Enter two strings and Concatenate It
Write an assembly language assignment program with a subroutine STRCAT which will ask the user for two different strings and will display the concatenated string back to the user. A sample run of the program is:
Solution:
EXPORT main ; this line is needed to interface with init.s
IMPORT GetCh ; Input one ASCII character from the UART #1 window (from keyboard)
IMPORT PutCh ; Output one ASCII character to the UART #1 window
IMPORT PutCRLF ; Output CR and LF to the UART #1 window
IMPORT UDivMod ; Perform unsigned division to obtain quotient and remainder
IMPORT GetDec ; Input a signed number from the UART #1 window
IMPORT PutDec ; Output a signed number to the UART #1 window
IMPORT GetStr ; Input a CR-terminated ASCII string from the UART #1 window
IMPORT PutStr ; Output null-terminated ASCII string to the UART #1 window
AREA MyCode, CODE, READONLY
ALIGN
main
PUSH {LR} ; save return address of caller in init.s
; Prompt user to enter first string
LDR R0, =Prompt1
BL PutStr
; Read string from keyboard, save in STR1
LDR R0, =STR1
MOV R1,#MaxSTR1
BL GetStr
; Prompt user to enter second string
LDR R0, =Prompt2
BL PutStr
; Read string from keyboard, save in STR2
LDR R0, =STR2
MOV R1,#MaxSTR2
BL GetStr
LDR R0,=STR1 ; point to start of first string
LDR R1,=STR2 ; point to start of second string
BL STRCAT ; concatenate strings using subroutine
; Print result message
LDR R0, =Result
BL PutStr
; Print concatenated string
LDR R0, =STR1
BL PutStr
POP {PC} ; return from main (our last executable instruction)
; Subroutine to concatenate the two strings with addresses R0 and R1
; by adding second string at the end of the first one
STRCAT
PUSH {LR} ; save return address of caller
LOOP1
LDRB R2, [R0] ; load character from first string
CMP R2, #0 ; check if it's end of string
BEQ LOOP2 ; if so, go to next loop
ADD R0, R0, #1 ; else, increment position in string
B LOOP1 ; repeat loop
LOOP2
LDRB R2, [R1], #1 ; load character from second string and increment
STRB R2, [R0], #1 ; save character in first string and increment
CMP R2, #0 ; check if it's end of string
BNE LOOP2 ; if not zero, repeat loop
POP {PC} ; return from subroutine
MaxSTR1 EQU 200 ; maximum string length for first string
MaxSTR2 EQU 100 ; maximum string length for second string
Prompt1 DCB "Enter STR1: ",0
Prompt2 DCB "Enter STR2: ",0
Result DCB "Your concatenated string: ",0
ALIGN
AREA MyData, DATA, READWRITE
STR1 SPACE MaxSTR1+1
STR2 SPACE MaxSTR2+1
ALIGN
;-------------------- END OF MODIFIABLE CODE ----------------------
ALIGN
END ; end of source program in this file