Introduction

In this project, I modified a 6502 Assembly program originally designed for addition and turned it into a subtraction calculator. I used and adapted code from Krins Kumar’s blog post. This post satisfies that requirement by clearly labeling borrowed, modified, and new code segments.

Source and Permission

Original Code: “SPO Week 3.1 ‐ Full Lab” by Krins Kumar.
Permission: Granted by the author, with freedom to skip attribution—but I’m including it as per my lab requirements.

Borrowed vs. Modified vs. New Code

In the annotated code below, I use the following markers:

[BORROWED]: Lines copied verbatim or nearly so from Krins Kumar’s code.
[MODIFIED]: Lines originally from Krins’s code but significantly changed (renamed labels, reorganized arithmetic).
[NEW]: Entirely new lines or sections I wrote.

All unlabeled lines are effectively the same classification as the preceding comment block. For brevity, I’m showing only an excerpt below; my complete code is posted in my repository.

assemblyCopyEdit; =================== ROM Routine Definitions ===================
; Original: define      SCINIT      $ff81 ; initialize/clear screen
;define      SCINIT      $ff81 ; [BORROWED - commented out original]
define      CLEAR_SCREEN $ff81   ; (changed name) [MODIFIED]

; Original: define      CHRIN       $ffcf ; input character from keyboard
;define      CHRIN       $ffcf ; [BORROWED - commented out original]
define      GET_CHAR    $ffcf   ; (changed name) [MODIFIED]

; Original: define      CHROUT      $ffd2 ; output character to screen
;define      CHROUT      $ffd2 ; [BORROWED - commented out original]
define      PUT_CHAR    $ffd2   ; (changed name) [MODIFIED]

; =================== Zero‑Page Variable Definitions ===================
; Original: define      PRINT_PTR   $10
;define      PRINT_PTR   $10 ; [BORROWED]
define      PRNT_PTR    $10   ; (renamed) [MODIFIED]

; Original: define      value       $14
;define      value       $14  ; [BORROWED]
define      SUM_BCD     $14   ; now holds the minuend/result (changed function) [MODIFIED]

; [NEW] define      SUB_NUM     $1E     ; new variable for subtrahend
; [NEW] define      TEMP_BCD    $1F     ; temporary storage for swapping

; =================== Main Routine: SUBTRACTNUMBERS ===================
; (changed from ADDNUMBERS)
SUBTRACTNUMBERS:
    ; [NEW] Prompt for first number
    JSR PRINT_STR
    dcb $0D,$0D,"Enter first number:",00

    LDA #$00
    ; [BORROWED] 'STA value_h' turned into:
    STA SUM_H       ; [MODIFIED]
    JSR GET_NUMBER  ; [MODIFIED, originally GETNUM]
    STA SUM_BCD     ; [MODIFIED, originally 'sta value']

    ; [NEW] Prompt for second number
    JSR PRINT_STR
    dcb "Enter second number:",00
    JSR GET_NUMBER
    STA SUB_NUM

    ; [NEW] Compare & swap if the first < second
    LDA SUM_BCD
    CMP SUB_NUM
    BCS DO_SUB
    LDA #$2D      ; ASCII '-'
    JSR PUT_CHAR
    LDA SUM_BCD
    STA TEMP_BCD
    LDA SUB_NUM
    STA SUM_BCD
    LDA TEMP_BCD
    STA SUB_NUM

DO_SUB:
    SED
    CLC
    SBC SUB_NUM
    CLD
    STA SUM_BCD

    ; [MODIFIED] Display result
    PHA
    JSR PRINT_STR
    dcb "Result:",32,32,32,32,32,00

    LDA SUM_H
    BEQ PROC_TENS
    AND #$0F
    ORA #$30
    JSR PUT_CHAR
    JSR UPDATE_GFX
    LDA #$10
    LDX CUR_X
    LDY CUR_Y
    JSR DRAW_IMAGE
    LDA CUR_X
    CLC
    ADC #$06
    STA CUR_X
    JMP PROC_TENS

    ; etc. ...
    RTS

(The full code includes the DRAW_IMAGE, PRINT_STR, GET_NUMBER, UPDATE_GFX subroutines, along with the pixel data for digits. I have placed [BORROWED] or [MODIFIED] markers on relevant lines, and [NEW] markers where I added lines that did not exist previously in Krins’s code.)

Key Changes and Additions

Arithmetic: Changed from ADC (addition) to SBC (subtraction) in BCD mode, with logic to handle negative results by swapping numbers.
Variable Renaming: e.g., value → SUM_BCD, XPOS → CUR_X, etc.
New Variables: SUB_NUM for the subtrahend, TEMP_BCD for swapping.
Updated Prompts: New strings asking the user for first and second numbers.
Minus Sign Output: If first < second, print “–” and swap.

Conclusion

This project involved modifying a 6502 assembly program to implement subtraction instead of addition while maintaining proper BCD arithmetic. The code structure was improved for readability, and key functions were optimized.

🔗 GitHub Repositoryhttps://github.com/samartho4/SPO-600/blob/main/Lab%203/calculator

Lab 3 - Follow up