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
commit 09af523f8c
27 changed files with 1029 additions and 0 deletions
@@ -0,0 +1,9 @@
+; hello21.asm -- the smallest useful INT 21h program: AH=09h then AH=4Ch with a return code
+bits 16
+org 0x100                   ; .COM program
+    mov ah, 09h             ; AH selects the DOS function: write '$'-terminated string
+    mov dx, msg             ; DS:DX -> string (DS = CS in a .COM file)
+    int 21h                 ; CD 21 -- the DOS entry point
+    mov ax, 4C07h           ; AH=4Ch terminate, AL=7 return code (ERRORLEVEL)
+    int 21h
+msg db 'Hello from INT 21h, AH=09h', 0Dh, 0Ah, '$'