PIC18FXX20
second word of the instruction is executed 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 pro-
gram example that demonstrates this concept is shown
in Example 4-3. Refer to Section 19.0 for further details
of the instruction set.
4.7.1
TWO-WORD INSTRUCTIONS
The PIC18FXX20 devices have four two-word instruc-
tions: MOVFF, CALL, GOTOand LFSR. The second
word of these instructions has the 4 MSBs set to ‘1’s
and is a special kind of NOPinstruction. The lower 12
bits of the second word contain data to be used by the
instruction. If the first word of the instruction is exe-
cuted, the data in the second word is accessed. If the
EXAMPLE 4-3:
TWO-WORD INSTRUCTIONS
CASE 1:
Object Code
Source Code
0110 0110 0000 0000 TSTFSZ
1100 0001 0010 0011 MOVFF
1111 0100 0101 0110
REG1
; is RAM location 0?
REG1, REG2 ; No, execute 2-word instruction
; 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
1100 0001 0010 0011 MOVFF
1111 0100 0101 0110
REG1
; is RAM location 0?
REG1, REG2 ; Yes
; 2nd operand becomes NOP
REG3 ; continue code
0010 0100 0000 0000 ADDWF
4.8.2
TABLE READS/TABLE WRITES
4.8
Lookup Tables
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.
Lookup tables are implemented two ways. These are:
• Computed GOTO
• Table Reads
4.8.1
COMPUTED GOTO
A computed GOTOis 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 PCLinstruction. The next
instruction executed will be one of the RETLW 0xnn
instructions, that returns the value 0xnnto the calling
function.
A description of the Table Read/Table Write operation
is shown in Section 5.0.
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.
DS39609A-page 46
Advance Information
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
