Question

In this problem, you will write a LC-3 assembly code that removes blank spaces from a string. Assume that the string starts at memory location 0x5000, and is terminated by a ‘\0’ character (ASCII value = 0). Your program should store the modified string in the memory location starting at 0x5100. You do not need to modify the original string stored at 0x5000. You can assume that the original string at 0x5000 will always be less than 100 characters in length, and it will always start with a letter (A-Z, lowercase or uppercase possible). Note: If the modified string has 7 characters, and the original string has 15 characters, the last 8 characters of your modified string should be all ‘%’ (ASCII value = 0x25). For example: If the original string at 0x5000 was “aa 12 d e f”, the modified string at 0x5100 after your program completes execution should be “aa12def%%%%”. Note that the original string has 4 blank space characters, and the modified string has 4 extra “%” in the end. Your code should start at memory location 0x3000.

Answer 1

Program:

; remblank.asm ORIG x3000 LEA RO, input LD R1, INPUT STRING L1 LDR R2, R0 , #0 ADD R3 , R2, #0 BRz done : start at memory location 0x3000 ; Load starting address of input string ; R1-x5000 ;loop to store input string at location x5000 ; read current character into R2 STR R2 ,R1, #0 ADD R0, R0, #1 ADD R1,R1,#1 BR L1 done STR R2,R1,#0 LD RO,INPUT STRING PUTS LD RO, NEW LINE OUT R3-R2+0 ; if current character is zero (null character) go to label done ; otherwise, store current charcter into current location ;go to next character ;go to next location ; go to L1 ; write also null character : Load input string location ; print input string ; print new line ; to track the location at the input string ; to track the location at the output string ; R6 32, ASCII value of blank space ' ' ; R6--32 LD RO, INPUT STRING LD R1, OUTPUT STRING LD R6 BLANK SPACE NOT R6, R6 ADD R6, R6, #1 AND R3, R3, #0 LOOP LDR R2 , R0 , #0 BRnz INPUT OVER ADD R0,R0 , #1 ADD R5, R2,R6 BRz SKIP STORING SPACE STR R2 ,R1, #0 ADD R1,R1, #1 BRnzp LOOP SKIP STORING SPACE ADD R3,R3,#1 BRnzp LOOP INPUT_OVER LD R2, REPLACE ; store %s LOOP! ADD R3, R3,#-1 R3-0 (to keep track number of blank spaces) ; R2 contains the character at location RO+0 :If current character is null goto INPUT_OVER Increment RO to access next character in input string R5 R2-32 : Check and skip storing if character is blank space ; Store current character (in R2) to output string Increment R1 to store next character jump to loop :jump to loop ; R2 = x25 Decrement R3 ;Store % to current location in output string BRz END STR R2 ,R1, #0 ADD R1,R1,#1 BRnzp LooP1 : Increment R1 to go to next location END STR R2,R1, #0 ;Store 0 at the end of string LD RO,OUTPUT STRING PUTS ; print the modified string HALT input .STRINGZ "aa 12 d e f"aa 12 d e f INPUT STRING .FILL x5000 OUTPUT STRING .FILL x5100 BLANK SPACE FILL x20 REPLACE .FILL x25

Sample output:

compile remblank.asm

LC3 Simulator File About Controls Next Step Continue Stop Suspended as remblank.asm Loaded object file DLC3lc3os.obj Loaded symbol file D:LC3lc3os.sym Assembly of remblank.asm' completed without errors or warnings. Registers R0 x0000 R1 x0000 R2 k0000 R3 |x0000 R4 x0000 R5 x0000 Memory R6 x0000 R7 x0000 PC x0200 MPR|x0000 PSR x8002 BP Address Value Instruction 1x01E5 1x01E6 □1x01E7 x0265 BRp x024B X0265 BRp x024C X0265 BRp TRAP PUTSP_S_LOOP x0265 BRp x024E x0265 BRp x024F x0265 BRp x0250 0265 BRp TRAP PUTSP MSB 0 X0265 BRp x0252 X0265 BRp x0253 X0265 BRp x0254 X0265 BRp x0255 X0265 BRp x0256 x0265 BRp x0257 X0265 BRp x0258 X0265 BRp TRAP PUTSP_DONE X0265 BRp x025A X0265 BRp x025B x0265 BRp x025C X0265 BRp x025D X0265 BRp x025E 0265 BRp TRAP HALT X0265 BRp x0260 X0265 BRp x0261 x0265 BRp x0262 x0265 BRp x0263 X0265 IBRp BAD TRAP X0265 BRp BAD INT x201A LD RO, MPR INIT 1x01E9 Devices x01EC x01EE X01EF x01F2 1x01F4 x01F6 □1x01F7 し1x01F8 x01F9 x01FA x01FC k01 FD x01FF □|X0200 OS-START

load remblank.obj

run

LC3 Simulator File About Controls Continue Hated as remblank.asm Assembly of remblank.asm' completed without errors or warnings Loaded object file D:LC31remblank.obj Loaded symbol file DALC3iremblank.sym Registers R0 x0000 R1 X7FFF Memory Address Instruction R7 x3028 x0248 x0249 x024B x5002 AND RO, RO, R2 x040FBRz TRAP PUTSP DONE «F021 TRAP #33 x5020 AND R0, R0, #0 x1628 ADD R3, R0, #8 MPRİXOFF8 PSR x8002 R4 x0000 R5 x0046 Devices [Ik024D TRAP PUTSP S L x1000 ADD R0R0. R0 x024E x14A0 x0601 x1021 ADD R2, R2, #0 BRzp TRAP PUTSP MSB 0 ADD R0, R0, #1 ADD R2, R2, R2 □ 0251 TRAP PUTSP-ISB-0 x1482 x0252 x16FF ADD R3, R3, #-1 X03F9 BRp TRAP PUTSP_S LOOP x1020 ADD R0, R0, #0 x0403 BRZ TRAP PUTSP_DONE «F021 TRAP #33 x1261 ADD R1. R1, #1 XOFED BR TRAP PUTSP LOOP x0255 x0256 x0257 x0259 TRAP PUTSP DONE x21B4 LD R0, OS SAVE RO x025E x025D x23B4 LD R1, OS_SAVE_R1 x25B4 LD R2, OS_SAVER2 x27B4 LD R3, OS SAVE R3 x2FB7 LD R7, OS SAVE R7 xC1C0 RET A1 AD İLDI R00S MCR x23B7 LD R1, MASK H x5001 AND RO, RO, R1 xB1AA İSTI RO, OS MCR X0F9C BR OS START aa 12 d ef aa12def%%%% x025E □|x025F TRAP-HALT x0260 x0261 11x0262 X0263

Code to copy:

;remblank.asm
.ORIG x3000           ; start at memory location 0x3000
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
LD R0,INPUT_STRING        ; Load input string location
PUTS               ; print input string
LD R0,NEW_LINE           ; print new line
OUT

LD R0, INPUT_STRING       ; to track the location at the input string
LD R1, OUTPUT_STRING       ; to track the location at the output string
LD R6, BLANK_SPACE       ; R6 = 32, ASCII value of blank space ' '
NOT R6, R6           ; R6 = -32
ADD R6, R6, #1  
AND R3,R3,#0           ; R3=0(to keep track number of blank spaces)   
LOOP LDR R2, R0, #0       ; R2 contains the character at location R0+0
BRnz INPUT_OVER           ; If current character is null goto INPUT_OVER
ADD R0,R0, #1           ; Increment R0 to access next character in input string  
ADD R5,R2,R6           ; R5=R2-32
BRz SKIP_STORING_SPACE       ; Check and skip storing if character is blank space
STR R2,R1,#0           ; Store current character (in R2) to output string
ADD R1,R1,#1           ; Increment R1 to store next character
BRnzp LOOP           ;jump to loop
SKIP_STORING_SPACE ADD R3,R3,#1
BRnzp LOOP           ;jump to loop
INPUT_OVER LD R2,REPLACE    ;R2 = x25
; store %s
LOOP1 ADD R3,R3,#-1       ;Decrement R3
   BRz END
   STR R2,R1,#0       ;Store % to current location in output string
   ADD R1,R1,#1       ;Increment R1 to go to next location
   BRnzp LOOP1
END STR R2,R1,#0       ;Store 0 at the end of string
  
LD R0,OUTPUT_STRING
PUTS               ; print the modified string
   HALT
input .STRINGZ "aa 12 d e f" ; aa 12 d e f
INPUT_STRING .FILL x5000
OUTPUT_STRING .FILL x5100
BLANK_SPACE .FILL x20
REPLACE .FILL x25
NEW_LINE .FILL xA