lab3

 This lab is for the student to understand how to use the math and string in 6502 assembly language, including displaying output, getting input from users and perform some operatiion based on the users' input. For this lab, I have designed and implement simple add calculator that accept the two input from the users. Check the input if they are number and then perform the add instruction and display the add output for the users.

The code:

define SCINIT   $ff81  ; Initialize/clear screen

define CHRIN    $ffcf  ; Input character from keyboard

define CHROUT   $ffd2  ; Output character to screen

define NUM1     = $01  ; Memory location for first number

define NUM2     = $02  ; Memory location for second number

define RESULT   = $03  ; Memory location for result


START:

       JSR SCINIT      ; Clear the screen

       

       ; Display message to enter the first number

       LDA #<MSG1

       LDY #>MSG1

       JSR PRINT_MSG


       ; Get the first number from the user

       JSR GETNUM      ; Call the number input routine

       STA NUM1        ; Store the first number in memory


       ; Display message to enter the second number

       LDA #<MSG2

       LDY #>MSG2

       JSR PRINT_MSG


       ; Get the second number from the user

       JSR GETNUM      ; Call the number input routine

       STA NUM2        ; Store the second number in memory


       ; Perform the addition

       CLC             ; Clear carry before addition

       LDA NUM1        ; Load the first number

       ADC NUM2        ; Add the second number

       STA RESULT      ; Store the result in memory


       ; Display the result message

       LDA #<MSG3

       LDY #>MSG3

       JSR PRINT_MSG


       ; Display the result

       LDA RESULT      ; Load the result

       CLC             ; Clear carry for ASCII conversion

       ADC #$30        ; Convert number to ASCII

       JSR CHROUT      ; Output the result as a character

       BRK             ; End of program


; Routine to get number input from keyboard

GETNUM:

        JSR CHRIN       ; Read a character from the keyboard

        CMP #$0D        ; Compare with ASCII code for Enter (carriage return)

        BEQ WAIT_ENTER  ; If it's Enter, wait for a valid number input

        CMP #$30        ; Compare with ASCII '0'

        BMI GETNUM      ; If input is less than '0', retry

        CMP #$3A        ; Compare with ASCII '9' + 1

        BPL GETNUM      ; If input is greater than '9', retry

        PHA             ; Save the input number on the stack

        JSR CHROUT      ; Echo the character back to the screen

        

    WAIT_ENTER:

          JSR CHRIN       ; Wait for Enter key

          CMP #$0D        ; Compare with ASCII code for Enter

          BNE WAIT_ENTER  ; If not Enter, keep waiting

          LDA #$0A        ; Load line feed (newline)

          JSR CHROUT      ; Print new line

          PLA             ; Retrieve the input number from the stack

          SEC             ; Set carry flag for proper subtraction

          SBC #$30        ; Convertdefine SCINIT    $ff81  ; Initialize/clear screen



PRINT_MSG:

        LDY #$00        ; Initialize Y to 0

CHARNEXT:

        LDA MSG1,Y       ; Load character from MSG+Y into A

        BEQ MSGDONE     ; If we've loaded a 0, we're done

        JSR CHROUT      ; Output the character

        INY             ; Move to next character

        JMP CHARNEXT    ; Repeat for next character

MSGDONE:

        RTS             ; Return from subroutine



MSG1:

   DCB "E","N","T","E","R",$20,"T","H","E",$20

   DCB "N","U","M","B","E","R",":"$00



For thsi program, I have implememnt the start to control the whole program flow. Then I implement the GETNUM function, which will wait for the user to input the number, check if the input from user is a number and convert the char to number for the assembly language to add them up. Also, I have implement function WAIT_ENTER to wait for the user for entering the number. Finally, I use the add instruction in assembly language to add these two number and convert the result back to char for display.

In this lab, I have tried to implement my first assembly language program. There are many things that can be improved. For example, this program can only perform add instruction. Unlike many modern calculators which can be used for minus, time and divide. Also, I didn't implement the promot for user if they enter the number incorrectly.

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