8086-ASSEMBLY-LANGUAGE-PROGRAMS/Source Code/Arithmetic/subtraction_8bit_with_user_input.asm at main · Amey-Thakur/8086-ASSEMBLY-LANGUAGE-PROGRAMS · GitHub

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; =============================================================================
; TITLE: 8-bit Interactive Subtraction with ASCII Adjustment
; DESCRIPTION: This program reads two single-digit decimal numbers from the
;              keyboard, performs subtraction (Minuend - Subtrahend), and
;              displays the result. It highlights the use of AAS (ASCII
;              Adjust for Subtraction) to handle multi-digit borrowing logic
;              in Unpacked BCD format.
; AUTHOR: Amey Thakur (https://github.com/Amey-Thakur)
; REPOSITORY: https://github.com/Amey-Thakur/8086-ASSEMBLY-LANGUAGE-PROGRAMS
; LICENSE: MIT License
; =============================================================================

.MODEL SMALL
.STACK 100H

; -----------------------------------------------------------------------------
; DATA SEGMENT
; -----------------------------------------------------------------------------
.DATA
    VAL_MINUEND    DB ?
    VAL_SUBTRAHEND DB ?
    VAL_DIFFERENCE DB ?

    ; User Prompts
    MSG_PROMPT1  DB 0DH, 0AH, "Enter minuend: $"
    MSG_PROMPT2  DB 0DH, 0AH, "Enter subtrahend: $"
    MSG_RESULT   DB 0DH, 0AH, "Result (A - B): $"

; -----------------------------------------------------------------------------
; CODE SEGMENT
; -----------------------------------------------------------------------------
.CODE
MAIN PROC
    ; --- Step 1: Initialize Data Segment ---
    MOV AX, @DATA
    MOV DS, AX

    ; --- Step 2: Read Minuend ---
    LEA DX, MSG_PROMPT1
    MOV AH, 09H
    INT 21H

    MOV AH, 01H
    INT 21H
    SUB AL, 30H
    MOV VAL_MINUEND, AL

    ; --- Step 3: Read Subtrahend ---
    LEA DX, MSG_PROMPT2
    MOV AH, 09H
    INT 21H

    MOV AH, 01H
    INT 21H
    SUB AL, 30H
    MOV VAL_SUBTRAHEND, AL

    ; --- Step 4: Subtraction and Adjustment ---
    MOV AL, VAL_MINUEND
    SUB AL, VAL_SUBTRAHEND
    MOV AH, 00H
    AAS                          ; ASCII Adjust for Subtraction

    ; Prepare for display
    ADD AH, 30H                  ; Borrow indicator / Tens digit
    ADD AL, 30H                  ; Ones digit
    MOV BX, AX

    ; --- Step 5: Display Result ---
    LEA DX, MSG_RESULT
    MOV AH, 09H
    INT 21H

    MOV AH, 02H
    MOV DL, BH
    INT 21H

    MOV DL, BL
    INT 21H

    ; --- Step 6: Shutdown ---
    MOV AH, 4CH
    INT 21H
MAIN ENDP

END MAIN

; =============================================================================
; TECHNICAL NOTES & ARCHITECTURAL INSIGHTS
; =============================================================================
; 1. SUBTRACTION LOGIC:
;    The CPU performs subtraction by adding the Two's Complement of the
;    subtrahend.
;
; 2. AAS (ASCII ADJUST FOR SUBTRACTION):
;    Checks the lower 4 bits of AL. If bit 4 borrow occurred (AF=1) or
;    digit > 9, it subtracts 6 from AL and 1 from AH.
;
; 3. BORROW DETECTION:
;    If the first digit is smaller than the second (e.g., 5-7), AAS adjusts
;    AH to FFH. When converted to ASCII (+30H), this results in character
;    2FH ('/'), signaling a negative wrap.
;
; 4. UNPACKED BCD:
;    A format where one digit occupies a full byte. Highly efficient
;    for character-based I/O.
;
; 5. INTERRUPT OVERHEAD:
;    INT 21H/AH=01H is a blocking call; the CPU sits in an idle loop waiting
;    for keyboard input.
; = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =