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All exception processing is performed at the supervisor level. All bus cycles generated
during exception processing are supervisor references, and all stack accesses use the
SSP.
Instructions that have important system effects can only be executed at supervisor level.
For instance, user programs are not permitted to execute STOP, LPSTOP, or RESET
instructions. To prevent a user program from gaining privileged access, except in a
controlled manner, instructions that can alter the S-bit in the SR are privileged. The TRAP
#n instruction provides controlled user access to operating system services.
5.4.2.2 USER PRIVILEGE LEVEL. If the S-bit in the SR is cleared, the processor
executes instructions at the user privilege level. The bus cycles for an instruction executed
at the user privilege level are classified as user references, and the values of the function
codes on FC2–FC0 specify user address spaces. While the processor is at the user level,
implicit references to the system SP and explicit references to address register seven (A7)
refer to the USP.
5.4.2.3 CHANGING PRIVILEGE LEVEL. To change from user privilege level to
supervisor privilege level, a condition that causes exception processing must occur. When
exception processing begins, the current values in the SR, including the S-bit, are saved
on the supervisor stack, and then the S-bit is set to enable supervisory access. Execution
continues at supervisor privilege level until exception processing is complete.
To return to user access level, a system routine must execute one of the following
instructions: MOVE to SR, ANDI to SR, EORI to SR, ORI to SR, or RTE. These
instructions execute only at supervisor privilege level and can modify the S-bit of the SR.
After these instructions execute, the instruction pipeline is flushed, then refilled from the
appropriate address space.
The RTE instruction causes a return to a program that was executing when an exception
occurred. When RTE is executed, the exception stack frame saved on the supervisor
stack can be restored in either of two ways.
If the frame was generated by an interrupt, breakpoint, trap, or instruction exception, the
SR and PC are restored to the values saved on the supervisor stack, and execution
resumes at the restored PC address, with access level determined by the S-bit of the
restored SR.
If the frame was generated by a bus error or an address error exception, the entire
processor state is restored from the stack.
5.5 EXCEPTION PROCESSING
An exception is a special condition that preempts normal processing. Exception
processing is the transition from normal mode program execution to execution of a routine
that deals with an exception. The following paragraphs discuss system resources related
to exception handling, exception processing sequence, and specific features of individual
exception processing routines.
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MC68340 USER’S MANUAL
MOTOROLA
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