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strcat routine in MIPS Homework Solution


Enter two strings and Concatenate It

Write an assembly language 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:

MIPS

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