Assembly Code Examples Disk Watch Disk Watch interrupts segment at 0h ; This is where the disk interrupt org 13h*4 ; holds the address of its service routine disk_int label dword interrupts ends screen segment at 0B000h ; A dummy segment to use as the Extra screen ends ; Segment so we can write to the display code_seg segment assume cs:code_seg org 0100h ; ORG = 100h to make this a .COM file first: jmp load_watch ; First time through jump to initialize routine msg_part_1 db 'Disk error: ' ; Here are the error messages msg_part_2 db 'No response Failed Seek NEC Error ' db 'Bad CRC SeenDMA Overrun Impos Sector' db 'No Addr MarkW. ProtectedErr Unknown ' first_position dw ? ; Position of 1st char on screen flags dw ? screen_seg_offset dw 0 ; 0 for mono, 8000h for graphics old_disk_int dd ? ; Location of old disk interrupt ret_addr label dword ; Used in fooling around with ret_addr_word dw 2 dup(?) ; the stack disk_watch proc far ; The disk interrupt will now come here assume cs:code_seg pushf ; First, call old disk interrupt call old_disk_int pushf ; Save the flags in memory location "FLAGS" pop flags ; (cunning name) jc error ; If there was an error, carry flag will have jmp fin ; been set by Disk Interrupt error: push ax ; AH has the status of the error push cx ; Push all used registers for politeness push dx push di push si push es lea si,msg_part_1 ; Always print "Disk Error: " part. assume es:screen ; Use screen as extra segment mov dx,screen mov es,dx mov di,screen_seg_offset ; DI will be pointer to screen position add di,first_position ; Add to point to desired area on screen call write_to_screen ; This writes 12 chars from [SI] to [DI] mov dx,80h ; Initialize for later comparisons mov cx,7 ; Loop seven times e_loop: cmp ah,dh ; Are error code and DH the same? je e_found ; If yes, Error has been found add si,12 ; Point to next error message shr dh,1 ; Divide DH by 2 loop e_loop ; Keep going until matched DH = 0 cmp ah,3 ; Error code no even number; 3 perhaps? je e_found ; If yes, have found the error add si,12 ; Err unknown; unknown error returned e_found:call write_to_screen ; Write the error message to the screen pop es ; Restore the registers pop si pop di pop dx pop cx pop ax fin: pop ret_addr_word ; Fooling with the stack. We want to pop ret_addr_word ; preserve the flags but the old flags add sp,2 ; are still on the stack. First remove push flags ; return address, then flags. Fill flags popf ; from "FLAGS", return to correct addr. jmp ret_addr disk_watch endp write_to_screen proc near ; Puts 12 characters on screen mov cx,12 ; Loop 12 times w_loop: movs es:byte ptr[di],cs:[si] ; Move to the screen mov al,7 ; Move screen attribute into screen buffer mov es:[di],al inc di ; Point to next byte in screen buffer loop w_loop ; Keep going until done ret write_to_screen endp load_watch proc near ; This procedure initializes everything assume ds:interrupts ; The data segment will be the interrupt area mov ax,interrupts mov ds,ax mov ax,disk_int ; Get the old interrupt service routine mov old_disk_int,ax ; address and put it into our location mov ax,disk_int ; OLD_DISK_INT so we can call it. mov old_disk_int,ax mov disk_int,offset disk_watch ; Now load the address of DskWatch mov disk_int,cs ; routine into the disk interrupt mov ah,15 ; Ask for service 15 of INT 10h int 10h ; This tells us how display is set up sub ah,25 ; Move to twenty five places before edge shl ah,1 ; Mult. by two (char & attribute bytes) mov byte ptr first_position,ah ; Set screen cursor test al,4 ; Is it a monochrome display? jnz exit ; Yes - jump out mov screen_seg_offset,8000h ; No, set up for graphics display exit: mov dx,offset load_watch ; Set up everything but this program to int 27h ; stay and attach itself to DOS load_watch endp code_seg ends end first ; END "FIRST" so 8088 will go to FIRST first.