MOTOROLA M68040 USER’S MANUAL B- 11
instructions. The processor generates exception vector number 11 for the
unimplemented F-line instruction exception vector, fetches the address of the F-line
exception handler from the exception vector table, and begins execution of the
handler after prefetching instructions to fill the pipeline. Refer to Section 8
Exception Processing for details about exception processing.
B.5.2 MC68EC040 Stack Frames
When the processor executes an RTE instruction, it examines the stack frame on top of
the active supervisor stack to determine if it is a valid frame and what type of context
restoration it requires. The set of stack frames included for exception processing are four-
and six-word stack frames, a four-word throwaway stack frame, an access error stack
frame, and a new eight-word unimplemented floating-point stack frame. The stack frame
that the MC68040 can generate and the MC68EC040 can process is the floating-point
post-instruction stack frame. Refer to Section 8 Exception Processing for details about
exception stack frames.
Eight-Word Stack Frame (Format $4)
Stack Frames Exception Types Stacked PC Points To
SP STATUS REGISTER
PC OF FAULTED
• The MC68040 cannot
generate or read this stack.
• Effective address field is
the address of the faulted
When the MC68EC040 writes or reads a stack frame, it uses long-word operand transfers
wherever possible. Using a long-word-aligned stack pointer greatly enhances exception
processing performance. The processor does not necessarily read or write the stack
frame data in sequential order. The system software should not depend on a particular
exception generating a particular stack frame. For compatibility with future devices, the
software should be able to handle any type of stack frame for any type of exception. The
MC68EC040 does not generate the floating-point post-instruction stack frame. The
MC68040 cannot accept the eight-word unimplemented stack frame. The MC68EC040
can handle all MC68040 stack frame formats.