Freescale Semiconductor StarCore SC140 - Manual (Page 296)

7-46 SC140 DSP Core Reference Manual
Dynamic Programming Rules
7.6.6 Rule Detection Across Exception Boundaries
The SC140 can take an exception at most VLES boundaries in a program, and return using a RTE//RTED/
instruction after completing the exception service routine. The programming rules ensure that the
transition from the program to the exception service routine, and the return back to the program are correct.
The return back to the program is covered by the RTE//RTED/ programming rules in this chapter.
The following rules cover the transition from the program to the exception service routine. They apply to
the first 1-2 VLES of the exception service routine, and do not apply to the program that was interrupted:
Rule SR.2a
Instructions that are affected by the C, T, S0, S1, or VF0-VF3 status bits in SR are not allowed in the first
two VLES of an exception service routine. This rule does not apply to instructions that are affected by the
EXP status bit in SR.
The assembler-mapped instruction CLR Dn is never affected by the SR status bits, even though it is
implemented as SUB Da,Da,Dn. Therefore, this rule applies to the SUB instruction, but not to CLR (SUB
Da,Da,Dn is taken as CLR in this context).
Rule SR.4b
MOVE-like instructions that reads or writes the EMR register are not allowed at the first two VLES of an
exception service routine. The exception to this rule is any bit mask instruction(s) that does not have a zero
value to the corresponding position of DOVF bit in the mask or that accesses the high portion of the EMR.
Example 7-85. EMR access at the start of an exception
; ISR Start
move.l emr.l,d0
; Not allowed
; ISR Start
bmclr #$0004,emr.l
; Not allowed
; ISR Start
bmclr #$fff0,emr.l
; Allowed
; ISR Start
bmtstc #$0020,emr.l
; Allowed
; ISR Start
move #$00000004,sr
; Not allowed
Rule SR.6
The following instructions are not allowed in the first two VLES of an exception service routine:
• DOENn/DOENSHn
• CONT/CONTD
• BREAK
• SKIPLS