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
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Claude Sonnet 5.5
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1 ; opcodes.asm -- which bytes does each spelling assemble to? (read the listing, never run)
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2 bits 16
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3 org 0x100
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4 00000000 CC int3 ; the one-byte breakpoint -> CC
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5 00000001 CD03 int 3 ; "INT n" with n = 3 in NASM -> CD 03
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6 00000003 CD03 db 0CDh, 03h ; the same two bytes, written out -> CD 03
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7 00000005 CD21 int 21h ; DOS entry point -> CD 21
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8 00000007 CD10 int 10h ; BIOS video -> CD 10
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9 00000009 CE into ; one-byte INTO -> CE
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Hello from INT 21h, AH=09h
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ERRORLEVEL is at least 7
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AH=30h DOS version : 5.0
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AH=2Ah/2Ch date+time ranges : ok ok
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AH=02h + AH=06h : AB
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AH=01h + AH=0Ah (stdin) : echo -> Ankur
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AH=01h got 0x5A ; AH=0Ah count = 5, text = Ankur
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AH=25h/35h vector 60h : ok
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AH=48h/49h alloc+free 4 KB : ok
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AH=48h ask for 1 MB (fails) : CF=1 AX=0008 ; largest block (paragraphs) = 8E6C
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AH=3Ch/40h/3Dh/3Fh/42h : 12 bytes read ; LSEEK(end) says size = 12
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AH=56h + AH=4Eh findfirst : U.TXT size = 12
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AH=41h delete, reopen fails : CF=1 AX=0002
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AH=39h/3Ah mkdir+rmdir : ok ; AH=19h drive = 2
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AH=4Bh exec child : Hello from INT 21h, AH=09h
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AH=4Dh return code = 7 type = 0
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CC (INT3): return IP = trap address + 1, IF pushed = 1, IF in handler = 0, TF in handler = 0, IF after IRET = 1
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CD 03 (INT 3): return IP = trap address + 2, IF pushed = 1, IF in handler = 0, TF in handler = 0, IF after IRET = 1
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before INT3
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after INT3 (program survived)
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-u 100 L9
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072E:0100 B81111 MOV AX,1111
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072E:0103 CC INT 3
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072E:0104 B82222 MOV AX,2222
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072E:0107 B8004C MOV AX,4C00
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-g
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Unexpected breakpoint interrupt
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AX=1111 BX=0000 CX=000C DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0104 NV UP EI PL ZR NA PE NC
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072E:0104 B82222 MOV AX,2222
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-r
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AX=1111 BX=0000 CX=000C DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0104 NV UP EI PL ZR NA PE NC
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072E:0104 B82222 MOV AX,2222
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-t
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AX=2222 BX=0000 CX=000C DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0107 NV UP EI PL ZR NA PE NC
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072E:0107 B8004C MOV AX,4C00
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-r
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AX=2222 BX=0000 CX=000C DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0107 NV UP EI PL ZR NA PE NC
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072E:0107 B8004C MOV AX,4C00
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-g
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Program terminated normally (0000)
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-q
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-u 100 L9
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072E:0100 B81111 MOV AX,1111
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072E:0103 CD03 INT 03
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072E:0105 B82222 MOV AX,2222
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072E:0108 B8004C MOV AX,4C00
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-g
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AX=1111 BX=0000 CX=000D DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0104 NV UP EI PL ZR NA PE NC
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072E:0104 03B82222 ADD DI,[BX+SI+2222] DS:2222=0000
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-r
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AX=1111 BX=0000 CX=000D DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0104 NV UP EI PL ZR NA PE NC
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072E:0104 03B82222 ADD DI,[BX+SI+2222] DS:2222=0000
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-t
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AX=1111 BX=0000 CX=000D DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0108 NV UP EI PL ZR NA PE NC
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072E:0108 B8004C MOV AX,4C00
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-r
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AX=1111 BX=0000 CX=000D DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000
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DS=072E ES=072E SS=072E CS=072E IP=0108 NV UP EI PL ZR NA PE NC
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072E:0108 B8004C MOV AX,4C00
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-g
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Program terminated normally (0000)
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-q
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clean run, no breakpoint:
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CX=3
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CC patch=CC trap: vector=3 return IP=7C03 (site=7C02, +1) debugger restores IP=7C02
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after resume: CX=3 (expected 3), final IP=7C05
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CD 03 patch=CD03 trap: vector=3 return IP=7C04 (site=7C02, +2) debugger restores IP=7C03
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after resume: CX=0 (expected 3), final IP=7C64
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RESULT: PASS
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Title: DEBUG
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Version: 2.50
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Entered-date: 2024-06-01
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Copying-policy: MIT License
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nasm: NASM version 2.16.01
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dosbox: DOSBox version 0.74-3
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unicorn: 2.1.4
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python: Python 3.11.15
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PASS int3 assembles to the single byte CC
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PASS NASM 'int 3' assembles to CD 03
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PASS int 21h assembles to CD 21
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PASS into assembles to CE
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PASS hello21 prints its string
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PASS AH=4Ch return code 7 visible as ERRORLEVEL
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PASS AH=30h reports a version
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PASS AH=2Ah/2Ch ranges ok
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PASS AH=02h+06h write AB
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PASS AH=01h returned 5A ('Z') and AH=0Ah counted 5 chars 'Ankur'
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PASS AH=25h/35h round trip
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PASS AH=48h/49h alloc+free ok
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PASS AH=48h 1 MB request fails CF=1 AX=0008
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PASS file I/O: 12 bytes read, LSEEK says 12
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PASS findfirst sees U.TXT size 12
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PASS reopening deleted file: CF=1 AX=0002
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PASS mkdir/rmdir ok
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PASS child exit code 7, termination type 0 via AH=4Dh
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PASS CC: return IP = trap + 1
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PASS CD 03: return IP = trap + 2
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PASS IF and TF are 0 inside the handler, IF back to 1 after IRET
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PASS IF pushed on the stack was 1
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PASS INT 3 with no handler: program survives under DOSBox
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PASS DEBUG + CC: 'Unexpected breakpoint interrupt' and IP=0104
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PASS DEBUG + CC: next instruction after trap is MOV AX,2222
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PASS DEBUG + CC: single-step then AX=2222
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PASS DEBUG + CD 03: no breakpoint message
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PASS DEBUG + CD 03: stops at IP=0104 inside the INT 03 instruction
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PASS DEBUG + CD 03: MOV AX,2222 never runs (AX stays 1111 after stepping)
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PASS Unicorn: CC patch trap return IP = site+1, resume gives CX=3
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PASS Unicorn: CD 03 patch trap return IP = site+2 and resume goes wrong
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