Motorola DSP56000, 24-Bit Digital Signal Processor manual Bootend Movec, Andi

NORMAL PROCESSING STATE

Case 2: One of the more common sequences where pipeline effects are apparent is as follows:

•	;Move a number into register Rn (n=0–7).
•
MOVE #xx,Rn
MOVE X:(Rn),A	;Use the new contents of Rn to address memory.
•
•

In this case, before the first MOVE instruction has written Rn during its execution cycle, the second MOVE has accessed the old Rn, using the old contents of Rn. This is because the address for indirect moves is formed during the decode cycle. This overlap- ping instruction execution in the pipeline causes the pipeline effect. One instruction cycle should be allowed after an address register has been written by a MOVE instruction before the new contents are available for use as an address register by another MOVE instruction. The proper instruction sequence is as follows:

•	;Move a number into register Rn.
•
MOVE X0,Rn
NOP	;Execute any instruction or instruction
•	;sequence not using Rn.
•
MOVE X:(Rn),A	Use the new contents of Rn.

Case 3: A situation related to Case 2 can be seen in the boot ROM code shown in AP- PENDIX A of the DSP56001 Technical Data Sheet. At the end of the bootstrap operation, the operation mode register (OMR) is changed to mode #2, and then the program that was loaded is executed. This process is accomplished in the last three instructions:

_BOOTEND MOVEC	#2,OMR	;Set the operating mode to 2
		;(and trigger an exit from
		;bootstrap mode).
ANDI	#$0,CCR	;Clear SR as if RESET and
		;introduce delay needed for
		;Op. Mode change.
JMP	<$0	;Start fetching from PRAM, P:$0000

The JMP instruction generates its jump address during its decode cycle. If the JMP instruction followed the MOVEC, the MOVEC instruction would not have changed the OMR before the JMP instruction formed the fetch address. As a result, the jump would fetch the instruction at P:$0000 of the bootstrap ROM (MOVE #$FFE9,R2). The OMR would then change due to the MOVEC instruction, and the next instruction would be the

MOTOROLA PROCESSING STATES 7 - 5