ECE 353 Introduction to Microprocessor Systems

1. ECE 353Introduction to Microprocessor Systems Michael J. Schulte Week 11

2. Topics • Interrupt Concepts • 80C188EB Interrupt Handling • Hardware Interrupts • 80C188EB Interrupt Control Unit (ICU) • Interrupt Service Routines (ISRs) • Interrupt Driven Systems • Software Interrupts and Exceptions • Interrupt Priority and Latency • Interrupt Controllers • Debugging Interrupt Hardware and Software

3. Why Use Interrupts? • Maximize processor utilization and efficiency • Allow use of sleep/idle states when nothing to do to save power • Minimize latency in responding to complex input/output structures • Facilitate event driven applications

4. Interrupt Primer • Terminology • Event – an occurrence that the processor must respond to • Asynchronous events – events that occur at unpredictable times while program is running • Pending interrupt – an IRQ signal has been received, but it has not been serviced yet. • Interrupt Service Routine (ISr) – the code executed in response to an IRQ signal. • Interrupt-driven I/O – I/O devices that use interrupts to signal when they require service • Critical code section – a section of code that cannot be interrupted

5. Basic Interrupt Hardware

6. Types of Interrupts • Maskable interrupt – an interrupt that can be disabled when desired • Nonmaskable interrupt (NMI) – an interrupt that cannot be disabled. • Level-sensitive interrupts – an interrupt request is said to exist whenever the IRQ signal is at the designated level • Edge-sensitive interrupts – an interrupt is only recognized on the signal’s transition

7. Multiple Interrupt Sources • Multiple interrupt sources handled by • Polled interrupts - single ISR is invoked for all interrupts and ISR must check all the possible interrupt sources • Vectored interrupts - each interrupt source is associated with a unique ISR and the processor hardware selects the correct ISR • The starting address of the ISR is obtained from a look-up table (vector table) • Interrupt processing begins at a fixed, unique location based on the interrupt source

8. Multiple Interrupt Structure

9. Interrupt Priority • Priority schemes determine the order in which interrupts are serviced

10. Software Interrupts & Exceptions • Software interrupts provide another mechanism for changing control flow • Interrupt instructions – Often used by applications to communicate with OS • x86 features the INT <type> instruction • Exceptions – software interrupts that are invoked by some unusual condition • Division by zero • Page fault

11. 80C188EB Interrupts

12. Interrupt Processing Sequence

13. Saved and Restored State State saved on interrupt Save restored with RETI

14. Interrupt Vector Table • Each interrupt is identified by an 8-bit type code • The interrupt vector table contain 256 32-bit ISR starting addresses (IP, CS) • Starts at 00000h • Not relocatable in the 80C188 • 1024 bytes long (1KB) • Structure

15. Initializing Interrupt Vectors mov di, INT_TYPE * 4 cli push ds xor ax, ax mov ds, ax ;segment 0000h mov ax, offset ISR_X mov [di], ax ;load offset mov ax, seg ISR_X mov [di+2], ax ;load segment pop ds sti

16. 80C1888EB Hardware Interrupts • Modular core interrupts • INTR – Generated by ICU • Interrupt Enable Flag (IF) • STI / CLI • NMI – Nonmaskable interrupt • HALT – Causes CPU to stop • Interrupt example • Hardware • Software (vector table)

17. 80C188EB ICU • Performs synchronization and prioritization of interrupts to CPU (block diagram) • Functions • Masking (IMASK, PRIMSK) • Priorities (TCU/SCU, INT0-1, INT2-4) • Default • Synchronization/prioritization (REQST, INSERV) • External interrupts (INT0-1, INT2-4) • Internal interrupts (TCU/SCU, INTSTS) • Polling interrupt sources (POLLSTS, POLL)

18. Interrupt Service Routines • ISR prerequisites • ISR implementation • Context save • Clear IRQ FF (if necessary) • Allow nesting (if desired) • Handle interrupt • Clear INSERV bit • Context restore • IRET • EOI register use (EOI) • Shared procedures and resources

19. Interrupt Driven Systems • Foreground vs. background tasks • Events determine order of execution

20. Software Interrupts & Exceptions • INT instruction • Exceptions • Divide error • Single-step • Breakpoint • INTO • BOUND • Invalid Opcodes • ESC

21. Interrupt Priority and Latency • Overall priorities and resolution • Defaults • INTR, NMI, and exceptions • Interrupt timing • Latency • Response time • 80C188EB ICU specifics

22. 82C59A PIC • 82C59A structure and capabilities • Can be cascaded • IBM PC used two of them to get 15 IRQ lines • Now usually part of chip set • Prioritization schemes • Fixed • Rotating • Hierarchical • Interrupt Acknowledge Bus Cycles • Run to get type number of interrupt source • 82C59A handles placing data on the bus

23. Interrupt Issues • Using periodic interrupts to perform iterative tasks • What to do when good interrupts go bad… • Software debugging • Hardware debugging • Real-time issues • Inter-process communication (IPC) issues

24. In-Class Assessment Quiz • What sort of safeguards might you need to design into NMI hardware? • For the 80C188EB, describe what happens between an IRQ being asserted and the actual execution of the ISR. • What are the differences between vectored interrupts and polled interrupts?

25. In-Class Assessment Quiz • What is a ‘level-sensitive’ interrupt? • What problems can arise when using a semaphore to control access to a resource used by the main program and an ISR? What 80C188 instructions help handle this? • Draw a flowchart for a periodic (1 KHz) ISR that will be used to generate precise delays. Only a single word variable is to be used to communicate with the ISR.

26. VectorTable

27. Interrupt Example - Hardware

28. IMASK

29. PRIMSK

30. REQST

31. INSERV

32. POLL

33. POLLSTS

34. EOI

35. INTSTS

36. REQST

37. TCUCON, SCUCON

38. I2CON,I3CON,I4CON

39. I0CON,I1CON

40. ICU Latency and Response Time

41. Default Interrupt Priorities Letters indicate relative priority within a level

42. ICU Block Diagram

43. INTABus Cycles