Exception Handling (2)

9/20/6 Lecture 3 - Instruction Set - Al 1

Exception Handling (2)

9/20/6 Lecture 3 - Instruction Set - Al 2

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

$00 0000 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 TableSystem reserves lowMemory for the table

If some vectors not used then no handler

Rather than no pointer, point to aSpurious exception handler

9/20/6 Lecture 3 - Instruction Set - Al 5

How to implement it? First answer is MUST be in ROM –

1st 2 longwords are initial supervisor stack pointer and initial program counter $00 0000 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

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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 $00 0000 to $00 0007

ROM is being addressed Otherwise RAM

9/20/6 Lecture 3 - Instruction Set - Al 7

Overlay scheme When reading $00 0000 or $00 0004

actually read $00 1000 or $00 1004

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Implementation of Overlay When location

$00 0000 or $00 0004 is address ROM is selected

9/20/6 Lecture 3 - Instruction Set - Al 9

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

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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Processing an Vectored Interrupt Complete instruction

currently executing Stack PC Continue according to

protocol

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IACK cycle During IACK

device provides vector number

The exception handler starts execution

9/20/6 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 25-31 are reserved for

autovectored interrupts on IRQ1* to IRQ7* Avoids having a lot of glue logic for these

older devices

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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

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Autovectored Hardware System must have

vectored interrupt hw Autovectored needed

only if 6800 family peripherals used