; int3_handler.asm -- install our own INT 3 handler (AH=25h), then trigger it twice:
;   once with the one-byte CC and once with the two-byte CD 03.
; Prints how far past the trapping instruction the pushed return IP points, and the
; state of IF (interrupt flag) inside the handler versus after IRET.
bits 16
org 0x100
    ; --- save the old vector with AH=35h, install ours with AH=25h ---
    mov ax, 3503h
    int 21h                 ; ES:BX = current INT 3 handler
    mov [old_off], bx
    mov [old_seg], es
    mov ax, 2503h
    mov dx, handler
    int 21h                 ; DS:DX = new handler for vector 3

    sti                     ; make sure IF=1 before we trap, so the handler's IF=0 is visible
    mov word [target], trap_cc
    mov dx, m_cc
    call puts
trap_cc:
    int3                    ; CC
    call report

    mov word [target], trap_cd
    mov dx, m_cd
    call puts
trap_cd:
    db 0CDh, 03h            ; CD 03
    call report

    ; --- restore the old vector and exit ---
    push ds
    mov dx, [old_off]
    mov ds, [old_seg]
    mov ax, 2503h
    int 21h
    pop ds
    mov ax, 4C00h
    int 21h

; ------------------------------------------------------------------
; The handler runs with IF=0 and TF=0 (the CPU cleared them when it took the interrupt).
handler:
    push bp
    mov bp, sp
    push ax
    ; stack now: [bp+0]=saved BP, [bp+2]=return IP, [bp+4]=return CS, [bp+6]=FLAGS
    mov ax, [bp+2]
    mov [ret_ip], ax
    mov ax, [bp+6]
    mov [flags_pushed], ax
    pushf
    pop ax                  ; FLAGS as the handler itself sees them
    mov [flags_inside], ax
    pop ax
    pop bp
    iret

report:
    mov dx, m_delta
    call puts
    mov ax, [ret_ip]
    sub ax, [target]        ; how many bytes past the trapping instruction?
    call putdec
    mov dx, m_ifpush
    call puts
    mov ax, [flags_pushed]
    mov cl, 9
    shr ax, cl
    and al, 1
    add al, '0'
    mov dl, al
    mov ah, 02h
    int 21h
    mov dx, m_ifin
    call puts
    mov ax, [flags_inside]
    mov cl, 9
    shr ax, cl
    and al, 1
    add al, '0'
    mov dl, al
    mov ah, 02h
    int 21h
    mov dx, m_tfin
    call puts
    mov ax, [flags_inside]
    mov cl, 8
    shr ax, cl
    and al, 1
    add al, '0'
    mov dl, al
    mov ah, 02h
    int 21h
    mov dx, m_ifafter
    call puts
    pushf
    pop ax
    mov cl, 9
    shr ax, cl
    and al, 1
    add al, '0'
    mov dl, al
    mov ah, 02h
    int 21h
    jmp crlf

m_cc      db 'CC    (INT3): $'
m_cd      db 'CD 03 (INT 3): $'
m_delta   db 'return IP = trap address + $'
m_ifpush  db ', IF pushed = $'
m_ifin    db ', IF in handler = $'
m_tfin    db ', TF in handler = $'
m_ifafter db ', IF after IRET = $'

old_off dw 0
old_seg dw 0
target  dw 0
ret_ip  dw 0
flags_pushed dw 0
flags_inside dw 0

%include "common.inc"