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4.8

Notes

4.8.1

Value of the Stack Pointer (SP)

An address error occurs if the stack is accessed for exception processing when the value of the

stack pointer (SP) is not a multiple of four. Therefore, a multiple of four should always be stored

in SP.

4.8.2

Value of the Vector Base Register (VBR)

An address error occurs if the vector table is accessed for exception processing when the value of

the vector base register (VBR) is not a multiple of four. Therefore, VBR should always be set to a

multiple of four.

4.8.3

Address Errors that Are Caused by Stacking During Address Error Exception

Processing

When the stack pointer is not a multiple of four, address errors will occur in the exception

processing (interrupt, etc.) stacking. After the exception processing ends, the CPU will then shift

to address error exception processing. An address error will also occur during the address error

exception processing stacking, but the CPU is set up to ignore the address error so that it can avoid

an infinite series of address errors. This allows it to shift program control to the address error

exception service routine and process the error.

When an address error does occur in exception processing stacking, the stacking bus cycle (write)

is executed. In SR and PC stacking, four is subtracted from each of the SPs so the SP values are

not multiples of four after stacking either. Since the address value output during stacking is the SP

value, the address that produced the error is exactly what is output. In such cases, the stacked write

data will be undefined.

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