Add interrupts module: INT 3 (CC breakpoint) vs INT 21h (DOS services)

NASM sources, DOSBox/FreeDOS Debug/Unicorn harness, captured output and 31 assertions backing the ankurm.com article "INT 3 vs INT 21h in 8086 Assembly". Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh
2026-09-30 19:10:56 +00:00
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+; common.inc -- tiny output helpers shared by the demo programs (DOS .COM, NASM syntax)
+
+; print the '$'-terminated string at DS:DX  (INT 21h, AH=09h)
+puts:
+    mov ah, 09h
+    int 21h
+    ret
+
+; print CRLF
+crlf:
+    mov dx, s_crlf
+    jmp puts
+s_crlf db 0Dh, 0Ah, '$'
+
+; print AX as four hex digits using INT 21h AH=02h
+puthex16:
+    push ax
+    mov al, ah
+    call puthex8
+    pop ax
+puthex8:
+    push ax
+    shr al, 1
+    shr al, 1
+    shr al, 1
+    shr al, 1
+    call nib
+    pop ax
+    and al, 0Fh
+nib:
+    add al, '0'
+    cmp al, '9'
+    jbe .ok
+    add al, 7
+.ok:
+    mov dl, al
+    mov ah, 02h
+    int 21h
+    ret
+
+; print AX as unsigned decimal
+putdec:
+    push bx
+    push cx
+    push dx
+    xor cx, cx
+    mov bx, 10
+.d1:
+    xor dx, dx
+    div bx
+    push dx
+    inc cx
+    test ax, ax
+    jnz .d1
+.d2:
+    pop dx
+    add dl, '0'
+    mov ah, 02h
+    int 21h
+    loop .d2
+    pop dx
+    pop cx
+    pop bx
+    ret