PIC18F242 datasheet(43/332 Pages) MICROCHIP

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DS39564C-page 41

PIC18FXX2

4.7.1

TWO-WORD INSTRUCTIONS

The PIC18FXX2 devices have four two-word instruc-

tions: MOVFF, CALL, GOTO and LFSR. The second

word of these instructions has the 4 MSBs set to 1’s

and is a special kind of NOP instruction. The lower 12

bits of the second word contain data to be used by the

instruction. If the first word of the instruction is exe-

cuted, the data in the second word is accessed. If the

second word of the instruction is executed by itself (first

word was skipped), it will execute as a NOP. This action

is necessary when the two-word instruction is preceded

by a conditional instruction that changes the PC. A pro-

gram example that demonstrates this concept is shown

in Example 4-3. Refer to Section 20.0 for further details

of the instruction set.

EXAMPLE 4-3:

TWO-WORD INSTRUCTIONS

4.8

Lookup Tables

Lookup tables are implemented two ways. These are:

• Computed GOTO

• Table Reads

4.8.1

COMPUTED GOTO

A computed GOTO is accomplished by adding an offset

to the program counter (ADDWF PCL).

A lookup table can be formed with an ADDWF

PCL

instruction and a group of RETLW 0xnn instructions.

WREG is loaded with an offset into the table before

executing a call to that table. The first instruction of the

called routine is the ADDWF PCL instruction. The next

instruction executed will be one of the RETLW 0xnn

instructions, that returns the value 0xnn to the calling

function.

The offset value (value in WREG) specifies the number

of bytes that the program counter should advance.

In this method, only one data byte may be stored in

each instruction location and room on the return

address stack is required.

4.8.2

TABLE READS/TABLE WRITES

A better method of storing data in program memory

allows 2 bytes of data to be stored in each instruction

location.

Lookup table data may be stored 2 bytes per program

word by using table reads and writes. The table pointer

(TBLPTR) specifies the byte address and the table

latch (TABLAT) contains the data that is read from, or

written to program memory. Data is transferred to/from

program memory, one byte at a time.

A description of the Table Read/Table Write operation

is shown in Section 3.0.

CASE 1:

Object Code

Source Code

0110 0110 0000 0000

TSTFSZ

REG1

; is RAM location 0?

1100 0001 0010 0011

MOVFF

REG1, REG2

; No, execute 2-word instruction

1111 0100 0101 0110

; 2nd operand holds address of REG2

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

1111 0100 0101 0110

; 2nd operand becomes NOP

0010 0100 0000 0000

ADDWF

REG3

; continue code

Note:

The ADDWF

PCL

instruction does not

update PCLATH and PCLATU. A read

operation on PCL must be performed to

update PCLATH and PCLATU.

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