DSPIC30F datasheet(26/248 Pages) MICROCHIP

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dsPIC30F

DS70083G-page 24

Preliminary

 2004 Microchip Technology Inc.

2.3

Instruction Flow

There are 8 types of instruction flows:

Normal one-word, one-cycle instructions: these

instructions take one effective cycle to execute

as shown in Figure 2-2.

FIGURE 2-2:

INSTRUCTION PIPELINE FLOW: 1-WORD, 1-CYCLE

One-word, two-cycle (or three-cycle) instruc-

tions that are flow control instructions: these

instructions include the relative branches, rela-

tive call, skips and returns. When an instruction

changes the PC (other than to increment it), the

pipelined fetch is discarded. This causes the

instruction to take two effective cycles to exe-

cute as shown in Figure 2-3. Some instructions

that change program flow require 3 cycles, such

as the RETURN, RETFIE and RETLW instruc-

tions, and instructions that skip over 2-word

instructions.

FIGURE 2-3:

INSTRUCTION PIPELINE FLOW: 1-WORD, 2-CYCLE

CY0TCY1TCY2TCY3TCY4TCY5

1. MOV.b #0x55,W0

Fetch 1

Execute 1

2. MOV.b #0x35,W1

Fetch 2

Execute 2

3. ADD.b W0,W1,W2

Fetch 3

Execute 3

CY0TCY1TCY2TCY3TCY4TCY5

1. MOV

#0x55,W0

Fetch 1

Execute 1

2. BTSC W1,#3

Fetch 2

Execute 2

Skip Taken

3. ADD

W0,W1,W2

Fetch 3

Flush

4. BRA

SUB_1

Fetch 4

Execute 4

5. SUB

W0,W1,W3

Fetch 5

Flush

6. Instruction @ address SUB_1

Fetch SUB_1

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