Motorola ColdFire MCF5281 - Manual (Page 67)

ColdFire Core
Freescale Semiconductor 2-21
3. The processor then generates a trace exception. The PC in the exception stack frame points to the
instruction after the stop, and the SR reflects the value loaded in the previous step.
If the processor is not in trace mode and executes a stop instruction where the immediate operand sets
SR[T], hardware loads the SR and generates a trace exception. The PC in the exception stack frame points
to the instruction after the stop, and the SR reflects the value loaded in step 2.
Because ColdFire processors do not support any hardware stacking of multiple exceptions, it is the
responsibility of the operating system to check for trace mode after processing other exception types. As
an example, consider a TRAP instruction execution while in trace mode. The processor initiates the trap
exception and then passes control to the corresponding handler. If the system requires that a trace exception
be processed, it is the responsibility of the trap exception handler to check for this condition (SR[T] in the
exception stack frame set) and pass control to the trace handler before returning from the original
exception.
2.3.4.7 Unimplemented Line-A Opcode
A line-A opcode is defined when bits 15-12 of the opword are 0b1010. This exception is generated by the
attempted execution of an undefined line-A opcode.
2.3.4.8 Unimplemented Line-F Opcode
A line-F opcode is defined when bits 15-12 of the opword are 0b1111. This exception is generated when
attempting to execute an undefined line-F opcode.
2.3.4.9 Debug Interrupt
See Chapter 30, “Debug Support , ” for a detailed explanation of this exception, which is generated in
response to a hardware breakpoint register trigger. The processor does not generate an IACK cycle, but
rather calculates the vector number internally (vector number 12). Additionally, SR[M,I] are unaffected by
the interrupt.
2.3.4.10 RTE and Format Error Exception
When an RTE instruction is executed, the processor first examines the 4-bit format field to validate the
frame type. For a ColdFire core, any attempted RTE execution (where the format is not equal to {4,5,6,7})
generates a format error. The exception stack frame for the format error is created without disturbing the
original RTE frame and the stacked PC pointing to the RTE instruction.
The selection of the format value provides some limited debug support for porting code from M68000
applications. On M68000 family processors, the SR was located at the top of the stack. On those
processors, bit 30 of the longword addressed by the system stack pointer is typically zero. Thus, if an RTE
is attempted using this old format, it generates a format error on a ColdFire processor.
If the format field defines a valid type, the processor: (1) reloads the SR operand, (2) fetches the second
longword operand, (3) adjusts the stack pointer by adding the format value to the auto-incremented address
after the fetch of the first longword, and then (4) transfers control to the instruction address defined by the
second longword operand within the stack frame.
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3