XA User Guide 5-4 3/24/97
Protection Via Dual Stack Pointers
The XA provides a two-level user/supervisor protection mechanism. These are the user or
application mode and the system or supervisor mode. In a multitasking environment, tasks in a
supervisor level are protected from tasks in the application level.
The XA has two stack pointers (in the register file) called the System Stack Pointer (SSP) and
the User Stack Pointer (USP). In multitasking systems one stack pointer is used for the
supervisory system and another for the currently active task. This helps in the protection
mechanism by providing isolation of system software from user applications. The two stack
pointers also help to improve the performance of interrupts. If the stack for a particular
application would exceed the space available in the on-chip RAM, or on-chip RAM is needed for
other time critical purposes (since on-chip RAM is accessed more quickly than off-chip
memory), the main stack can be put off-chip and the interrupt stack (using the System SP) may
be put in on-chip RAM.
These features of the XA place it well above the competition in suitability to multi-tasking
applications.
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