1. Chapter 15 BIOS-Level Programming
Assembly Language for
Intel-Based Computers,
4th edition
Kip R. Irvine

2. Chapter Overview Review Chapter 13 Keyboard Input with INT 16h
VIDEO Programming with INT 10h
Drawing Graphics Using INT 10h
Memory-Mapped Graphics
Mouse Programming

3. Interrupts:
Hardware interrupt: in response to a request by a hardware device that needs attention. Hardware interrupts occur at "unexpected" times.
Software interrupt: A call to DOS or BIOS in response to a interrupt instruction (INT) in the program being processed.
Exception: An automatically generated trap in response to an exceptional condition such as division by zero.

4. Hardware Interrupt Steps
0. The program is executing in the CPU
1. The hardware device needs attention and requests an interrupt.
2. The CPU finishes processing the current instruction. It the saves its "state" (flags and CS:IP) on the stack.
3. The address of the interrupt handler is obtained from the Interrupt Vector Table.

5. Hardware Interrupt Steps
4. The CPU loads the address of the interrupt handler into the IP.
5. The interrupt handler code is processed.
6. When the interrupt handler is finished, the CPU pops the "state" (CS:IP and Flags) back from the stack.
7. Execution of original program continues beginning at the next instruction.

6. Hardware Interrupts 1 Device Program Interrupt table Interrupt Code
Stack 4 3 1 1 2 5 7 6

7. Hardware Interrupt Comments
Some interrupt operations are so critical that the interrupt driver may disable other interrupts until the critical operation is completed.
In IBM terminology, hardware interrupts are known as an IRQ. Each device is assigned a number which determines the Interrupt Vector entry of the appropriate interrupt handler.

8. Software Interrupt
Software interrupt. Works much the same way but steps 1 and 2 are replaced by an INT n instruction in the code. The number n determines the table entry specifying location of the interrupt handler desired.

9. Common Software Interrupts
INT 10h Video services
INT 16h Keyboard services
INT 21h DOS services - "DOS function calls" Most interrupts use AH to specify the desired operation (function)

10. INT 21h functions INT 21 functions Example Program
4Ch – terminate process
mov ah,4Ch
mov al,0
int 21h
02h – character output
advance cursor
mov ah,02h
mov dl,’A’
06h – character output
mov ah,06h
mov dl,’a’
09h – print $ terminated string
mov ah,09h
mov dx,OFFSET string
int 21h

11. INT 21h functions (continues)
max characters allowed (including <CR>)
number of characters input (excluding <CR>)
characters input …
INT 21 functions
Example Program
01h – read a char from stdin
mov ah,01h
int 21h
mov char,al
06h – character input
no waiting
mov ah,06h
mov dl,0FFh
0Ah – input string
mov ah,0Ah
mov dx,OFFSET kybdData
0Bh – status of input buffer
mov ah,0Bh
int 21h
(al = 0/0FFh)

12. Keyboard Input functions Summary
INT 1 6
0Ah
0Bh
Wait Y N
Filters
Echo -
Ctrl-Break
String

13. INT 21h functions (continues)
INT 21 functions
Example Program
3Fh – read from stdin or a disk file;
BX = file or device handle (keyboard = 0);
CX = number of bytes to read;
DS:DX = address of array
mov ah,3Fh
mov bx,0
mov cx,127
mov dx,OFFSET inBuffer
int 21h
mov bytesRead,ax
40h – Write to stdout or a disk file;
BX = file or device handle (console = 1);
CX = number of bytes to write;
mov ah,40h
mov bx,1
mov cx,LENGTHOF message
mov dx,OFFSET message
int 21h
mov bytesWritten,ax

14. INT 21h functions (continues)
INT 21 functions
Example Program
2Ah – Get system date
Year: 1980 – 2099
Month: 1 – 12
Day:
mov ah,2Ah
int 21h
mov year,cx
mov month,dh
mov day,dl
mov dayOfWeek,al
2Bh – Set system date
AL = 0 if successful; otherwise AL = 0FFh
mov ah,2Bh
mov cx,year
mov dh,month
mov dl,day
cmp al,0
jne failed

15. INT 21h functions (continues)
INT 21 functions
Example Program
2Ch – Get system time
Hour: 0 – 23
Minute: 0 – 59
Second:
mov ah,2Ch
int 21h
mov hours,ch
mov minutes,cl
mov seconds,dh
2Dh – Set system time
AL = 0 if successful; otherwise AL = 0FFh
mov ah,2Dh
mov ch,hours
mov cl,minutes
mov dh,seconds
cmp al,0
jne failed

16. INT 21h functions (continues)
INT 21 functions
Example Program
716Ch – open/create a file
Set CF if fail
Page 479
3Eh – close a file
Page 480
42h – move file pointer
Page 481
5706h – file creation data/time
62h – get program segment prefix
Page 486

17. BIOS I/O functions Advantages: Disadvantages Faster More control
"Understands" video monitors
Disadvantages
Cannot be redirected
Somewhat harder to use

18. Keyboard Input with INT 16h
How the Keyboard Works
INT 16h Functions
Set Typematic Rate (03h)
Push Key into Keyboard Buffer (05h)
Wait for Key (10h)
Check Keyboard Buffer (11h)
Get Keyboard Flags
Clearing the Keyboard Buffer

19. Keyboard input: ASCII and Scan Codes
ASCII code: one of the 127 characters in ASCII alphabet. Values from 1 to 127.
Many keys do not have an ASCII code. Examples: All Alt keys like Alt A, F1, Shift F2, Control F3, right arrow, Home, Insert, ...
Non-ASCII keys have an ASCII code of 00 or E0h.
BIOS functions provide both ASCII code and Scan code

20. Scan Codes: Examples F1 3B 00 Shift F1 54 00 ^ F1 5E 00 Alt F1 68 00
Character Scan ASCII A 1E a E ^A 1E Alt A 1E
F1 3B Shift F ^ F1 5E Alt F
Home 47 E  4D E0

21. How the Keyboard Works Input Port sc Keyboard sc sc, ac
Typeahead Buffer
INT 9h handler
sc, ac ac
INT 16h handler
INT 21h handler

22. INT 16h Functions Set Typematic Rate (03h)
Push Key into Keyboard Buffer (05h)
Wait for Key (10h)
Check Keyboard Buffer (11h)
Get Keyboard Flags
Clearing the Keyboard Buffer

23. Set Typematic Rate (03h) Set Typematic repeat rate AH = 3 AL = 5
BH = repeat delay
0 = 250ms
1 = 500ms
2 = 750ms
3 = 1000ms
BL = repeat rate
0 = fastest
1Fh = slowest
Mov AX, 0305h
Mov BH, 1
Mov BL, 0Fh
INT 16h

24. Push Key into Keyboard Buffer (05h)
Push key into typeahead buffer*
A key consists of ac and sc
AH = 5, CH = scan code, CL = ASCII code
mov AH, 5
mov CH, 3Bh ; scan code for F1 key
mov CL, 0 ; ASCII code
INT 16h
* If buffer is full, CF=1 and AL =1; otherwise CF = 0, AL = 0

25. Wait for Key 10h Remove a key from the buffer
If no key is in the buffer, wait for a key
AH = 10h
Return sc in AH, ac in AL
Every key has a scan code
Keys for non-ASCII characters have 00h or E0h as the ASCII code
Does not echo, can't be redirected.
mov AH, 10h
INT 16h
mov scanCode, AH
mov ASCIICode, AL

26. Sample Program
Get input keystrokes and display both the ASCII code and the scan code of each key. Terminates as Esc is pressed
TITLE Keyboard Display (keybd.asm)
Include Irvine16.inc
.code
main PROC
mov
mov ds,ax
call ClrScr ; clear screen
L1: mov ah,10h ; keyboard input
int 16h ; using BIOS
call DumpRegs ; look at AH, AL
cmp al,1Bh ; ESC key pressed?
jne L1 ; no: repeat the loop
exit
main ENDP
END main

27. Scan Codes: BIOS vs. DOS
BIOS: You get scan code and ASCII value every time! Special keys have a ASCII code of either 00h or E0h
DOS: You only get the scan code if the ASCII code is 00h. Codes come one at a time with the ASCII code first.

28. Processing scan codes in DOS28
Processing scan codes in DOS
mov ah, 01h ; Input char - DOS int 21h ; Input the ASCII code cmp al, 00h ; is it ASCII 0? jne processASCIIchar int 21h ; read scancode mov aScanCode, al

29. Processing scan codes in BIOS
mov ah, 10h ; Input char – BIOS
int 16h ; get scan and ASCII
cmp al, 0E0h ; is it an extended key?
je scankey
cmp al, 00h ; is it an ASCII key?
jne ASCIIKey
scankey: ; process scancode in ah

30. Check Keyboard Buffer 11h
Check if any keys are waiting
Returns the ac and sc of the next available key
Not remove the key from the buffer
mov ah, 11h
int 16h
jz NoKeyWaiting ;no key in buffer
mov scanCode, ah
mov ASCIICode, al

31. Get Keyboard Flags 12h Returns current state of the keyboard flags
Returns copy of the keyboard flags in AX
Keyboard flags are in the BIOS data area
mov ah, 12h
int 16h
mov keyFlags, ax

32. Get Keyboard Flags 12h Bit Description R Shift  8 L Ctr  1 L Shift 
R Shift  8
L Ctr  1
L Shift  9
L Alt  2
L/R Ctr  10
R Ctr  3
L/R Alt  11
R Alt  4
Scroll Lock on 12
Scroll key  5
Num Lock on 13
Num Lock  6
Caps Lock on 14
Caps Lock  7
Insert on 15
SysReq 

33. Three Levels of Video Level Compatibility Speed Redirection
DOS high slow yes
BIOS high medium no
Direct medium high no
DOS output is generic. There are "no" special video effects
BIOS video "understands" video and allows special effects
Direct video allows everything but do it yourself

34. Text Attributes A Color text
Background Foreground White character Black background A
0 Do not blink

35. Pages Most adapters and most modes allow multiple pages
BIOS allows writing to any of the pages even if it is not currently displayed
Example uses:
Write to hidden page and then display that page to create an "instantaneous" switch in the display
Save old screen data while doing a special page

36. Text Mode Colors
Code Color Code Color 0000 black gray 0001 blue light blue 0010 green light green 0011 cyan light cyan 0100 red light red 0101 magenta light magenta 0110 brown yellow 0111 light gray bright white

37. Function 0Fh: Get Video Mode Function 00h: Set Video Mode
mov ah, 0Fh ; Get video mode int 10h mov vmodeOld, al mov columnOld, ah
mov pageOld, bh mov ah, 00h ; Set video mode mov al, vmodeNew int 10h

38. Function 03h: Get Cursor Location Function 02h: Set Cursor Location
mov ah, 3 ; get cursor location
mov bh, vpage
int 10h
mov cursor, CX ; CH = starting scan line, CL = ending
mov position, DX ; DH = row, DL = col
mov ah, 2 ; set cursor location
mov dh, row
mov dl, col

39. Function 08h: Read character and attribute Function 09h: Write character and attribute
mov ah, 08h ; Read char. and attr mov bh, videoPage int 10h
mov aChar, al mov theAttribute, ah
mov ah, 09h ; Write char. and attr mov al, aChar mov bh, videoPage mov bl, theAttribute mov cx, repeatCount int 10h

40. Function 0Ah: Write character
Just like 09h except that the attribute is unchanged
mov ah, 0Ah ; Write char mov al, aChar mov bh, videoPage mov cx, repeatCount int 10h

41. Functions 06h (07h): Scroll up (down)
Specify a window by its corners
Specify the number of lines to scroll in window
Number lines = 0 means all lines
Row and columns numbering starts with 0
mov ah, 06h ; scroll window up mov al, numLines mov ch, topRow mov cl, leftColumn mov dh, bottomRow mov dl, rightColumn mov bh, theAttribute int 10h AB
Scrool up one line AB AB

42. Restore the screen
Recall that after changing the video mode, pages, or attributes we should restore the screen before terminating the program
mov ah, 0h ; Set video mode mov al, vmodeOld int 10h mov ah, 05h ; Set video page mov al, pageOld int 10h

43. Toggle Blinking and Intensity Modes
Set the highest bit of a color attribute to either control the color intensity or blink the character
AL = 3; BL: 0 = intesity, 1 = blinking
mov ah, 10h ; blinking/intensity mov al, 3
mov bl, 1 ; enable blinking int 10h
* Video display must run in a full screen mode

44. Function 0Eh: Teletype output
Print the character, move cursor to right, go to beginning of next line if needed
mov ah, 0Eh ; Write char and advance cursor mov al, aChar mov bh, videoPage int 10h

45. Function 13h: String Teletype output
Print the string at a given row and column
String may contain both characters and attribute values
.data
colorStr byte ‘A’, 1Fh, ‘B’, 1Ch, ‘C’, 1Bh
row byte 10
col byte 20
.code
mov ax, SEG colorStr
mov es, ax
mov ah, 13h ; Write string mov al, ; write mode mov bh, videoPage mov CX, (SIZEOF colorStr)/2
mov dh, row
mov dl, col
mov bp, OFFSET colorStr
int 10h

46. Function 0Ch: Write Graphics Pixel
Draw a pixel on the screen in graphics mode
Slow, most write directly into video memory
mov ah, 0Ch
mov al, pixelValue
mov bh, videoPage
mov CX, x_coord
mov DX, y_coord
int 10h

47. Function 0Dh: Read Graphics Pixel
Read a pixel from the screen at a given row and column
mov ah, 0Dh
mov bh, videoPage
mov CX, x_coord
mov DX, y_coord
int 10h
mov pixelValue, al

48. Mouse Programming INT 33h
Requires device driver program to be installed
Mouse movement are tracked in mickeys
1 mickey = 1/200 of an inch of mouse travel
8 mickeys = 1 horizontal pixel
16 mikeys = 1 vertical pixel

49. Reset Mouse and Get Status
AX = 0FFFFh; BX = number of mouse buttons if mouse support is available
AX = 0 if no mouse found
Centered on the screen, video page 0
Mouse pointer hidden
Internal counter is set to -1
mov ax, 0
int 33h
cmp ax, 0
je MouseNotAvailable

50. Showing/Hiding the Mouse Pointer
mov ax, 1 ;show mouse pointer
int 33h ;internal counter is incremented
mov ax, 2 ;hide mouse pointer
int 33h ;internal counter is decremented

51. Get Mouse Position and Status
mov ax, 3
int 33h
mov x_coord, CX ; x-coord in pixels
mov y_coord, DX
test bx, 1 ; test mouse button status
jnz Left_Button_Down
test bx, 2
jnz Right_Button_Down
test bx, 4
jnz Middle_Button_Down

52. Set Mouse Position
mov ax, 4
mov CX, x_coord
mov DX, y_coord
int 33h

53. Get Button Presses (Drag)
Return the status of all mouse buttons and the position of the last button press
BX: button ID – 0 = left, 1 = right, 2 = center
CX: X-coord of last button press
DX: Y-coord of last button press
mov ax, 5
mov bx, 0; button ID
int 33h
test ax, 1 ; left button?
jz skip
mov X_coord, CX
mov Y_coord, DX

54. Get Button Release (Click/End of Drag)
BX: button ID – 0 = left, 1 = right, 2 = center
CX: X-coord of last button release
DX: Y-coord of last button release
mov ax, 6
mov bx, 0; button ID
int 33h
test ax, 1 ; left button?
jz skip
mov X_coord, CX
mov Y_coord, DX

55. Setting Horizontal Limits
CX: Min X-coord (in pixels)
DX: Max X-coord
mov ax, 7
mov CX, Min_X
mov DX, Max_X
int 33h

56. Setting Vertical Limits
CX: Min Y-coord (in pixels)
DX: Max Y-coord
mov ax, 8
mov CX, Min_Y
mov DX, Max_Y
int 33h

57. Miscellaneous Mouse Functions
Description
I/O parameters
0Fh
Set Mickeys to 8 pixel ratio
In:CX = h_mickey, DX = v_mickey
10h
Set exclusive area
In: CX/DX = x/y_coord UL, SI/DI = x/y_coord BR
13h
Set double speed threshold
In: DX = threshold speed m/s
1Ah
Set mouse sensitivity
In: BX/CX = h/v_speed, DX = double speed threshold
1Bh
Get mouse sensitivity
Out: BX, CX, DX
1Fh
Disable mouse driver
Out: AX = 0FFFFh if fails
20h
Enable mouse driver
24h
Get mouse information
CH = mouse type, CL = IRQ number

58. CSCE 380
Department of Computer Science and Computer Engineering
Pacific Lutheran University
11/24/2002