz END_STRING ;jump back to END_STRING if al is not 0
dec edi ;edi points to end of string
shr ecx, 1 ;ecx is loop count (shift one = length/2)
L1:
mov bl, [esi] ;load characters
mov al, [edi]
mov [esi], al ;swap characters
mov [edi], bl
inc esi ;update forward pointer by 1
dec edi ;decrement backward pointer by 1
loop L1 ;and loop
; display the string
mov edx,OFFSET source
call WriteString
call Crlf ; print\r\n
;Use nested loops to reverse word for word
mov ecx, LENGTHOF source ;set outer loop count ecx= entire length
mov esi, OFFSET source ;esi points to start of string
mov edi, OFFSET source
dec edi
mov ecxbkp, ecx ;save outer loop count
L2: ; go through beginning to end of string copy space character for length
inc edi
mov al, [edi] ;move edi address into al register
cmp al, ' ' ;find space character
loop L2
mov ecx, 16 ;modify inner loop count (length/2) (for swap)
dec edi
L3: ; reverse/swap word for word
mov bl,[esi]
mov al,[edi] ;load words
mov [esi], al
mov [edi],bl ;swap words
inc esi ;update forward pointer by 1
dec edi ;decrement backward pointer by 1
loop L3 ;and loop
mov ecxbkp, ecx ;restore outer loop count
; display the string
;mov edx,OFFSET source
;call WriteString
;call Crlf ; print\r\n
exit
main ENDP
END main
enter code here
Some changes to get you on the way:
Change loop L2 into jne L2
Don't use a fixed value of 16 in ECX but calculate the exact length of the word.
Save the position in EDI in a variable to be able to continu with the next word after you will have reversed this one in your L3 loop.
L2:
inc edi
mov al, [edi] ;move edi address into al register
cmp al, ' ' ;find space character
jne L2
mov SavedPointer, edi
lea ecx,[edi-1]
sub ecx,esi ;ECX is length of current word