8086处理器Tasm4.1科学计算器项目：跳转越界及加法计算问题求助

Hey there! Nice work tackling an 8086-based scientific calculator for your university exam—those assembly projects are always a great learning experience. Let's walk through why adding .386 fixed your jump issue but broke your addition, and how to fix it.

Why .386 Might Be Causing Addition Errors #

When you add .386 to your TASM code, you're switching the assembler into 32-bit addressing mode (even though you're targeting an 8086, which is 16-bit). This changes a few key behaviors that can mess up your 16-bit-focused addition logic:

• TASM may default to 32-bit operands/registers (like EAX instead of AX) if you don't explicitly specify 16-bit sizes.
• Memory accesses might use 32-bit offsets instead of 16-bit, leading to reading/writing the wrong memory locations for your numbers.
• Some DOS interrupts (like the ones you're probably using in your PRINT macro) rely on 16-bit registers—if the macro doesn't preserve these, your addition registers could get overwritten.

Step-by-Step Debugging & Fixes #

1. Audit Your Addition Code for 32-bit vs 16-bit Mismatches #

Double-check every line of your addition logic to ensure you're using 16-bit registers and explicit 16-bit memory operands:

• Use AX, BX, CX, DX (16-bit) instead of EAX, EBX, etc. (32-bit).
• If you're accessing memory values, explicitly mark them as 16-bit words with WORD PTR to override TASM's 32-bit default in .386 mode.

Example Fix:
If your original code was:

ADD AX, [NUM1]

Change it to:

ADD AX, WORD PTR [NUM1] ; Explicitly specify 16-bit word access

(Assuming NUM1 is defined as a DW (16-bit word) in your .DATA section.)

2. Check Your PRINT Macro for Register Overwrites #

Your PRINT macro likely uses DOS interrupt 09h (string display), which requires AH = 09h and DX pointing to the string. If this macro doesn't save/restore registers like AX or DX, it can overwrite values you're using for addition.

Fix the Macro to Preserve Registers:
Modify your PRINT macro to push registers before using them, then pop them afterward:

PRINT MACRO MSG
    PUSH AX       ; Save AX before modifying it
    PUSH DX       ; Save DX if you're using it elsewhere
    LEA DX, MSG
    MOV AH, 09H
    INT 21H
    POP DX        ; Restore DX
    POP AX        ; Restore AX
ENDM

This way, any registers your macro uses won't interfere with the values you're calculating in your addition logic.

3. Verify the Placement of .386 #

Make sure .386 is placed after your model directive, not before. The correct order ensures TASM uses 16-bit segments (compatible with 8086) while allowing 32-bit instructions if needed:

.MODEL SMALL
.386          ; Correct placement: after model, before stack/data/code
.STACK 100H
.DATA
; Your variables here (e.g., NUM1 DW 0, NUM2 DW 0)
.CODE
; Your main code and procedures here

4. Use TASM Debugger (TD.exe) to Step Through Addition #

Grab TASM's Turbo Debugger (TD.exe) and single-step through your addition code. Watch the registers (AX, BX, memory values) before and after the ADD instruction:

• Is the input value in the correct register/memory location?
• Does the ADD instruction produce the expected result?
• Is the result being stored correctly (e.g., back to a 16-bit memory word)?

This will help you pinpoint exactly where the calculation goes wrong.

Final Notes #

The .386 directive fixed your jump issue by allowing longer jump offsets, but it introduced 32-bit mode quirks that clash with your 8086-focused code. By explicitly enforcing 16-bit operands and preserving registers in macros, you should be able to get your addition working again.

内容的提问来源于stack exchange，提问作者kabir khan