SH7713 datasheet(119/904 Pages) RENESAS

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Section 3 DSP Operating Unit

Rev.1.50 Aug. 30, 2006 Page 79 of 860

REJ09B0288-0150

To control the repeat loop, the DSP extended control registers, such as the RE register and RS

specified by the LDRE, LDRS, and SETRC instructions.

• Repeat end register (RE)

The RE register is specified by the LDRE instruction. The RE register specifies (repeat

detection instruction address +4). In a repeat loop consisting of 4 or more instructions, an

instruction three instructions prior to the repeat end instruction is regarded as the repeat

detection instruction. A repeat loop consisting of three or less instructions is described later.

• Repeat start register (RS)

The RS register is specified by the LDRS instruction. In a repeat loop consisting of 4 or more

instructions, the RS register specifies the repeat start instruction address. In a repeat loop

consisting of three or less instructions, a specific address is specified in the RS. This is

described later.

• Repeat counter (RC[11:0] bits of the SR)

The repeat counter is specifies the number of repetitions by the SETRC instruction. During

repeat loop execution, the RC holds the remaining number of repetitions.

• Repeat flags (RF[1:0] bits of the SR)

The repeat flags are automatically specified according to the RS and RE register values during

SETRC instruction execution. The repeat flags store information on the number of instructions

included in the repeat loop. Normally, the user cannot modify the repeat flag values.

The CPU always executes instructions by comparing the RE register to program counter values.

Because the PC stores (the current instruction address +4), if the RE matches the PC during repeat

instruction detection execution, a repeat detection instruction can be detected. If a repeat detection

instruction is executed without branching and if RC[11:0] > 0, then repeat control is performed. If

RC[11:0]

≥ 2 when the repeat end instruction is completed, the RC[11:0] is decremented by 1 and

then control is passed to the address specified by the RS register.

Examples 2 to 4 show program examples of the repeat loop consisting of three instructions, two

instructions, and one instruction, respectively. In these examples, an instruction immediately prior

to the repeat start instruction is regarded as a repeat detection instruction. The RS register specifies

the specific value that indicates the number of repeat instructions.

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