Motorola MC68340 manual Fault Recovery

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

attempting to read the stack frame. The version number is located in the most significant byte (bits 15–8) of the internal register word at location SP + $14 in the stack frame. The validity check ensures that stack frame data will be properly interpreted in multiprocessor systems.

If a frame is invalid, a format error exception is taken. If it is inaccessible, a bus error exception is taken. Otherwise, the processor reads the entire frame into the proper internal registers, de-allocates the stack (12 words), and resumes normal processing. Bus error frames for faults during exception processing require the RTE instruction to rewrite the faulted stack frame. If an error occurs during any of the bus cycles required by rewrite, the processor halts.

If a format error occurs during RTE execution, the processor creates a normal four-word fault stack frame below the frame that it was attempting to use. If a bus error occurs, a bus-error stack frame will be created. The faulty stack frame remains intact, so that it may be examined and repaired by an exception handler or used by a different type of processor (e.g., MC68010, MC68020, or future M68000 processor) in a multiprocessor system.

5.5.3 Fault Recovery

There are four phases of recovery from a fault: recognizing the fault, saving the processor state, repairing the fault (if possible), and restoring the processor state. Saving and restoring the processor state are described in the following paragraphs.

The stack contents are identified by the special status word (SSW). In addition to identifying the fault type represented by the stack frame, the SSW contains the internal processor state corresponding to the fault.

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
TP	MV	0	TR	B1	B0	RR	RM	IN	RW	LG	SIZ			FUNC

TP—BERR frame type

MV—MOVEM in progress

TR—Trace pending

B1—Breakpoint channel 1 pending

B0—Breakpoint channel 0 pending

RR—Rerun write cycle after RTE

RM—Faulted cycle was read-modify-write

IN—Instruction/other

RW—Read/write of faulted bus cycle

LG—Original operand size was long word

SIZ—Remaining size of faulted bus cycle

FUNC—Function code of faulted bus cycle

5- 52MC68340 USER’S MANUALMOTOROLA

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