18F8680 datasheet(60/496 Pages) MICROCHIP

60 / 496 page

PIC18F6585/8585/6680/8680

DS30491C-page 58

 2004 Microchip Technology Inc.

4.7.1

TWO-WORD INSTRUCTIONS

The PIC18F6585/8585/6680/8680 devices have four

two-word instructions: 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 instruc-

tion is executed, the data in the second word is

accessed. If the second word of the instruction is exe-

cuted 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 program example that demon-

strates this concept is shown in Example 4-3. Refer to

Section 25.0 “Instruction Set Summary” for further

details of the instruction set.

EXAMPLE 4-3:

TWO-WORD INSTRUCTIONS

4.8

Look-up Tables

Look-up 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 look-up 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.

Look-up 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 5.0 “Flash Program Memory”.

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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