Freescale Semiconductor StarCore SC140 - Manual (Page 298)

7-48 SC140 DSP Core Reference Manual
Programming Guidelines
7.7 Programming Guidelines
The rules in this section cannot be detected within the visibility of the assembler and simulator. For
example, the assembler and simulator cannot determine if the computed JMP below has a valid COF
destination.
Example 7-87. Invalid COF Destination Cannot be Detected
jmp r0 ;assembler and simulator cannot determine
; if COF destination is the start of a VLES
The following rules must be detected by the programmer, and can be avoided by good programming
practices. In addition to these rules, some good programming practices are presented below to assist the
programmer in writing robust, compatible SC140 code.
Rule J.1
A COF destination must be the start (lowest) address of a VLES. A COF destination cannot jump into the
middle of a VLES. This rule applies to all types of COF destinations described in
Section 7.4.8,
“Change-Of-Flow Destinations.”
Example 7-88. COF Destination in the Middle of a VLES
jmp _dest+2 ;not allowed
...
_dest mac d0,d1,d2 mac d3,d4,d5
The assembler evaluates the address of a VLES label as the start (lowest) address of a VLES, regardless of
its source position in the VLES. Good programming practice always places COF destination labels before
or at the start of a VLES. Programmers should be careful that computed COF destinations are the start of a
VLES. This will ensure that rule J.1 is enforced.
Rule J.2
A COF destination cannot be a delay slot.
Example 7-89. COF Destination in a Delay Slot
move.l #_dest,r0
...
jmp r0
;not allowed - COF delay slot
jmpd _dest2
_dest
add d0,d1,d2
Good programming practice never places COF destination labels before or inside a delay slot VLES.
Programmers should be careful that computed COF destinations are not a delay slot. This will ensure that
rule J.2 is enforced.
Rule J.3
Code should not be written to rely on the fact that delayed COF instructions and their delay slots are
non-interruptible. In order to create non-interruptible sequences, the user should use other mechanisms
such as the BMTSET.W instruction or encapsulating the code with the DI and EI instructions. Complying
with this rule will help to insure compatibility with future StarCore architectures.