Repeat-bit based, compact system and method for implementing zero- overhead loops

摘要

A repeat-bit based system and method for executing zero overhead loops, or repeat loops, in an information processing chip that does not require a repeat end register or a dedicated comparator. Executing repeat loops requires a processor to iterate N times a code fragment of loop instructions. All systems providing this capability must know when to refetch the first loop instruction at the end of a repeat. To do this, the present invention adds a repeat bit to the processor's instruction set. This bit is set by the assembler/compiler that generates the executable code fragment comprising the repeat loop. Where the repeat loop includes plural instructions, the assembler sets the repeat bit of the penultimate loop instruction. As each loop instruction is fetched, decoded and executed, the decoder detects the repeat bit and passes it to loop control circuitry. If the code fragment has not been iterated N times and the repeat bit is set, the program counter (PC) is loaded with the address of the first repeat loop instruction, which is refetched. Otherwise, the PC is incremented and the next instruction is fetched. Where the repeat loop has a single instruction, a nop instruction must be added after the instruction to be repeated. Two systems and methods for maintaining the repeat count are disclosed. The first requires a decrementor that decrements the repeat count from N each time the loop is iterated. Another replaces the decrementor with the PC incrementor, which increments the repeat counter from -N or -(N-1).