PIC18C442 datasheet(39/296 Pages) MICROCHIP

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PIC18CXX2

Preliminary

DS39026B-page 39

4.7.1

TWO-WORD INSTRUCTIONS

The PIC18CXX2 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.

EXAMPLE 4-3:

TWO-WORD INSTRUCTIONS

4.8

Lookup 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 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 5.0.

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

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