Freescale Semiconductor SC140 Loop Initiation and Execution, Loop Type, Location Functionality

Hardware Loops

Table 5-9 illustrates the location of these marker bits and their functionality in both short and long loops. Refer to Appendix A, “SC140 DSP Core Instruction Set,” for further details.

Table 5-9. LPMARKA and LPMARKB Bits in Short and Long Loops

Loop Type		LPMARKA		LPMARKB
	Location	Functionality	Location	Functionality
Short loop	SA	Identifies a single-execution	SA	Identifies a two-execution
		set loop. Causes no timing		set loop. Causes no timing
		overhead.		overhead.
Long loop	LA	Identifies a jump to SA after	LA-2	Identifies a jump to SA
		executing the set at LA, if the		after executing the sets at
		loop is repeated. Causes a		LA-2, LA-1, and LA, if the
		timing overhead.		loop is repeated. Causes
				no timing overhead.

5.4.3 Loop Initiation and Execution

The following steps are required to initiate a hardware loop:

1.Execute a DOSETUPn instruction at some stage before the loop starts (except in the case of a short loop). This instruction writes the start address of the loop to the corresponding SAn register.

2.Execute a DOENn or DOENSHn instruction to load the corresponding LCn register with the number of iterations for the loop. The corresponding loop flag bit is implicitly set when LCn is loaded with the loop iteration value. The SLF is set if the loop is initialized by a DOENSHn instruction.

3.Execute a SKIPLS instruction before entering the loop to check the value of LCn. If the value of LCn is less than or equal to zero, then the loop is skipped and the program counter (PC) is loaded with the address specified in the SKIPLS instruction. If it is guaranteed that LCn is greater than zero (for example, if the loop is initialized by an immediate value), the SKIPLS instruction can be omitted. The SKIPLS instruction provides the additional flexibility of skipping the steps in the loop completely if the loop count is zero initially.

After the LCn is loaded and the LFn bit is set with the DOENn or DOENSHn instruction, the hardware loop is ready for operation. In long loops, whenever the program reaches the execution set marked by LPMARKB [which appears two execution sets before the last execution set of the loop (LA-2)], LCn is compared to the value one in order to detect loop termination. If the value of LCn is greater than one, the program effectively jumps to the start address while executing the two execution sets at LA-1 and LA. The LCn is decremented by one and the loop is repeated. If the value of LCn is equal or less than one, the loop terminates and the loop flag bit is cleared. Execution of instructions continues in sequence.

In short loops, one or two execution sets are stored in internal buffers and repeated the appropriate number of times according to the value stored in LCn. No program fetches are required for short loops.

SC140 DSP Core Reference Manual 5-27