MOTOROLA M68040 USER’S MANUAL A- 7
A.5.2 MC68LC040 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 MC68LC040 provides five different stack frames for exception
processing and allows for an MC68040-specific stack frame. The set of frames includes
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 MC68LC040 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 MC68LC040 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 format of stack frame for any type of exception. The
MC68LC040 does not generate the floating-point post-instruction stack frame. The
MC68040 cannot accept the eight-word unimplemented stack frame. The MC68LC040
can handle all MC68040 stack frame formats.
A.6 MC68LC040 ELECTRICAL CHARACTERISTICS
The following paragraphs provide information on the maximum rating and thermal
characteristics for the MC68LC040. This section is subject to change. For the most recent
specifications, contact a Motorola sales office or complete the registration card at the end
of this manual.