PIC18F258 Datasheet, PDF (45/384 Pages) Microchip Technology – High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN

PIC18FXX8
4.7.1 TWO-WORD INSTRUCTIONS
The PIC18FXX8 devices have 4 two-word instructions:
MOVFF, CALL, GOTO and LFSR. The 4 Most Signifi-
cant bits of the second word are set to ‘1’s, and indicate
a special NOP instruction. The lower 12 bits of the sec-
ond word contain the 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 4-4. Refer to Section 25.0 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).
The ADDWF PCL instruction does not update PCLATH/
PCLATU. A read operation on PCL must be performed
prior to the 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.
Warning: The LSb of PCL is fixed to a value of ‘0’.
Hence, computed GOTO to an odd
address is not possible.
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
transferred to/from program memory, one byte at a
time.
A description of the Table Read/Table Write operation
is shown in Section 6.1.
EXAMPLE 4-4: TWO-WORD INSTRUCTIONS
CASE 1:
Object Code
Source Code
0110 0110 0000 0000 TSTFSZ
1100 0001 0010 0011 MOVFF
1111 0100 0101 0110
0010 0100 0000 0000 ADDWF
REG1
; is RAM location 0?
REG1, REG2 ; No, execute 2-word instruction
; 2nd operand holds address of REG2
REG3
; continue code
CASE 2:
Object Code
0110 0110 0000 0000 TSTFSZ
1100 0001 0010 0011 MOVFF
1111 0100 0101 0110
0010 0100 0000 0000 ADDWF
Source Code
REG1
; is RAM location 0?
REG1, REG2 ; Yes
; 2nd operand becomes NOP
REG3
; continue code
 2002 Microchip Technology Inc.
Preliminary
DS41159B-page 43