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
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+PASS  int3 assembles to the single byte CC
+PASS  NASM 'int 3' assembles to CD 03
+PASS  int 21h assembles to CD 21
+PASS  into assembles to CE
+PASS  hello21 prints its string
+PASS  AH=4Ch return code 7 visible as ERRORLEVEL
+PASS  AH=30h reports a version
+PASS  AH=2Ah/2Ch ranges ok
+PASS  AH=02h+06h write AB
+PASS  AH=01h returned 5A ('Z') and AH=0Ah counted 5 chars 'Ankur'
+PASS  AH=25h/35h round trip
+PASS  AH=48h/49h alloc+free ok
+PASS  AH=48h 1 MB request fails CF=1 AX=0008
+PASS  file I/O: 12 bytes read, LSEEK says 12
+PASS  findfirst sees U.TXT size 12
+PASS  reopening deleted file: CF=1 AX=0002
+PASS  mkdir/rmdir ok
+PASS  child exit code 7, termination type 0 via AH=4Dh
+PASS  CC: return IP = trap + 1
+PASS  CD 03: return IP = trap + 2
+PASS  IF and TF are 0 inside the handler, IF back to 1 after IRET
+PASS  IF pushed on the stack was 1
+PASS  INT 3 with no handler: program survives under DOSBox
+PASS  DEBUG + CC: 'Unexpected breakpoint interrupt' and IP=0104
+PASS  DEBUG + CC: next instruction after trap is MOV AX,2222
+PASS  DEBUG + CC: single-step then AX=2222
+PASS  DEBUG + CD 03: no breakpoint message
+PASS  DEBUG + CD 03: stops at IP=0104 inside the INT 03 instruction
+PASS  DEBUG + CD 03: MOV AX,2222 never runs (AX stays 1111 after stepping)
+PASS  Unicorn: CC patch trap return IP = site+1, resume gives CX=3
+PASS  Unicorn: CD 03 patch trap return IP = site+2 and resume goes wrong