PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 45

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• Manufacturer: MICROCHIP [Microchip Technology]

4.7.1
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 are implemented two ways. These are:
• Computed GOTO
• Table Reads
4.8.1
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.
EXAMPLE 4-4:
 2002 Microchip Technology Inc.
CASE 1:
CASE 2:
0110 0110 0000 0000
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
0110 0110 0000 0000
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
Object Code
Object Code
Lookup Tables
TWO-WORD INSTRUCTIONS
COMPUTED GOTO
TWO-WORD INSTRUCTIONS
TSTFSZ
MOVFF
ADDWF
TSTFSZ
MOVFF
ADDWF
REG1
REG1, REG2 ; No, execute 2-word instruction
REG3
REG1
REG1, REG2 ; Yes
REG3
Preliminary
; is RAM location 0?
; 2nd operand holds address of REG2
; continue code
; is RAM location 0?
; 2nd operand becomes NOP
; continue code
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
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.
Source Code
Warning: The LSb of PCL is fixed to a value of ‘0’.
Source Code
TABLE READS/TABLE WRITES
Hence, computed GOTO to an odd
address is not possible.
PIC18FXX8
DS41159B-page 43