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

2. Lecture 3 - Instruction Set - Al
68000 Exception Handling
The 68000’s exception vector table
9/20/6
Lecture 3 - Instruction Set - Al

3. Exception vector table
256 longwords in low memory
$ to $00 03FF (1024 bytes)
Allow specialized routine to handle each exception type
Some 8-bit uprocessors even have a small table
9/20/6
Lecture 3 - Instruction Set - Al

4. Exception Vector Table
System reserves low
Memory for the table
If some vectors not used then no handler
Rather than no pointer, point to a
Spurious exception handler
9/20/6
Lecture 3 - Instruction Set - Al

5. Lecture 3 - Instruction Set - Al
How to implement it?
First answer is MUST be in ROM –
1st 2 longwords are initial supervisor stack pointer and initial program counter $ to ?
These must be there on start up
ROM is good as they will be there at startup
ROM is also bad as not easily modified
Changes require a new ROM
9/20/6
Lecture 3 - Instruction Set - Al

6. Lecture 3 - Instruction Set - Al
Solutions
Fixed vector in ROM points to a second vector in RAM that points to routine
WHY? Not possible to get a 16 byte ROM
Possibly overlay a ROM and RAM at same addresses?
When address is in range $ to $ ROM is being addressed
Otherwise RAM
9/20/6
Lecture 3 - Instruction Set - Al

7. Lecture 3 - Instruction Set - Al
Overlay scheme
When reading $ or $ actually read $ or $
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Lecture 3 - Instruction Set - Al

8. Implementation of Overlay
When location $ or $ is address ROM is selected
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Lecture 3 - Instruction Set - Al

9. Lecture 3 - Instruction Set - Al
2nd Method – Shadow ROM
RAM and ROM are located in the same address space
At startup POR* selects ROM
POR* does a reset of FF1
On Reads ROM is selected, on Writes RAM
Once vector table copied FF1 is set so future reads are from RAM
9/20/6
Lecture 3 - Instruction Set - Al

10. Lecture 3 - Instruction Set - Al
Implementation
ROM is addressed until RAM* becomes low
After RAM* goes low the RS FF enables the RAM
RAM* would be a signal from an I/O port
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Lecture 3 - Instruction Set - Al

11. Processing an Vectored Interrupt
Complete instruction currently executing
Stack PC
Continue according to protocol
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Lecture 3 - Instruction Set - Al

12. Lecture 3 - Instruction Set - Al
IACK cycle
During IACK device provides vector number
The exception handler starts execution
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Lecture 3 - Instruction Set - Al

13. Hardware interface of device
68000 family peripheral having IACK input
Note that IACK has unique Function Code, FC7, i.e., 111
9/20/6
Lecture 3 - Instruction Set - Al

14. Autovectored interrupt
Older devices and non-family devices unable to respond with appropriate vector number
Another pin, VPA*, Valid Peripheral Address is used to signal this.
Vector numbers are reserved for autovectored interrupts on IRQ1* to IRQ7*
Avoids having a lot of glue logic for these older devices
9/20/6
Lecture 3 - Instruction Set - Al

15. Timing of autovectored interrupts
Example
IRQ2* asserted
Followed by VPA* being asserted
Vector 26 used to respond
Autovectored are 25 to 31 in vector table
9/20/6
Lecture 3 - Instruction Set - Al

16. Autovectored Hardware
System must have vectored interrupt hw
Autovectored needed only if 6800 family peripherals used
9/20/6
Lecture 3 - Instruction Set - Al