1. Unit:08 Software Interrupts
Kip Irvine: Assembly Language for Intel-Based Computers

2. Kip Irvine: Assembly Language for Intel-Based Computers
Overview
INT Instruction
Redirecting Input/Output
MS-DOS (INT 21h) Function Calls
Output Functions
Input Functions
Date /Time Functions
BIOS Keyboard Input (INT 16h)
BIOS Video Control (INT 10h)
Recursion
Kip Irvine: Assembly Language for Intel-Based Computers

3. Kip Irvine: Assembly Language for Intel-Based Computers
Interrupts
Hardware interrupts
occur as a response to a hardware device
routed through the Intel 8259 Interrupt Controller
Software interrupts
calls to operating system functions, located in BIOS and resident portion of DOS
activated by the INT instruction
Kip Irvine: Assembly Language for Intel-Based Computers

4. Kip Irvine: Assembly Language for Intel-Based Computers
INT Instruction
The INT instruction is always followed by a hexadecimal number that identifies its type
Common examples:
INT 10h - video BIOS
INT 14h - Serial I/O
INT 16h - keyboard BIOS
INT 17h - printer services
INT 1Ah - Time of day
INT 1Ch - User timer
INT 21h - DOS services
Kip Irvine: Assembly Language for Intel-Based Computers

5. Interrupt Vectoring Process
CPU processes an interrupt instruction using the interrupt vector table, a table of addresses in the lowest 1,024 bytes of memory.
Each entry in the table is a 32-bit segment offset address that points to an OS subroutine.
The actual address in this table vary from one machine to another.
Fig. shows the steps taken by CPU when INT instruction is invoked by a program.
Kip Irvine: Assembly Language for Intel-Based Computers

6. Interrupt Vectoring Process
Kip Irvine: Assembly Language for Intel-Based Computers

7. Redirecting Input-Output
Standard Input device and standard output device are collectively called the console, which involves the keyboard for input and the video display for output.
From DOS command line, we can redirect standard input so it is read from a file or hardware port rather than a keyboard.
Standard output can be redirected so it writes to a file or hardware port rather than the video display.
Kip Irvine: Assembly Language for Intel-Based Computers

8. Kip Irvine: Assembly Language for Intel-Based Computers
Example
An executable program named prog1.exe can have its I/O redirected with the following command lines:
Prog1 > prn output to printer
Prog1 < infile.txt Input from file called infile.txt
Prog1 < infile.txt > prn input from file, output to printer
Kip Irvine: Assembly Language for Intel-Based Computers

9. Standard DOS device names
Description
CON
Console (video display or keyboard)
LPT1 or PRN
First parallel printer
LPT2, LPT3
Parallel port 2 & 3
COM1, COM2
Serial port 1 & 2
NUL
Nonexistent or dummy device
Kip Irvine: Assembly Language for Intel-Based Computers

10. DOS Function Calls (INT 21h)
The INT 21h instruction activates a DOS function call
The function number (0-255) is placed in the AH register before invoking INT 21h
Some functions require that you assign values to certain registers before invoking INT 21h
Some functions return values in registers
Kip Irvine: Assembly Language for Intel-Based Computers

11. Kip Irvine: Assembly Language for Intel-Based Computers
Simple Console I/O
mov ah, ; single character input
int 21h
mov ah, ; select Dos function 2
mov dl,'A‘ ; character to be displayed
int 21h ; call dos to do the job
mov ah, ; string output
mov dx,offset message
Kip Irvine: Assembly Language for Intel-Based Computers

12. Kip Irvine: Assembly Language for Intel-Based Computers
INT 21h: Standard Input
• 01h Filtered Input With Echo
• 06h Direct Input Without Waiting
• 07h Direct Input, No Ctrl-Break
• 08h Direct Input with Ctrl-Break
• 0Ah Buffered Input
• 0Bh Get Input Status
• 0Ch Clear Input Buffer, Invoke Input Function
• 3Fh Read From File or Device
Kip Irvine: Assembly Language for Intel-Based Computers

13. Comparison of Standard Input
Kip Irvine: Assembly Language for Intel-Based Computers

14. Keyboard Parameter Record (Function 0Ah)
Kip Irvine: Assembly Language for Intel-Based Computers

15. 3Fh: Read from File or Device
When the user presses Enter at the end of the input, two bytes (0Dh,0Ah) are appended to the string in the input buffer and the count in AX includes the extra characters.
buffer db 127 dup(0) .
mov ah,3Fh ; read from file/device
mov bx, ; device = keyboard
mov cx, ; request 127 bytes maximum
mov dx,offset buffer
int 21h ; AX = number chars typed + 2
Kip Irvine: Assembly Language for Intel-Based Computers

16. 40h: Write to File or Device
buffer db 127 dup(0)
count dw ? .
mov ah,40h ; read from file/device
mov bx, ; device = console
mov cx,count ; number of chars to write
mov dx,offset buffer
int 21h
Kip Irvine: Assembly Language for Intel-Based Computers

17. Date/Time Functions (Get Date 2Ah)
Function 2Ah returns the current system date, placing the year number in CX the month number in DH and the day of the week in AL.
The day number in AL uses the value of 0 for Sunday, 1 for Monday and so on
The following statements call the function and saves the value:
mov ah,2Ah
int 21h
mov year,cx
mov month,dh
mov day,dl
mov dayOfWeek,al
Kip Irvine: Assembly Language for Intel-Based Computers

18. Date/Time Functions (Set Date 2Bh)
Function 2Bh sets the current date, using the same registers as function 2Ah (get date)
The function returns the value 0 in AL if the change was successful.
Example
mov ah,2Bh
mov cx,year
mov dh,month
mov dl,day
int 21h
cmp al,0
jne badDate
Kip Irvine: Assembly Language for Intel-Based Computers

19. Date/Time Functions (Get Time 2Ch)
Function 2Ch returns the current system time, placing the hour in CH the minutes in CL and the seconds in DH and the hundredth of seconds in DL.
The following statements call the function and saves the value:
mov ah,2Ch
int 21h
mov hours,ch
mov minutes,cl
mov seconds,dh
Kip Irvine: Assembly Language for Intel-Based Computers

20. Date/Time Functions (Set Time 2Dh)
Function 2Dh sets the current time, using the same registers as function 2Ch (get time)
The function returns the value 0 in AL if the change was successful.
Example
mov ah,2Dh
mov ch,hours
mov cl,minutes
mov dh,seconds
int 21h
cmp al,0
jne badTime
Kip Irvine: Assembly Language for Intel-Based Computers

21. INT 16h BIOS Keyboard Input
A direct way to retrieve keyboard input is through the INT 16h keyboard services.
The function code is placed in AH before calling INT 16h.
The following example shows how to wait for a keystroke with INT 16h. Suppose F1 key were pressed .
Mov ah,10h ;requests BIOS keyboard input
Int 16h ;AH =3Bh, AL= 0
Kip Irvine: Assembly Language for Intel-Based Computers

22. INT 16h BIOS Keyboard Input
Kip Irvine: Assembly Language for Intel-Based Computers

23. Kip Irvine: Assembly Language for Intel-Based Computers
Keyboard Status Byte
Kip Irvine: Assembly Language for Intel-Based Computers

24. Keyboard Input Using INT 16h
Use INT 16h to input any key, including function keys, arrow keys, and other extended keys.
mov ah,10h ; wait for key
int 16h ; AH=scan code, AL=ASCII code
Kip Irvine: Assembly Language for Intel-Based Computers

25. Kip Irvine: Assembly Language for Intel-Based Computers
Keyboard Scan Codes
A keyboard scan code is a unique 8-bit binary number associated with a particular keyboard key.
A list of frequently used codes is inside the front cover of the book. Here are samples:
Home 47h
End 4Fh
PgUp 49h
PgDn 51h
F1 function key 3Bh
F2 function key 3Ch
F3 function key 3Dh
F4 function key 3Eh
Kip Irvine: Assembly Language for Intel-Based Computers

26. Common ASCII Control Characters
Kip Irvine: Assembly Language for Intel-Based Computers

27. Video Attribute Layout
(MSDOS mode only)
If your program is running in an MS-DOS window under Windows/NT, your background color is stored in bits 4-7 of the attribute bit, and blinking is disabled.
Kip Irvine: Assembly Language for Intel-Based Computers

28. 3-bit Background Colors
The following background colors are used only when running in full-screen mode or in pure MSDOS mode (by rebooting).
Kip Irvine: Assembly Language for Intel-Based Computers

29. 4-bit Foreground Colors
Kip Irvine: Assembly Language for Intel-Based Computers

30. 4-bit Background Colors
Kip Irvine: Assembly Language for Intel-Based Computers

31. Table 9. Listing of INT 10h Functions (1 of 2)
Kip Irvine: Assembly Language for Intel-Based Computers

32. Table 9. Listing of INT 10h Functions (2 of 2)
Kip Irvine: Assembly Language for Intel-Based Computers

33. INT 10h (09h) Write Character and Attribute
This function does not advance the cursor, so you have to do that separately
mov ah, ; write character and attribute
mov al,0Ah ; ASCII character 0Ah
mov bh, ; video page 0
mov bl,2 ; color (attribute) = green
mov cx, ; display it one time
int 10h
Kip Irvine: Assembly Language for Intel-Based Computers

34. Example: Write a Color String
string db "ABCDEFGHIJKLMOP"
count = ($-string)
color db 1 .
mov cx,count
mov si,offset string
L1:
push cx ; save loop counter
mov ah, ; write character and attribute
mov al,[si] ; character to display
mov bh, ; video page 0
mov bl,color ; get the color
mov cx, ; display it one time
int 10h
call AdvanceCursor
inc color ; next color
inc si ; next character position
pop cx ; restore loop counter
Loop L1
Kip Irvine: Assembly Language for Intel-Based Computers