HCS08 datasheet(64/316 Pages) NXP

64 / 316 page

Chapter 5 Resets, Interrupts, and General System Control

MC9S08SG8 MCU Series Data Sheet, Rev. 8

Freescale Semiconductor

recommended for anyone other than the most experienced programmers because it can lead to subtle

program errors that are difficult to debug.

The interrupt service routine ends with a return-from-interrupt (RTI) instruction which restores the CCR,

A, X, and PC registers to their pre-interrupt values by reading the previously saved information from the

stack.

NOTE

For compatibility with M68HC08 devices, the H register is not

automatically saved and restored. It is good programming practice to push

H onto the stack at the start of the interrupt service routine (ISR) and restore

it immediately before the RTI that is used to return from the ISR.

If more than one interrupt is pending when the I bit is cleared, the highest priority source is serviced first

(see Table 5-2).

5.5.1

Interrupt Stack Frame

Figure 5-1 shows the contents and organization of a stack frame. Before the interrupt, the stack pointer

(SP) points at the next available byte location on the stack. The current values of CPU registers are stored

on the stack starting with the low-order byte of the program counter (PCL) and ending with the CCR. After

stacking, the SP points at the next available location on the stack which is the address that is one less than

the address where the CCR was saved. The PC value that is stacked is the address of the instruction in the

main program that would have executed next if the interrupt had not occurred.

Figure 5-1. Interrupt Stack Frame

When an RTI instruction is executed, these values are recovered from the stack in reverse order. As part

of the RTI sequence, the CPU fills the instruction pipeline by reading three bytes of program information,

starting from the PC address recovered from the stack.

CONDITION CODE REGISTER

ACCUMULATOR

INDEX REGISTER (LOW BYTE X)

PROGRAM COUNTER HIGH

* High byte (H) of index register is not automatically stacked.

PROGRAM COUNTER LOW

UNSTACKING

ORDER

STACKING

ORDER

TOWARD LOWER ADDRESSES

TOWARD HIGHER ADDRESSES

SP BEFORE

SP AFTER

INTERRUPT STACKING

THE INTERRUPT

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