asmhatreandClaude Sonnet 5.5
09af523f8c
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
124 lines
2.7 KiB
NASM
124 lines
2.7 KiB
NASM
; int3_handler.asm -- install our own INT 3 handler (AH=25h), then trigger it twice:
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; once with the one-byte CC and once with the two-byte CD 03.
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; Prints how far past the trapping instruction the pushed return IP points, and the
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; state of IF (interrupt flag) inside the handler versus after IRET.
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bits 16
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org 0x100
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; --- save the old vector with AH=35h, install ours with AH=25h ---
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mov ax, 3503h
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int 21h ; ES:BX = current INT 3 handler
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mov [old_off], bx
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mov [old_seg], es
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mov ax, 2503h
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mov dx, handler
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int 21h ; DS:DX = new handler for vector 3
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sti ; make sure IF=1 before we trap, so the handler's IF=0 is visible
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mov word [target], trap_cc
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mov dx, m_cc
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call puts
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trap_cc:
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int3 ; CC
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call report
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mov word [target], trap_cd
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mov dx, m_cd
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call puts
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trap_cd:
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db 0CDh, 03h ; CD 03
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call report
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; --- restore the old vector and exit ---
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push ds
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mov dx, [old_off]
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mov ds, [old_seg]
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mov ax, 2503h
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int 21h
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pop ds
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mov ax, 4C00h
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int 21h
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; ------------------------------------------------------------------
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; The handler runs with IF=0 and TF=0 (the CPU cleared them when it took the interrupt).
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handler:
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push bp
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mov bp, sp
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push ax
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; stack now: [bp+0]=saved BP, [bp+2]=return IP, [bp+4]=return CS, [bp+6]=FLAGS
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mov ax, [bp+2]
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mov [ret_ip], ax
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mov ax, [bp+6]
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mov [flags_pushed], ax
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pushf
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pop ax ; FLAGS as the handler itself sees them
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mov [flags_inside], ax
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pop ax
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pop bp
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iret
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report:
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mov dx, m_delta
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call puts
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mov ax, [ret_ip]
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sub ax, [target] ; how many bytes past the trapping instruction?
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call putdec
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mov dx, m_ifpush
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call puts
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mov ax, [flags_pushed]
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mov cl, 9
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shr ax, cl
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and al, 1
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add al, '0'
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mov dl, al
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mov ah, 02h
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int 21h
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mov dx, m_ifin
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call puts
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mov ax, [flags_inside]
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mov cl, 9
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shr ax, cl
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and al, 1
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add al, '0'
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mov dl, al
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mov ah, 02h
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int 21h
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mov dx, m_tfin
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call puts
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mov ax, [flags_inside]
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mov cl, 8
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shr ax, cl
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and al, 1
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add al, '0'
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mov dl, al
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mov ah, 02h
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int 21h
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mov dx, m_ifafter
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call puts
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pushf
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pop ax
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mov cl, 9
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shr ax, cl
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and al, 1
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add al, '0'
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mov dl, al
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mov ah, 02h
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int 21h
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jmp crlf
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m_cc db 'CC (INT3): $'
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m_cd db 'CD 03 (INT 3): $'
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m_delta db 'return IP = trap address + $'
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m_ifpush db ', IF pushed = $'
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m_ifin db ', IF in handler = $'
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m_tfin db ', TF in handler = $'
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m_ifafter db ', IF after IRET = $'
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old_off dw 0
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old_seg dw 0
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target dw 0
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ret_ip dw 0
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flags_pushed dw 0
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flags_inside dw 0
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%include "common.inc"
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