AT75C datasheet(27/171 Pages) ATMEL

27 / 171 page

AT75C DSP Subsystem

1368C–INTAP–08/02

Interrupt Context

Switching

When a program is interrupted by an interrupt service routine, it is necessary to save

those registers used by the service routine so that the interrupted program resumes

execution correctly. To reduce the involved overhead, a context-switching mechanism

can be used for each of the following interrupts: NMI, INT0, INT1 and/or INT2. Whether

a specific interrupt should use the context-switching mechanism is determined by the

corresponding bit in the internal configuration register (ICR). Refer to the comments on

ICR contents on page 29 and to the MOV instruction in “Instruction Set” on page 31.

When an interrupt that uses context switching is accepted, context switching occurs

automatically without any impact on interrupt latency. When returning from this interrupt

service routine, context switching should be used to restore the original register values

automatically. Refer to the RETI and CNTX instructions in “Instruction Set” on page 31.

Context switching involves three parallel mechanisms: push/pop to/from dedicated

shadow bits, swap of a dedicated page register and swap between two specific

accumulators.

The following register bits are saved automatically as shadow bits, i.e., one stack level

pushed to or popped from the shadow registers.

The page bits ST1[7:0] are swapped to an alternative register. This means that when an

interrupt is accepted, the current page is saved into the alternative register while the

previous (stored) value of the page is restored so that it can be used without additional

initialization. When returning from the interrupt, the interrupt page is saved again into the

alternative register for the next interrupt, and the page used before entering the interrupt

service routine is swapped back to ST1.

The A1- and B1-accumulators are automatically swapped. Therefore, it is very conve-

nient to use B1 to store data needed for interrupt routines. This data will be transferred

automatically into the A1-accumulator on interrupt service for interrupts using the con-

text-switching mechanism and transferred back while returning from the interrupt

service routine.

A context-switching activation instruction is also available. For details refer to the CNTX

instruction in “Instruction Set” on page 31.

AT75C Datasheet(PDF) 27 Page - ATMEL Corporation