Today's Topics

1. Interrupt-Driven I/O
   • Interrupting a Microprocessor
   • Interrupting an 80x86
2. Interrupt Service Routines (ISR's)
   • Writing an ISR
   • Installing an ISR
3. Demonstrate Timer Interrupt
4. Terminate & Stay Resident (TSR) Programs

Interrupt-Driven I/O

• Polling
  • Repetitively check to see if something has happened.

  • Burn CPU cycles until event occurs (eg: Delay in MP3)

  • Analogy: Checking your watch, standing at the door ...

• Interrupt-Driven
  • Do something else until an event occurs

  • Process event when it happens, then resume normal activities

  • Analogy: Alarm clock, doorbell, telephone ring ...

Interrupting a Microprocessor (General Terms)

• Hardware Diagram on Board
  • Interrupt signal (pin) CPU

    

  • Interrupt Controller

  • System Bus

  • Periphial Devices (Timer, Serial Ports, Parallel Ports, Soundcard, Scanner, SCSI device ...)

• Response Procedure
  1. Complete current instruction

  2. Preserve current context

  3. Identify Source

  4. Activate Interrupt Service Routine

  5. Return to original program

Interrupting an 80x86

• Update Hardware Diagram on Board
  • Interrupt signal (pin) on the 80x86 CPU

    

  • 8259 Programmable Interrupt Controller (PIC)
    (typically built into motherboard chipset)

  • ISA/PCI System Bus (pins defined for IRQ signals)

  • Perphial Devices (static, switchable, or Plug'N'Play)
    (Current configuration of Lockwood's Computer)
    • IRQ 0: Timer (triggered 18.2/second)
    • IRQ 1: Keyboard (keypress)
    • IRQ 2: PIC
    • IRQ 3,4: Serial Ports (com2/com1 & com4/com3 data RX)
    • IRQ 5: SoundBlaster (transfer request)
    • IRQ 6: Floppy (read/write completed)
    • IRQ 8: Real-time Clock
    • IRQ 9: Ethernet (packet arrived/transmitted)
    • IRQ 11: CDROM (data read)
    • IRQ 13: Math Co-Processor
    • IRQ 15: IDE Hard-Drive Controller (read/write completed)

  • Recall IRQ->INT Mapping (from Lecture 13)
    • IRQ 0..7 = INT 8..Fh
    • IRQ 8..15 = INT 70h..77h

• Response Procedure
  1. Complete current instruction

  2. Preserve current context
     • PUSHF Store flags to stack
     • Clear Trap Flag (TF) & IF (Interrupt Flag)
     • PUSH CS Store return address to stack
     • PUSH IP

  3. Identify Source
     • Read 8259 PIC status register
     • Determine which device (N) triggered interrupt

  4. Activate Interrupt Service Routine
     • Use N to index vector table
     • Read CS/IP from table
     • Jump to instruction

  5. Execute Interrupt Service Routine
     • Your code goes here

  6. Indicate End-Of-Interrupt (EOI) to 8259 PIC
     • MOV AL,20h
     • OUT 20h,AL

  7. Return (IRET)
     • POP IP (Far Return)
     • POP CS
     • POPF (Restore Flags)

Interrupt Service Routines (ISR) - General Notes

• Hardware Interrupts can happen at any time
  (Diagram on board)

• Currently running program should have no idea that it was interrupted.

• ISRs are meant to be short.

• ISRs can be interrupted.
  (Diagram on board)

• ISRs should never make DOS calls
  (Non-reentrant: Expects single execution)
  (Diagram on board)

• ISRs must be in memory
  • Option 1: Redefine interrupt only while your program is running
  • Option 2: Use DOS Terminate-and-Stay-Resident (TSR) command

Writing an ISR

• Save ALL registers you modify (Push to stack)

• Set DS=CS (Your DS is unknown)

• Service hardware that caused the interrupt

• Reset PIC (or let other routine handle hardware)

• Restore Registers w/original values (POP)

• IRET: Pop Flags & return

Installing an ISR

• Let N==interrupt to service

• Read current function pointer in vector table
  • Use DOS function 35h

  • Set AL=N
  • Call DOS Function AH=35h, INT 21h
  • Returns: ES:BX = Address stored at vector

• Set new function pointer in vector table
  • Use DOS function 25h

  • Set DS:DX=New Routine
  • Set AL=N
  • DOS Function AH=25h, INT 21H

• Interrupts can be installed, chained, or called
  • Install New interrupt
    (replace old interrupt)

    MyIntVector PROC FAR .. Save Registers Service Hardware Reset PIC Restore Registers .. IRET MyIntVector ENDP

  • Chain into interrupt
    Service MyCode first
    

    MyIntVector PROC FAR .. Save Registers MyCode Restore Registers .. JMP CS:Old_Vector MyIntVector ENDP

  • Call Original Interrupt
    Service MyCode last

    MyIntVector PROC FAR .. PUSHF CALL CS:Old_Vector .. Save Registers MyCode Restore Registers .. IRET MyIntVector ENDP

Timer Interrupt Demonstration

• In-class demonstration

• Program Overview
  • Install on IRQ0 (INT 8)
  • Call Original Timer Interrupt
  • Increments counter on every interrupt (count)
  • Increments second counter (sCount) when Count=18
  • Increments minute Counter (mCount) when sCount=60
  • Running Program prints SecCount
  • Termination Restores Interrupt

• Source Code (Summary)
• Source Code (Complete)
• INT8.EXE (80x86 Executable)

Terminate & Stay-Resident (TSR)

• Purpose: Install an ISR that remains on the system even after you have terminated your program.

• Must be a .COM program
  • Begins at offset 0x0100h (DOS fills in header)
    As with .EXE, can be relocated to any segment
  • Has no stack (uses system stack)

• Includes variables, ISR routine, and an Installation procedure. The installation procedure updates the vector table to "hook" the interrupt. Upon completion of the installation procedure, the program terminates but the variables and ISR will remain in memory.

• Sample Code (Summary)

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