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C167CS

Derivatives

Interrupt and Trap Functions

User’s Manual

5-19

V2.0, 2000-07

Context Switching

An interrupt service routine usually saves all the registers it uses on the stack, and

restores them before returning. The more registers a routine uses, the more time is

wasted with saving and restoring. The C167CS allows to switch the complete bank of

CPU registers (GPRs) with a single instruction, so the service routine executes within its

own, separate context.

The instruction “SCXT CP, #New_Bank” pushes the content of the context pointer (CP)

on the system stack and loads CP with the immediate value “New_Bank”, which selects

a new register bank. The service routine may now use its “own registers”. This register

bank is preserved, when the service routine terminates, i.e. its contents are available on

the next call.

Before returning (RETI) the previous CP is simply POPped from the system stack, which

returns the registers to the original bank.

Note: The first instruction following the SCXT instruction must not use a GPR.

Resources that are used by the interrupting program must eventually be saved and

restored, e.g. the DPPs and the registers of the MUL/DIV unit.

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