page 66,132
;============================================================================
; CD_CHECK.COM
code segment
assume cs:code
org 100h
main: jmp initialize
int21h dd -1 ;Int 21 vector (DOS)
;============================================================================
; DOSINT processes calls to interrupt 21h
;============================================================================
dosint proc far
assume cs:code,ds:nothing,es:nothing
cmp ah,4eh ;Check for find first
jne goto_dos ;No, nevermind
push di ;Save DI
mov di,dx ;copy DX pointer to index reg
cmp byte ptr [di],'@' ;is it our 'fake' drive?
jne goto_dos_2 ;No, nevermind
cmp byte ptr [di+1],':' ;is it our 'fake' drive
jne goto_dos_2 ;No, nevermind
push es ;Yes, Save ES
push ds ;Point ES to DS for scasb
pop es
push si ;Save SI
push cx ;Save CX
push bx ;Save BX
mov cx,64
cld
mov al,0 ;find the length
repne scasb ;di points past the 0
mov si,di ;Save pointer
mov cx,si ;calculate path length
sub cx,dx ; as SI-DX
jcxz exit ;Abort if length 0
std ;Scan backwords
mov al,'\' ;find the last '\'
repne scasb ;
je got_it ; Found one
mov di,dx ;otherwise,point to drive letter
got_it: inc di ;back up the pointer
inc di ; to the actual filename
mov cx,si ;calculate the length
sub cx,di ; as SI-DI
jcxz exit ;Abort if length 0
push cx ;Save Filename length
push di ;Save Filename Pointer
mov ah,2fh ;Get DTA address [ES:BX]
pushf
call cs:[int21h] ;call the old INT 21
mov di,bx ;Copy DTA to DI
cld ;Copy forward
mov cx,1eh ;fill the first 30 bytes
xor al,al ; with zeros
rep stosb ;Stuff 0's, point to filename
pop si ;Restore Filename Pointer
pop cx ;Restore Filename length
loop1: rep movsb ;(ES:DI) < (DS:SI)
xor ax,ax ;Yes, say the file exists
pop bx ;Restore BX
pop cx ;Restore CX
pop si ;Restore SI
pop es ;Restore ES
pop di ;Restore DI
iret ;Return
exit: pop bx ;Restore BX
pop cx ;Restore CX
pop si ;Restore SI
pop es ;Restore ES
goto_dos_2: pop di ;Restore DI
goto_dos: jmp cs:[int21h] ;else pass the call to DOS
dosint endp
;----------------------------------------------------------------------------
; Initialization routine.
;----------------------------------------------------------------------------
initialize proc near
assume cs:code,ds:code,es:code
mov ax,3521h ;Get interrupt 21 (DOS)
int 21h ; vector.
mov word ptr [int21h],bx
mov word ptr [int21h+2],es
mov ax,2521h ;Point int 21 to internal
mov dx,offset dosint ; routine.
int 21h
mov ax,ds:[2ch] ;Get environment segment.
mov es,ax
mov ah,49h ;Free up environment.
int 21h
tsr: mov dx,OFFSET initialize
mov ax,3100h ;Terminate and stay resident
int 27h
initialize endp
code ends
end main
