SH7721 datasheet(151/1524 Pages) RENESAS

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Section 3 DSP Operating Unit

Rev. 3.00 Jan. 18, 2008 Page 89 of 1458

REJ09B0033-0300

3.3

CPU Extended Instructions

3.3.1

DSP Repeat Control

In DSP mode, a specific function is provided to execute repeat loops efficiently. By using this

function, loop programs can be executed without overhead caused by the compare and branch

instructions.

(1)

Examples of Repeat Loop Programs

Examples of repeat loop programs are shown below.

• Example 1: Repeat loop consisting of 4 or more instructions

LDRS RptStart

; Sets repeat start instruction address

to the RS register

LDRE RptDtct +4

; Sets (repeat detection instruction

address + 4) to the RE register

SETRC #4

; Sets the number of repetitions (4) to

the RC[11:0] bits of the SR register

Instr0

; At least one instruction is required

from SETRC instruction to [Repeat start

instruction]

RptStart: instr1

; [Repeat start instruction]

... ...

;

... ...

;

RptDtct:

instr(N-3)

;Three instruction prior to the repeat

end instruction is regarded as repeat

detection instruction

RptEnd2:

instr(N-2)

;

RptEnd1:

instr(N-1)

;

RptEnd:

instrN

; [Repeat end instruction]

In the above program example, instructions from the RptStart address (instr1 instruction) to the

RptEnd address (instrN instruction) are repeated four times. These repeated instructions in the

program are called repeat loop. The start and end instructions of the repeat loop are called the

repeat start instruction and repeat end instruction, respectively. The CPU sequentially executes

instructions and starts repeat loop control if the CPU detects the completion of a specific

instruction. This specific instruction is called the repeat detection instruction. In a repeat loop

consisting of four or more instructions, an instruction three instructions prior to the repeat end

instruction is regarded as the repeat detection instruction. In a repeat loop consisting of four or

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