
Introduction
MOTOROLA
MCF5206 USERS MANUAL Rev 1.0
1-5
when executing instructions, fetching instructions and operands, and storing instruction
results.
Exception processing is the transition from program processing to system, interrupt, and
exception handling; it includes fetching the exception vector, stacking operations, and
refilling the instruction fetch pipe after an exception. The processor enters exception
processing when an exceptional internal condition arises, such as tracing an instruction, an
instruction resulting in a trap, or executing specific instructions; (External conditions, such
as interrupts and access errors, also cause exceptions) and ends when the first instruction
of the exception handler enters the operand execution pipeline.
Stopped mode is a reduced power operation mode that causes the processor to remain
quiescent until either a reset or nonmasked interrupt occurs. The STOP instruction is used
to enter this operation mode.
The processor halts when it receives an access error or generates an address error while in
the exception processing state. For example, if during exception processing of one access
error another access error occurs, the MCF5206 processor cannot complete the transition
to normal processing nor can it save the internal machine state. The processor assumes that
the system is not operational and halts. Only an external reset can restart a halted
processor. When the processor executes a STOP instruction, it is in a special type of normal
processing state, e.g., one without bus cycles. The processor stops but it does not halt.
The processor can also halt in a restart mode because of Background-Debug mode events.
1.3.1.2 PROGRAMMING MODEL. The ColdFire programming model is separated into two
privilege modes: supervisor and user. The S-bit in the status register (SR) indicates the
current privilege mode. The processor identifies a logical address by accessing either the
supervisor or user address space, which differentiates between supervisor and user modes.
User programs can access only registers specific to the user mode. System software
executing in the supervisor mode can access all registers using the control registers to
perform supervisory functions. User programs are thus restricted from accessing privileged
information. The operating system performs management and service tasks for user
programs by coordinating their activities. This difference allows the supervisor mode to
protect system resources from uncontrolled accesses.
Most instructions execute in either mode but some instructions that have important system
effects are privileged and can execute only in the supervisor mode. For instance, user
programs cannot execute the STOP instructions. To prevent a program executing in user
mode from entering the supervisor mode, instructions that can alter the S-bit in the SR are
privileged. The TRAP instructions provide controlled access to operating system services
for user programs.
When in normal processing, the processor employs the user mode and the user
programming model . During exception processing, the processor changes from user to
supervisor mode. The current SR value on the stack is saved and then the S-bit is set,
forcing the processor into the supervisor mode. To return to the user mode, a system routine
must execute a MOVE to SR, or an RTE, which operate in the supervisor mode, modifying
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Freescale Semiconductor, Inc.
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