1. 9/20/6Lecture 3 - Instruction Set - Al1 Exception Handling (2)

2. 9/20/6Lecture 3 - Instruction Set - Al2 68000 Exception Handling  The 68000’s exception classification  Register that support OS and exceptions  Protecting OS memory  The various exceptions  The exception vectors

3. 9/20/6Lecture 3 - Instruction Set - Al3 The 68000’s family exceptions

4. 9/20/6Lecture 3 - Instruction Set - Al4 Supervisory State Registers Registers across The processor Family

5. 9/20/6Lecture 3 - Instruction Set - Al5 Registers  68000 SSP – Supervisor Stack Pointer System Byte of Status Word  68010 adds Vector Base Register Source Function Code Destination Function Code  etc

6. 9/20/6Lecture 3 - Instruction Set - Al6 Supervisor Memory Space

7. 9/20/6Lecture 3 - Instruction Set - Al7 Exceptions and Interrupts  Reset Externally Generated Force both the RESET* and HALT* pins low for at least 10 clock cycles Force the 68000 into a known start-up state No RTE

8. 9/20/6Lecture 3 - Instruction Set - Al8 Exceptions and Interrupts  Bus Error Externally Generated Initiated by hardware driving BERR* pin low A catchall exception Can be used to indicated non-populated memory has been addressed

9. 9/20/6Lecture 3 - Instruction Set - Al9 Exceptions and Interrupts  Interrupt Hardware generated on pins IPL0*, IPL1*, IPL2* Pins activated indicated level of interrupt Bits 8, 9, 10 of the status word indicates level of interrupt needed for service Interrupting device generates level code through a 8-line-to-3-bit encoder

10. 9/20/6Lecture 3 - Instruction Set - Al10 Interrupt Hardware

11. 9/20/6Lecture 3 - Instruction Set - Al11 Interrupt HW Level encoder

12. 9/20/6Lecture 3 - Instruction Set - Al12 Exceptions and Interrupts  Address Error Exception occurs when 68000 attempts to access a 16-bit word or 32-bit longword at an odd address Consider  LEA $7000,A0 Load A0 with $00 7000  MOVE.B (A0)+,D0 Load D0 with byte at (A0)n incr  MOVE.W (A0)+,D0 Load D0 with word Final instruction has A0 containing $7001

13. 9/20/6Lecture 3 - Instruction Set - Al13 Exceptions and Interrupts  Illegal Instruction What happens when you try to execute and op- code that is not defined? 8-bit uprocessor days 68000?  Illegal Instruction Exception

14. 9/20/6Lecture 3 - Instruction Set - Al14 Exceptions and Interrupts  Divide by Zero Trying to divide by 0 generates this exception  CHK Instruction Check Register against bounds CHK D1,D0 generates an exception if  [D0(0:15)] < 0  [D0(0:15)] > [D1(0:15)] Point – useful for compiler writers

15. 9/20/6Lecture 3 - Instruction Set - Al15 Exceptions and Interrupts  Privilege Violation Processor in user state (S-bit is clear) Attempt to execute a privileged instruction  Trace Controlled by the trace bit of the status word Contents of all registers output after each instruction

16. 9/20/6Lecture 3 - Instruction Set - Al16 Exceptions and Interrupts  Line 1010 Emulator Allow creation of special instructions When this op code is encountered an exception occurs Remaining bits of instruction word can be used to further specify “user” created instruction  Line 1111 Emulator A second version

17. 9/20/6Lecture 3 - Instruction Set - Al17 Exceptions and Interrupts  Un-initialized Interrupt Vector When a 68000 device causes and interrupt During IACK device supplies ID If not yet initialized $0F is sent so device can be initialized 68000 Devices respond with $0F when reset

18. 9/20/6Lecture 3 - Instruction Set - Al18 Exceptions and Interrupts  Spurious Interrupt IACK generated but no response? Prevents hanging when no device responds to IACK after an IPL0-IPL2 pin pulled low  TRAP (Software Interrupt) TRAP #x where x is from 0 to 15 ($0 to $F) Allows creation of unique instructions

19. 9/20/6Lecture 3 - Instruction Set - Al19 Exceptions and Interrupts  TRAPV Instruction Trap on overflow “IF V=1 THEN exception ELSE continue”  Double Bus Fault Example: A bus error occurs and while responding to this a second bus error occurs? 68000 halts!!!

20. 9/20/6Lecture 3 - Instruction Set - Al20 Exception Vectors  256 longwords (512 words)  Extend form address $00 0000 to $00 03FF

21. 9/20/6Lecture 3 - Instruction Set - Al21 Exception Vector Table  Table located in low memory  Schemes allow relocation to other addresses

22. 9/20/6Lecture 3 - Instruction Set - Al22  How would you implement the table in memory?  How would you allow the movement of the table to other addresses of memory?