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@ramntry ramntry/dbg.asm
Last active Aug 29, 2015

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Debugger
model tiny
.code
.486
org 100h
_:
fm_aligned = (offset free_memory - _ + 100h + 15) / 16 * 16
load_address = fm_aligned + 100h
start: call get_fname
call open_file
call get_fsize
call read_file
call close_file
call set_int
mov al, int_mode
cmp al, 1
jz victim_enter
call set_breakpoint
jmp victim_enter
int_raport: push bp
mov bp, sp
mov dx, offset line_msg
mov bx, ax
mov ah, 9
int 21h
mov dx, offset ip_eq_msg
int 21h
mov ax, bx
call hex4
mov dx, offset segment_msg
mov ah, 9
int 21h
mov ax, [bp + 4]
call hex4
mov dx, offset ax_eq_msg
mov ah, 9
int 21h
mov ax, [bp + 6]
call hex4
mov dx, offset bx_eq_msg
mov ah, 9
int 21h
mov ax, [bp + 8]
call hex4
mov dx, offset cx_eq_msg
mov ah, 9
int 21h
mov ax, [bp + 10]
call hex4
mov dx, offset dx_eq_msg
mov ah, 9
int 21h
mov ax, [bp + 12]
call hex4
mov dx, offset line_msg
mov ah, 9
int 21h
xor ax, ax
int 16h
cmp al, 'q'
jz victim_ret
pop bp
ret
int_handler: push bp
mov bp, sp
push dx
push cx
push bx
push ax
push ds
push cs
pop ds
xor cx, cx
mov cl, int_mode
shr cx, 1
mov ax, [bp + 2]
sub ax, cx
mov [bp + 2], ax
call int_raport
or cl, cl
mov al, broken_byte
mov bx, breakpoint
pop ds
mov byte ptr [bx], al
pop ax
pop bx
pop cx
pop dx
pop bp
iret
set_int: mov al, int_mode
mov ah, 35h
int 21h
mov cx, bx
mov bx, offset int_old
mov [bx], cx
mov cx, es
mov [bx + 2], cx
mov dx, offset int_handler
mov ah, 25h
int 21h
ret
restore_int: mov bx, offset int_old
mov dx, [bx]
mov cx, [bx + 2]
mov al, int_mode
push ds
mov ds, cx
mov ah, 25h
int 21h
pop ds
ret
set_breakpoint: mov ax, breakpoint
mov bx, offset fm_aligned
add bx, ax
mov al, byte ptr [bx]
mov dx, bx
mov bx, offset broken_byte
mov byte ptr [bx], al
mov bx, dx
mov byte ptr [bx], 0CCh
ret
victim_enter: pushf
push cs
push offset victim_ret
push 0
pushf
jnz bp_mode
pop ax
or ax, 100h
push ax
bp_mode: mov ax, cs
xor bx, bx
add bx, offset fm_aligned
mov byte ptr [bx], 0CFh
shr bx, 4
add ax, bx
mov es, ax
push ax
push 100h
push 0
push 0
push 0
push 0
push ax
mov ax, 100h
mov cl, int_mode
cmp cl, 1
jnz bp_mode2
call int_raport
bp_mode2: pop ds
add sp, 8
xor ax, ax
xor bx, bx
xor cx, cx
xor dx, dx
iret
victim_ret: push cs
pop ax
mov ds, ax
mov es, ax
mov dx, offset line_msg
mov ah, 9
int 21h
mov dx, offset debug_endmsg
int 21h
mov dx, offset line_msg
int 21h
call restore_int
int 20h
print_cmd: mov bx, 80h ; PSP:80 - command line length
xor cx, cx
mov cl, [bx]
or cx, cx
jz print_cmd_ret
dec cx
mov dx, 82h ; PSP:81 - command line itself
mov bx, 1 ; stdout handle
mov ah, 40h ; write to a file
int 21h
print_cmd_ret: ret
get_fname: mov bx, 80h
xor cx, cx
mov cl, [bx]
mov di, offset filename
or cx, cx
jz print_usage
dec cx
mov si, 82h
cld
get_fname_loop: mov al, byte ptr [si]
cmp al, ' '
jz get_fname_addr
movsb
loop get_fname_loop
xor ax, ax
stosb
jmp this_is_trapmode
get_fname_addr: xor ax, ax
stosb
xor bx, bx
dec cx
get_fname_lp2: inc si
mov bl, byte ptr [si]
sub bx, 30h
cmp bx, 9h
jle get_fname_sml
sub bx, 7h
get_fname_sml: shl ax, 4
or ax, bx
loop get_fname_lp2
mov bx, offset breakpoint
mov [bx], ax
ret
this_is_trapmode:
mov bx, offset int_mode
mov byte ptr [bx], 1
ret
open_file: mov dx, offset filename
mov ax, 3D00h ; open file in read mode
int 21h
jc open_file_err
ret
open_file_err: mov dx, offset fopen_errmsg
mov ah, 9
int 21h
int 20h
close_file: mov bx, ax
mov ah, 3Eh ; close file
int 21h
ret
get_fsize: mov bx, ax
mov ax, 4201h ; seek from current position
xor cx, cx
xor dx, dx
int 21h
mov di, dx
mov si, ax ; DI:SI <- original position
mov ax, 4202h ; seek from end of file
xor cx, cx
xor dx, dx
int 21h
push ax
mov ax, 4200h ; seek from start of file
mov cx, di
mov dx, si
int 21h ; restore original position
pop cx ; CX <- size of file
mov ax, bx
ret
read_file: push ax
mov bx, ax
mov ah, 3fh ; read file
mov dx, load_address
int 21h
jc read_file_err
pop ax
ret
read_file_err: mov dx, offset fread_errmsg
mov ah, 9
int 21h
int 20h
hex4: push ax
mov al, ah
call hex2
pop ax
hex2: push ax
shr al, 4
call hex1
pop ax
hex1: push ax
and al, 0Fh
cmp al, 10
sbb al, 69h
das
mov dl, al
mov ah, 2h
int 21h
pop ax
ret
print_usage: mov dx, offset usage_str
mov ah, 9
int 21h
int 20h
filename db 13 dup (?)
usage_str db "Usage: dbg <filename>[ <breakpoint>]"
db 13,10," If a breakpoint isn't specified,"
db 13,10," dbg will go to tracing mode. Use keys:"
db 13,10," [Enter] -> next point"
db 13,10," [q] -> quit",13,10
db "Written by Roman Tereshin (tereshin.roman@gmail.com)$"
fopen_errmsg db "Can not open file$"
alloc_errmsg db "Can not allocate memory$"
fread_errmsg db "Can not read file$"
line_msg db 13,10,"---------",13,10,'$'
ip_eq_msg db "ip = $"
segment_msg db 13,10,"ds = $"
ax_eq_msg db 13,10,"ax = $"
bx_eq_msg db 13,10,"bx = $"
cx_eq_msg db 13,10,"cx = $"
dx_eq_msg db 13,10,"dx = $"
debug_endmsg db "END$"
int_old dw 2 dup (?)
breakpoint dw 0
broken_byte db 0CFh
int_mode db 3
free_memory:
end start
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