section 7 - Index of

The DO instruction is another instruction that begins execution during the decode cycleof the pipeline. As a result, there are a number of restrictions concerning access contentionwith the program controller registers accessed by the DO instruction. The ENDDOinstruction has similar restrictions. APPENDIX A - INSTRUCTION SET DETAILS containsadditional information on the DO and ENDDO instruction restrictions.Case 5: A resource contention problem can occur when one instruction is using a registerduring its decode while the instruction executing is accessing the same resource. One exampleof this is as follows:MOVECDOX:$100,SSH#$10,ENDThe problem occurs because the MOVEC instruction loads the contents of X:$100 intothe system stack high (SSH) during its execution cycle. The DO instruction that followspushes the stack (LA -7 SSH, LC -7 SSL) during its decode cycle. Therefore, the twoinstructions try writing to the SSH simultaneously and conflict.-7.2.2 Summary of Pipeline-Related RestrictionsThe following paragraphs give a summary of the instruction sequences that cause pipelineeffects. Additional information about the individual instructions can be found inAPPENDIX A - INSTRUCTION SET DETAILS.DO Instruction restrictions:The DO instruction must not be immediately preceded by any of the following instructions:BCHG/BCLRlBSET LA, LC, SSH, SSL, or SPMOVEC/MOVEM to LA, LC, SSH, SSL, or SPMOVEC/MOVEM from SSHThe DO instruction cannot specify SSH as a source register, as in the following example:DO SSH,xxxxRestrictions 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) PCas a destination register, or specifies SSH as a source or a destination register.