Motorola MC68030 User Manual page 75

Data Organization and Addressing Capabilities
(UNABLE TO LOCATE ART. MUST BE RECREATED.)
Figure 2-15. M68000 Family Address Extension Words
A subroutine call saves the program counter on the active system stack, and the return
restores it from the active system stack. During the processing of traps and interrupts, both
the program counter and the status register are saved on the supervisor stack (either master
or interrupt). Thus, the execution of supervisor code is independent of user code and the
condition of the user stack; conversely, user programs use the user stack pointer
independently of supervisor stack requirements.
To keep data on the system stack aligned for maximum efficiency, the active stack pointer
is automatically decremented or incremented by two for all byte-sized operands moved to
or from the stack. In long-word-organized memory, aligning the stack pointer on a long-word
address signed significantly increases the efficiency of stacking exception frames,
subroutine calls and returns, and other stacking operations.
MOTOROLA MC68030 USER'S MANUAL 2-37