CS2422 Assembly Language

1
CS2422 Assembly Language System Programming

• November 2, 2006

2
Todays Topics

• INT (Interrupt).
• How Does a PC Boot?

3
Study Guide

• Interrupt Sections 13.1.3 and 13.1.4.
• (Advanced) Interrupt Handler Section 16.4.
• Boot Process http//www.pcguide.com/ref/mbsys/bio
  s/boot.htm

4
Interrupt

• If you call your friend and his/her mom says
  he/she is not home, what do you do?
• Do something else, and get interrupted when I/O
  events happen.
• May be triggered by hardware or software.
• INT is also used for system calls (low-level
  procedures).

5

• Note that we will use the 16-bit mode in the
  following slides.

6
Examples of INT I/O

• Put parameters in registers
• Reference chapter 8
• Example INT 10
• Function Code in AH
• Function 06H clear

Example Clear Screen MOV AX, 0600 MOV BH, 17 MOV
CX, 0000 MOV DX, 184F INT 10
7
Examples of I/O INT

• Another Example INT 21H
• Function Code in AH
• Function 09H display
• Assume the string hello is at DS200

Example Display hello MOV AH, 09 MOV DX,
200 INT 21H
8
INT Vectors

• In 000h-3FFh (1KB)
• For INT 00-FF
• Each INT uses a 4-byte vector (CSIP)
• 2 bytes for IP
• 2 bytes for CS
• Actual code (Service Routine) is in CSIP
• IRET at the end of INT Service Routine

9
INT Vectors Example
Main proc MOV AX,_at_data MOV DS, AX
INT 21h Main endp
PUSH DX PUSH CX
IP
CS
0h
4h


84h


3FFh
10
Interrupt Vectoring Process
11
Common Interrupts

• INT 10h Video Services
• INT 16h Keyboard Services
• INT 17h Printer Services
• INT 1Ah Time of Day
• INT 1Ch User Timer Interrupt
• INT 21h MS-DOS Services

12
Boot Process (1)

• CPU Reset
• Blank memory, except ROM
• Start running from address FFFF0 (only 16 bytes
  left!)

13
Boot Process (2)

• BIOS Boot and POST (Power-On Self Test)
• BIOS of other device may be checked too. (e.g.,
  Video)
• Flash ROM Upgrade Possible
• If booting from hard disk ? 1st Sector (MBR)
  loaded

14
System Boot (3)

• Master Boot Record (MBR) loaded.
• Heard about LILO?
• Remember that PC ? DOS/Windows
• OS (e.g., Linux, MS-DOS or Windows) is then
  loaded.
• INT Services may be in ROM or set up while
  loading OS.

15
For Further Information

• PC Guide BIOS System Boot http//www.pcguide.com
  /ref/mbsys/bios/boot.htm