PIC18F8722IPTSQTP datasheet(73/446 Pages) MICROCHIP

73 / 446 page

DS39646C-page 71

PIC18F8722 FAMILY

5.2.4

TWO-WORD INSTRUCTIONS

The standard PIC18 instruction set has 8 two-word

instructions: CALL, MOVFF, GOTO, LSFR, ADDULNK,

CALLW,

MOVSS

and SUBULNK. In all cases, the

second word of the instructions always has ‘1111’ as

its four Most Significant bits; the other 12 bits are literal

data, usually a data memory address.

The use of ‘1111’ in the 4 MSbs of an instruction spec-

ifies a special form of NOP. If the instruction is executed

in proper sequence – immediately after the first word –

the data in the second word is accessed and used by

the instruction sequence. If the first word is skipped for

some reason and the second word is executed by itself,

a NOP is executed instead. This is necessary for cases

when the two-word instruction is preceded by a condi-

tional instruction that changes the PC. Example 5-4

shows how this works.

EXAMPLE 5-4:

TWO-WORD INSTRUCTIONS

Note:

See

Section 5.6

“PIC18

Instruction

Execution and the Extended Instruc-

tion Set” for information on two-word

instructions in the extended instruction set.

CASE 1:

Object Code

Source Code

0110 0110 0000 0000

TSTFSZ

REG1

; is RAM location 0?

1100 0001 0010 0011

MOVFF

REG1, REG2

; No, skip this word

1111 0100 0101 0110

; Execute this word as a NOP

0010 0100 0000 0000

ADDWF

REG3

; continue code

CASE 2:

Object Code

Source Code

0110 0110 0000 0000

TSTFSZ

REG1

; is RAM location 0?

1100 0001 0010 0011

MOVFF

REG1, REG2

; Yes, execute this word

1111 0100 0101 0110

; 2nd word of instruction

0010 0100 0000 0000

ADDWF

REG3

; continue code

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