Question

In this problem, you will write an assembly program (LC-3 assembly language) to process a string starting at memory location x5000, and is terminated by a ‘\0’ character (ASCII value = 0). For a given string beginning at memory location x5000, convert all the upper-case English letters (A-Z) to lower-case, and all the lower case (a-z) to upper case. For example, if the string is “iLOVEComputerScience252”, the string after the processing should be: “IlovecOMPUTERsCIENCE252”. Store the string after the processing to memory location defined by x5100. Your program should start at memory location x3000.

Answer 1

Program:

Sample output:

load lc3os.obj

then compile q.asm (as q.asm)

load q.obj

Run by clicking on continue button

Code to copy:

; q3.asm
.ORIG x3000                  ; start at memory location 0x3000
;---------------------------------------------------------------------------
; for testing storing the string “IlovecOMPUTERsCIENCE252” at location x5000
;---------------------------------------------------------------------------
LEA R0,input                  ; Load starting address of input string
LD R1, INPUT_STRING          ; R1=x5000
L1                           ; loop to store input string at location x5000
LDR R2,R0,#0                  ; read current character into R2
ADD R3,R2,#0                  ; R3=R2+0
BRz done                       ; if current character is zero(null character)
                            ; go to label done
STR R2,R1,#0                  ; otherwise, store current charcter into current location
ADD R0,R0,#1                  ; go to next character
ADD R1,R1,#1                  ; go to next location
BR L1                      ; go to L1
done STR R2,R1,#0          ; write also null character
;---------------------------------------------------------------------------
; print input string
;---------------------------------------------------------------------------
LD R0,INPUT_STRING           ; Load input string location
PUTS                           ; print input string
LD R0,NEW_LINE              ; print new line
OUT
;---------------------------------------------------------------------------
; process the input string and store the converted string at 0x5100
;---------------------------------------------------------------------------
LD R0, INPUT_STRING          ; R0=starting address of the input string
LD R1, OUTPUT_STRING          ; R1=starting address of the output string

LOOP LDR R2, R0, #0          ; R2 contains the character at location R0+0
BRnz PROCESS_OVER           ; If current character is null goto label PROCESS_OVER
; check whether the letter is upper case or not(for upper case x, 41<=x<=5A)
LD R3,UCS      
NOT R3,R3
ADD R3,R3,#1                ; R3=-41
ADD R3,R2,R3
BRn CHECK_UPPER             ; if x-41<0, go to CHECK_UPPER
LD R3,UCE
AND R4,R4,#0
ADD R4,R4,R2
NOT R4,R4
ADD R4,R4,1                 ; R4=-x
ADD R3,R3,R4
BRn CHECK_UPPER             ; if 5A-x < 0, go to CHECK_UPPER
; if the letter is upper case add gap(32) to it to convert into lower case letter
LD R4,GAP
ADD R4,R4,R2
STR R4,R1,#0                  ; Store current character (in R2) to output string
BRnzp NEXT                    ; go to next iteration
; check whether the letter is lower case or not(for lower case x, 61<=x<=7A)
CHECK_UPPER LD R3,LCS
NOT R3,R3
ADD R3,R3,#1                ; R3=-61
ADD R3,R2,R3
BRn STORE                   ; if x-41<0, go to STORE(to store the character as it is)
LD R3,LCE
AND R4,R4,#0
ADD R4,R4,R2
NOT R4,R4
ADD R4,R4,1                 ; R4=-x
ADD R3,R3,R4
BRn STORE                   ; if 7A-x<0, go to STORE(to store the character as it is)
; if the letter is lower case subtract gap(32) from it to convert into upper case letter
LD R4,GAP
NOT R4,R4
ADD R4,R4,#1
ADD R4,R4,R2
STR R4,R1,#0
BRnzp NEXT                    ; go to next iteration
STORE STR R2,R1,#0         ; Store current character as it is, to output string
NEXT ADD R0,R0, #1          ; Increment R0 to access next character of the input string
ADD R1,R1,#1                  ; Increment R1 to store next character into output string
BRnzp LOOP                  ; jump to loop
PROCESS_OVER STR R2,R1,#0    ; Store null(0) at the end of the output string
;---------------------------------------------------------------------------
; print converted string
;---------------------------------------------------------------------------
LD R0,OUTPUT_STRING
PUTS                           ; print the modified string
   HALT                     ; exit the program
;---------------------------------------------------------------------------
; declare the necessary data
;---------------------------------------------------------------------------
input .STRINGZ "iLOVEComputerScience252"
INPUT_STRING .FILL x5000
OUTPUT_STRING .FILL x5100
NEW_LINE .FILL xA
LCS .FILL x61       ; (97)dec
LCE .FILL x7A       ; (122)dec
UCS .FILL x41       ; (65)dec
UCE .FILL x5A       ; (90)dec
GAP .FILL x20       ; (32)dec