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Assembly Source File | 1987-04-07 | 42.1 KB | 841 lines

;XDIR.COM for the IBM Personal Computer - 1986 by Jeff Prosise code segment para public 'code' assume cs:code org 100h begin: jmp initialize ;goto initialization code ; copyright db 'Copyright 1986 Ziff-Davis Publishing Co.',1Ah global db '\*.*',0 ;global directory filespec dos_segment dw ? ;DOS segment busy_flag dw ? ;offset of DOS BUSY_FLAG program_status db 0 ;XDIR processing status flag_13h db 0 ;status of interrupt 13h request_flag db 0 ;status of processing request adapter db 2 ;0 = MDA, 1 = CGA, 2 = EGA video_segment dw 0B800h ;video segment address video_page db ? ;current video page border_attr db 4Fh ;window border attribute text_attr db 0Fh ;window text attribute video_address dw ? ;window start address cursor_mode dw ? ;cursor shape cursor_pos dw ? ;cursor position maxlen db ? ;maximum input string length max_page db ? ;number of highest directory page dir_page db ? ;current directory page end_flag db ? ;status of text write routines error_flag db ? ;critical error status default_cursor dw 0607h ;default cursor shape (color) addr_6845 dw ? ;CRT Controller base address search_attr dw 0 ;file search attribute ; path dw 0 ;pointer to pathname buffer dta dw 64 ;pointer to Disk Transfer Area screen_buffer dw offset initialize ;pointer to screen buffer text_buffer dw offset initialize+1536 ;pointer to filename buffer ; keyboard_int label dword ;old interrupt 9 vector old9h dw 2 dup (?) timer_int label dword ;old interrupt 1Ch vector old1ch dw 2 dup (?) bdisk_int label dword ;old interrupt 13h vector old13h dw 2 dup (?) bp_int label dword ;old interrupt 28h vector old28h dw 2 dup (?) ; old_dta_segment dw ? ;old DTA segment address old_dta_offset dw ? ;old DTA offset address old24h_segment dw ? ;old interrupt 24h routine segment old24h_offset dw ? ;old interrupt 24h routine offset enable_values db 2Ch,28h,2Dh,29h ;values to enable CGA output db 2Ah,2Eh,1Eh errtext db 'No Files Found',0 ; ;------------------------------------------------------------------------------ ;Execution comes here thru interrupt 9 every time a key is pressed or released. ;------------------------------------------------------------------------------ keyboard proc near sti ;set interrupt enable flag push ax ;save AX in al,60h ;get scan code from keyboard cmp al,52 ;was the '.' key pressed? jne kb2 ;no, then exit to normal handler mov ah,2 ;check shift key status int 16h test al,8 ;is the Alt key pressed? je kb2 ;no, then exit call kb_reset ;reset keyboard, issue EOI pop ax ;restore AX cmp program_status,0 ;XDIR routine already active? jne kb1 ;yes, then don't set request flag mov request_flag,1 ;set request flag kb1: iret ;end interrupt routine kb2: pop ax ;restore AX jmp keyboard_int ;goto original keyboard routine keyboard endp ; ;------------------------------------------------------------------------------ ;Interrupt 1Ch handling routine. ;This procedure will now be used to handle int 8 instead of int 1c ;------------------------------------------------------------------------------ timer proc near pushf ;call original routine call timer_int cmp request_flag,0 ;request flag set? je timer1 ;no, then exit push es ;save ES and DI push di mov es,dos_segment ;get DOS segment in ES mov di,busy_flag ;address of DOS BUSY_FLAG in DI cmp byte ptr es:[di],0 ;DOS service currently active? pop di ;clean up the stack pop es jne timer1 ;yes, then we must wait cmp flag_13h,0 ;BIOS disk service active? jne timer1 ;yes, then don't interrupt it test_8259_status: push AX ;Save altered registers push DX mov DX,20h ;Port address of 8259 OCW3 mov AL,03 ;Set the In-Service Reg. read ;bits RR=1 and RIS=1 out DX,AL ;Tell 8259 to send ISR on next read jmp short get_mask ;This causes a 7 clock delay period ;on the AT to allow time for the 8259 ;to setup the ISR status for us get_mask: in AL,DX ;Get the ISR or AL,AL ;Set zero flag if no interrupts are ;being serviced pop DX ;Restore the altered registers pop AX jnz timer1 ;A hard interrupt is in progress mov request_flag,0 ;reset request flag call directory ;invoke directory routine timer1: iret ;done - exit timer endp ;------------------------------------------------------------------------------ ;------------------------------------------------------------------------------ ;------------------------------------------------------------------------------ ;Interrupt 13h handling routine. ;------------------------------------------------------------------------------ bdisk proc far inc flag_13h ;Set the busy flag pushf ;Simulate an interrupt call to the call bdisk_int ;original disk interrupt routine pushf ;Save returned flags from disk interrupt dec flag_13h ;Clear the busy flag popf ;Restore disk I/O status flags ret 2 ;Far return and discard flags on the stack bdisk endp ; ;------------------------------------------------------------------------------ ;Interrupt 28h handling routine. ;------------------------------------------------------------------------------ backproc proc near pushf ;call original routine call bp_int cmp request_flag,0 ;request flag clear? je bp1 ;yes, then exit mov request_flag,0 ;clear request flag call directory ;execute directory routine bp1: iret ;done - exit backproc endp ; ;------------------------------------------------------------------------------ ;Interrupt 24h handling routine. ;------------------------------------------------------------------------------ ioerr proc near sti ;restore interrupts mov error_flag,1 ;set external error flag mov al,0 ;tell DOS to ignore the error iret ;give control back to DOS ioerr endp ; ;------------------------------------------------------------------------------ ;DIRECTORY is called by other routines to pop up and control the window. ;------------------------------------------------------------------------------ directory proc near mov program_status,1 ;set program active flag sti ;enable interrupts push ax ;save registers push bx push cx push dx push si push di push ds push es push cs ;set DS to the code segment pop ds assume ds:code push cs ;set ES to the code segment pop es ; ;Make sure the current video mode is a text mode. ; mov ah,15 ;get video mode and page int 10h cmp al,2 ;mode 2? je dir1 ;yes, then continue cmp al,3 ;mode 3? je dir1 ;yes, then continue cmp al,7 ;mode 7? je dir1 ;yes, then continue exit: mov program_status,0 ;clear status flag pop es ;restore registers and exit pop ds pop di pop si pop dx pop cx pop bx pop ax ret ; ;Save video parameters that must be used now or restored later. ; dir1: mov video_page,bh ;save current video page mov ah,3 ;get cursor mode int 10h mov cursor_mode,cx ;save it call cursor_address ;get cursor address from CRTC mov cursor_pos,ax ;save it cld ;clear DF for string operations ; ;Save the screen contents and open the directory window. ; cmp adapter,1 ;disable video if CGA installed jne dir2 call disable_cga dir2: call save_screen ;save memory to be overwritten call open_window ;draw window to the display cmp adapter,1 ;re-enable CGA video jne dir3 call enable_cga ; ;Set the DTA and interrupt 24h vector to areas inside XDIR. Then get a ;directory path string from the keyboard. ; dir3: call ioset ;set DTA and 24h vector dir4: mov di,path ;point DI to directory path buffer mov dx,020Ah ;specify input line location mov cl,59 ;specify max length of 59 call readln ;get path string from keyboard cmp al,27 ;was ESC pressed? jne dir5 ;no, then continue jmp escape ;ESC was pressed - exit dir5: push cx ;save character count mov ah,1 ;hide the cursor mov ch,20h int 10h pop cx ;retrieve count mov dx,offset global+1 ;point DX to '*.*' text or cl,cl ;any characters entered? je dir7 ;yes, then skip ahead cmp byte ptr [di-1],'\' ;is last character a backslash? jne dir6 ;no, then append backslash to path dec di ;decrement path string pointer dir6: lea si,global ;append '\*.*' text to path mov cx,5 rep movsb mov dx,path ;point DX to path string ; ;Read filename information from the specified directory. ; dir7: call get_dir ;read directory information mov dir_page,0 ;initialize page number or cx,cx ;any files found? jne dir9 ;yes, then continue ; ;No files were found - print message and return to input loop. ; mov dx,0721h ;set cursor position lea si,errtext ;point SI to error message call write_string ;write error message call getkey ;wait for a keypress call clear_input_line ;clear input line call clear_window ;then clear the window jmp dir4 ;loop back for another try ; ;One or more files were found. Display them and look for keystrokes. ; dir9: call write_dir ;write one directory page dir10: call getkey ;wait for a keypress cmp al,0 ;extended code entered? je dir11 ;yes, then branch to handler cmp al,27 ;ESC pressed? jne dir10 ;no, then ignore keypress call clear_input_line ;clear window and loop back call clear_window jmp dir4 dir11: cmp ah,81 ;PgDn pressed? jne dir12 ;no, then continue testing mov al,dir_page ;last page displayed? cmp al,max_page je dir10 ;yes, then ignore keypress inc dir_page ;advance page number call clear_window ;clear window jmp dir9 ;go back and display new page dir12: cmp ah,73 ;PgUp pressed? jne dir10 ;no, then ignore keypress cmp dir_page,0 ;first page displayed? je dir10 ;yes, then ignore keypress dec dir_page ;update page indicator call clear_window ;clear window jmp dir9 ;loop back ; ;Reset the 24h vector and DTA address, close the window, and exit. ; escape: call ioreset ;restore DTA and 24h vector cmp adapter,1 ;disable CGA video jne esc1 call disable_cga esc1: call restore_screen ;restore screen contents cmp adapter,1 ;re-enable CGA video jne esc2 call enable_cga esc2: mov ah,2 ;set cursor position mov bh,video_page mov dx,cursor_pos int 10h mov ah,1 ;then unblank the cursor mov cx,cursor_mode int 10h jmp exit ;exit directory endp ; ;------------------------------------------------------------------------------ ;SAVE_SCREEN saves the contents of the screen that underlie the window. ;------------------------------------------------------------------------------ save_screen proc near mov dx,0208h ;first window row and column mov bl,video_page ;retrieve active video page xor bh,bh ;byte to word in BX call video_offset ;determine video memory offset mov video_address,di ;save offset address mov si,di ;transfer it to SI push ds ;save DS mov ds,video_segment ;then set it to the video segment assume ds:nothing mov di,screen_buffer ;point DI to storage buffer mov cx,12 ;12 lines to save save1: push cx ;save line count mov cx,64 ;64 characters per line rep movsw ;transfer one line to storage pop cx ;retrieve line count add si,32 ;point SI to next video line loop save1 ;loop until all lines are saved pop ds ;restore DS assume ds:code ret ;exit save_screen endp ; ;------------------------------------------------------------------------------ ;RESTORE_SCREEN restores the saved contents of video memory. ;------------------------------------------------------------------------------ restore_screen proc near push es ;save ES register value mov di,video_address ;point DI to starting video offset mov es,video_segment ;point ES to video memory mov si,screen_buffer ;point SI to storage buffer mov cx,12 ;12 lines to restore restore1: push cx ;save line count mov cx,64 ;64 characters per line rep movsw ;restore one line pop cx ;retrieve line count add di,32 ;set DI to next video line loop restore1 ;loop until done pop es ;restore ES ret restore_screen endp ; ;------------------------------------------------------------------------------ ;VIDEO_OFFSET calculates the offset address in video memory that corresponds ;to the indicated row, column, and video page. ;Entry: DH,DL - row, column | Exit: DI - offset ; BX - video page | ;------------------------------------------------------------------------------ video_offset proc near mov al,160 ;row * 160 mul dh shl dl,1 ;column * 2 xor dh,dh ;byte to word in DX add ax,dx ;add the results mov di,ax ;save result in DI mov ax,1000h ;length of one video page mul bx ;page * 1000h add di,ax ;add result to DI ret video_offset endp ; ;------------------------------------------------------------------------------ ;DISABLE_CGA and ENABLE_CGA disable and enable CGA video output. ;------------------------------------------------------------------------------ disable_cga proc near mov dx,3DAh ;address of Status Register disable1: in al,dx ;get status test al,8 ;vertical retrace active? je disable1 ;no, then wait sub dx,2 ;MSR address in DX mov al,25h ;value to disable video out dx,al ;disable video output ret disable_cga endp ; enable_cga proc near mov ah,15 ;get video mode int 10h lea bx,enable_values ;get value to enable display xlat ;value in AL mov dx,3D8h ;MSR address out dx,al ;enable video output ret enable_cga endp ; ;------------------------------------------------------------------------------ ;KB_RESET resets the keyboard and issues an EOI to the 8259 PIC. ;------------------------------------------------------------------------------ kb_reset proc near in al,61h ;get current control value mov ah,al ;save it in AH or al,80h ;set the high bit out 61h,al ;send it to the control port mov al,ah ;recover original value out 61h,al ;send it out cli ;suspend interrupts mov al,20h ;load EOI value out 20h,al ;send it to the 8259 sti ;restore interrupts ret kb_reset endp ; ;------------------------------------------------------------------------------ ;SHOW_CURSOR sets the cursor to its default state. ;------------------------------------------------------------------------------ show_cursor proc near mov ah,1 ;interrupt 10h service 1 mov cx,default_cursor ;set scan line definition int 10h ;set cursor shape ret show_cursor endp ; ;------------------------------------------------------------------------------ ;CURSOR_ADDRESS reads the current cursor position from the video controller. ;------------------------------------------------------------------------------ cursor_address proc near mov dx,addr_6845 ;get CRTC Address Register port mov al,14 ;OUT register number out dx,al inc dx ;point DX to Data Register in al,dx ;read high byte of cursor address mov ah,al ;save it in AH dec dx ;point DX back to Address Register mov al,15 ;OUT next register number out dx,al inc dx ;point DX to Data Register in al,dx ;read low byte of address and ax,07FFh ;strip 'page' bits from address mov bl,80 ;then divide by 80 div bl xchg ah,al ;reverse bytes for proper form ret cursor_address endp ; ;------------------------------------------------------------------------------ ;GETKEY waits for a keypress and returns the keycode in AX. ;Exit: AX - keycode ;------------------------------------------------------------------------------ getkey proc near mov ah,1 ;check keyboard buffer int 16h jne getkey1 ;jump if buffer contains a keycode int 28h ;no key pressed - issue int 28h jmp getkey ;loop back to try again getkey1: mov ah,0 ;get keycode from buffer int 16h ret ;exit with keycode in AX getkey endp ; ;------------------------------------------------------------------------------ ;OPEN_WINDOW writes the blank directory window to display memory. ;------------------------------------------------------------------------------ open_window proc near push es ;save ES mov es,video_segment ;set ES:DI to video memory mov di,video_address mov al,218 ;write first character mov ah,border_attr stosw mov cx,62 ;then do the next 62 mov al,32 rep stosw mov al,191 ;finish the first line stosw add di,32 ;set DI to start of next line mov cx,10 ;10 lines to do open1: push cx ;save line counter mov al,179 ;write first character on line mov ah,border_attr push ax ;save character/attribute pair stosw ;write them to video memory mov cx,62 ;write next 62 characters mov al,32 mov ah,text_attr rep stosw pop ax ;retrieve saved word for final char stosw ;finish the line add di,32 ;set DI to start of next line pop cx ;retrieve line counter loop open1 ;loop until 10 lines are done mov al,192 ;first character of last line stosw mov cx,62 ;write the next 62 characters mov al,196 rep stosw mov al,217 ;finish the last line stosw pop es ;restore ES ret open_window endp ; ;------------------------------------------------------------------------------ ;READLN accepts input of a string entered from the keyboard. ;Entry: ES:DI - buffer address | Exit: CL - string length ; DH,DL - cursor start position | ; CL - max length accepted | ;------------------------------------------------------------------------------ readln proc near mov maxlen,cl ;save max length mov ah,2 ;set cursor to start position mov bh,video_page int 10h call show_cursor ;make sure cursor is visible xor cl,cl ;initialize counter read1: call getkey ;get a character cmp al,13 ;ENTER key? je read_exit ;yes, then exit cmp al,27 ;ESC key? je read_exit ;yes, then exit cmp al,8 ;backspace key? je backspace ;yes, then do backspace function cmp al,32 ;ASCII 32 or greater? jb read1 ;no, then ignore it cmp cl,maxlen ;room for another entry? je read1 ;no, then ignore it push ax ;save character just entered push cx ;save character count mov ah,10 ;print the character mov cx,1 int 10h inc dl ;advance the cursor mov ah,2 int 10h pop cx ;retrieve count pop ax ;retrieve character stosb ;deposit entry in buffer inc cl ;update count jmp read1 ;go back for more backspace: or cl,cl ;any characters to delete? je read1 ;no, then ignore keystroke push cx ;save count dec dl ;move cursor back one space mov ah,2 int 10h mov ah,10 ;print a space character mov al,32 mov cx,1 int 10h pop cx ;retrieve count dec cl ;decrement it dec di ;decrement buffer pointer jmp read1 ;go back for more read_exit: ret ;exit readln endp ; ;------------------------------------------------------------------------------ ;IOSET saves the current DTA address and interrupt 24h vector, then replaces ;them with pointers to XDIR routines. IORESET restores the original values. ;------------------------------------------------------------------------------ ioset proc near push es ;save ES mov ah,2Fh ;get current DTA address int 21h mov old_dta_segment,es ;save it mov old_dta_offset,bx mov ah,1Ah ;set new DTA address mov dx,dta int 21h mov ah,35h ;get interrupt 24h vector mov al,24h int 21h mov old24h_segment,es ;save it mov old24h_offset,bx mov ah,25h ;then set it to IOERR routine lea dx,ioerr int 21h pop es ;restore ES ret ioset endp ; ioreset proc near mov ah,25h ;restore interrupt 24h vector mov al,24h mov dx,old24h_offset push ds assume ds:nothing mov ds,old24h_segment int 21h mov ah,1Ah ;restore original DTA address mov dx,old_dta_offset mov ds,old_dta_segment int 21h pop ds assume ds:code ret ioreset endp ; ;------------------------------------------------------------------------------ ;GET_DIR reads the specified directory and stores the ASCIIZ filename text. ;Entry: DS:DX - pathname | Exit: CX - number of files ;------------------------------------------------------------------------------ get_dir proc near mov error_flag,0 ;initialize critical error flag mov ah,4Eh ;find first filename mov cx,search_attr ;set search attribute int 21h ;initiate file search mov cx,0 ;zero CX in case no files found jc getdir4 ;done if no files found cmp error_flag,0 ;critical error flag clear? jne getdir4 ;no, then exit immediately inc cx ;initialize file count mov di,text_buffer ;set buffer address call copy_filename ;copy filename to buffer getdir1: mov ah,4Fh ;continue file search int 21h jc getdir2 ;done if nothing found cmp error_flag,0 ;did a critical error occur? jne getdir2 ;yes, then exit call copy_filename ;copy next filename inc cx ;update counter cmp cx,360 ;buffer full? jne getdir1 ;no, go back for more getdir2: sub di,13 ;go back to start of last filename getdir3: inc di ;point DI to next byte cmp byte ptr es:[di],0 ;is this a zero byte? jne getdir3 ;no, then advance to next byte dec byte ptr es:[di] ;mark end of text with a 255 mov ax,cx ;calculate number of pages dec ax mov bl,40 div bl mov max_page,al ;save highest page number getdir4: ret get_dir endp ; ;------------------------------------------------------------------------------ ;COPY_FILENAME copies an ASCIIZ filename from the DTA to the indicated address. ;Entry: ES:DI - destination address ;------------------------------------------------------------------------------ copy_filename proc near mov si,dta ;get DTA address add si,30 ;point SI to start of filename push cx ;save CX mov cx,13 ;filename length is 13 bytes rep movsb ;copy filename to storage pop cx ;restore entry value of CX ret copy_filename endp ; ;------------------------------------------------------------------------------ ;WRITE_DIR writes one page of directory data to the directory window. ;------------------------------------------------------------------------------ write_dir proc near mov end_flag,0 ;initialize END_FLAG mov ax,520 ;520 bytes per directory page mov bl,dir_page ;get page number in BL xor bh,bh ;byte to word in BX mul bx ;find offset into TEXT_BUFFER mov si,ax ;transfer offset to SI add si,text_buffer ;complete address calculation mov dx,030Ah ;specify starting cursor position mov cx,10 ;do 10 lines wdir4: push cx ;save line counter call write_line ;write one line inc dh ;set cursor to next line mov dl,10 pop cx ;restore line count cmp end_flag,0 ;END_FLAG set? jne wdir5 ;yes, then terminate loop wdir4 ;loop until all lines are done wdir5: ret write_dir endp ; ;------------------------------------------------------------------------------ ;WRITE_LINE writes a single line of directory text to the display. ;Entry: DH,DL - starting row and column ;------------------------------------------------------------------------------ write_line proc near mov cx,4 ;4 entries per line wline1: push cx ;save counter push si ;save text address push dx ;save cursor address call write_string ;write one entry pop dx ;retrieve cursor address add dl,16 ;set cursor to next field pop si ;retrieve text address add si,13 ;set it for next write pop cx ;retrieve count cmp end_flag,0 ;END_FLAG set? jne wline2 ;yes, then exit loop wline1 ;loop until done wline2: ret write_line endp ; ;------------------------------------------------------------------------------ ;WRITE_STRING writes an ASCIIZ string to the display. ;Entry: DH,DL - starting row and column ; DS:SI - string address ;------------------------------------------------------------------------------ write_string proc near mov ah,2 ;set cursor to start position mov bh,video_page int 10h mov cx,1 ;output one character at a time write1: lodsb ;get a character or al,al ;is it a zero? je write3 ;yes, then exit cmp al,255 ;is it 255? je write2 ;yes, then set END_FLAG and exit mov ah,10 ;print character int 10h mov ah,2 ;advance cursor inc dl int 10h jmp write1 ;loop until done write2: mov end_flag,1 ;set END_FLAG write3: ret write_string endp ; ;------------------------------------------------------------------------------ ;CLEAR_WINDOW clears the contents of the directory window. ;------------------------------------------------------------------------------ clear_window proc near mov ah,6 ;clear window with BIOS routine mov al,0 ;specify clear function mov cx,030Ah ;specify window coordinates mov dx,0C45h mov bh,text_attr ;attribute to be used int 10h ret clear_window endp ; ;------------------------------------------------------------------------------ ;CLEAR_INPUT_LINE clears the window input line. ;------------------------------------------------------------------------------ clear_input_line proc near mov ah,2 ;set cursor to start of input line mov dx,020Ah mov bh,video_page int 10h mov ah,10 ;then write a string of spaces mov al,32 mov cx,60 int 10h ret clear_input_line endp ; ;------------------------------------------------------------------------------ ;INITIALIZE prepares the program for residency. ;------------------------------------------------------------------------------ initialize proc near ; ;Determine what type of video adapter is installed. ; mov ah,12h ;prepare for call to int 10h mov bl,10h ;function - request EGA info int 10h cmp bl,10h ;BL=10h? je init1 ;yes, then no EGA installed or bh,bh ;BH=0? jne init2 ;no, then it's a monochrome system jmp init3 init1: dec adapter ;decrement ADAPTER value mov ah,15 ;get video mode int 10h cmp al,7 ;is it mode 7? jne init3 ;no, then it's a color system dec adapter ;set ADAPTER to 0 ; ;Modify video parameter values for monochrome, then initialize the cursor. ; init2: sub video_segment,800h ;set VIDEO_SEGMENT for monochrome mov border_attr,70h ;change attributes for monochrome mov text_attr,07h mov default_cursor,0C0Dh ;set monochrome cursor definition init3: call show_cursor ;set cursor to default mode ; ;Determine the port address of the CRT Controller and store it. ; mov ax,40h ;point ES to BIOS data segment mov es,ax mov di,63h ;point DI to address word mov dx,es:[di] ;get CRTC address mov addr_6845,dx ;save it ; ;Get and save the address of the DOS BUSY_FLAG. ; mov ah,34h ;function 34h int 21h ;get address mov dos_segment,es ;save segment mov busy_flag,bx ;save offset ; ;Save and replace all required interrupt vectors. ; mov ah,35h ;get interrupt 9 vector mov al,9 int 21h mov old9h,bx ;save it mov old9h[2],es mov ah,25h ;point it to KEYBOARD routine lea dx,keyboard int 21h mov ah,35h ;get interrupt 1Ch vector mov al,8 ;patched vh int 21h mov old1ch,bx ;save it mov old1ch[2],es mov ah,25h ;then point it to TIMER lea dx,timer int 21h mov ah,35h ;get interrupt 13h vector mov al,13h int 21h mov old13h,bx ;save it mov old13h[2],es mov ah,25h ;point it to BDISK lea dx,bdisk int 21h mov ah,35h ;get interrupt 28h vector mov al,28h int 21h mov old28h,bx ;save it mov old28h[2],es mov ah,25h ;point it to BACKPROC lea dx,backproc int 21h ; ;Terminate but remain resident in memory. ; mov dx,offset initialize+6216 ;point DX to end of resident code int 27h ;terminate-but-stay-resident initialize endp ; code ends end begin