Digital Photo Keychain User Manual

NORMAL PROCESSING STATE
7 - 8 PROCESSING STATES MOTOROLA
The DO instruction is another instruction that begins execution during the decode cycle
of the pipeline. As a result, there are a number of restrictions concerning access conten-
tion with the program controller registers accessed by the DO instruction. The ENDDO
instruction has similar restrictions. APPENDIX A - INSTRUCTION SET DETAILS con-
tains additional information on the DO and ENDDO instruction restrictions.
Case 5: A resource contention problem can occur when one instruction is using a register
during its decode while the instruction executing is accessing the same resource. One ex-
ample of this is as follows:
MOVEC X:$100,SSH
DO #$10,END
The problem occurs because the MOVEC instruction loads the contents of X:$100 into
the system stack high (SSH) during its execution cycle. The DO instruction that follows
pushes the stack (LA → SSH, LC → SSL) during its decode cycle. Therefore, the two
instructions try writing to the SSH simultaneously and conflict.
7.2.2 Summary of Pipeline-Related Restrictions
The following paragraphs give a summary of the instruction sequences that cause pipe-
line effects. Additional information about the individual instructions can be found in
APPENDIX A - INSTRUCTION SET DETAILS.
DO instruction restrictions:
The DO instruction must not be immediately preceded by any of the following instruc-
tions:
BCHG/BCLR/BSET LA, LC, SSH, SSL, or SP
MOVEC/MOVEM to LA, LC, SSH, SSL, or SP
MOVEC/MOVEM from SSH
The DO instruction cannot specify SSH as a source register, as in the following example:
DO SSH,xxxx
Restrictions near the end of DO loops:
Proper DO loop operation is guaranteed if no instruction starting at address LA-2, LA-1,
or LA specifies the program controller registers SR, SP, SSL, LA, LC, or (implicitly) PC
as a destination register, or specifies SSH as a source or a destination register.