PIC18LC658T-ESLASHCL datasheet(47/366 Pages) MICROCHIP

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 2000 Microchip Technology Inc.

Advanced Information

DS30475A-page 47

PIC18CXX8

4.7.1

TWO WORD INSTRUCTIONS

The PIC18CXX8 devices have 4 two word instructions:

MOVFF, CALL, GOTO

and LFSR. The second word of

these instructions has the 4 MSB’s 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 executed, 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 neces-

sary when the two word instruction is preceded by a

conditional instruction that changes the PC. A program

example that demonstrates this concept is shown in

Example 4-3. Refer to Section 19.0 for further details of

the instruction set.

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 exe-

cuting a call to that table. The first instruction of the called

routine is the ADDWF PCL instruction. The next instruc-

tion executed will be one of the RETLW 0xnn instruc-

tions 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 as 2 bytes per pro-

gram 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 trans-

ferred to/from program memory one byte at a time.

A description of the Table Read/Table Write operation

is shown in Section 5.0.

EXAMPLE 4-3:

TWO WORD INSTRUCTIONS

Warning:

The LSb of PCL is fixed to a value of ‘0’.

Hence, computed GOTO to an odd

address is not possible.

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

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