PIC18F2331/2431/4331/4431
6.4.1
TWO-WORD INSTRUCTIONS
6.4
Instructions in Program Memory
The standard PIC18 instruction set has four two-word
instructions: CALL, MOVFF, GOTO and LSFR. In all
cases, the second word of the instructions always has
‘1111’ as its four Most Significant bits; the other 12 bits
are literal data, usually a data memory address.
The program memory is addressed in bytes. Instructions
are stored as two bytes or four bytes in program memory.
The Least Significant Byte of an instruction word is
always stored in a program memory location with an
even address (LSB = 0). Figure 6-5 shows an example of
how instruction words are stored in the program memory.
To maintain alignment with instruction boundaries, the
PC increments in steps of 2 and the LSB will always read
‘0’.
The use of ‘1111’ in the four MSbs of an instruction
specifies a special form of NOP. If the instruction is exe-
cuted in proper sequence, immediately after the first
word, the data in the second word is accessed and
used by the instruction sequence. If the first word is
skipped for some reason and the second word is
executed by itself, a NOP is executed instead. This is
necessary for cases when the two-word instruction is
preceded by a conditional instruction that changes the
PC. Example 6-4 shows how this works.
The CALL and GOTO instructions have the absolute
program memory address embedded into the instruction.
Since instructions are always stored on word boundaries,
the data contained in the instruction is a word address.
The word address is written to PC<20:1>, which
accesses the desired byte address in program memory.
Instruction 2 in Figure 6-5 shows how the instruction,
‘GOTO 000006h’, is encoded in the program memory.
Program branch instructions, which encode a relative
address offset, operate in the same manner. The offset
value stored in a branch instruction represents the num-
ber of single-word instructions that the PC will be offset
by. Section 24.0 “Instruction Set Summary” provides
further details of the instruction set.
Note:
For information on two-word instructions
in the extended instruction set, see
Section 24.2 “Instruction Set”.
FIGURE 6-5:
INSTRUCTIONS IN PROGRAM MEMORY
Word Address
LSB = 1
LSB = 0
Program Memory
Byte Locations
000000h
000002h
000004h
000006h
000008h
00000Ah
00000Ch
00000Eh
000010h
000012h
000014h
Instruction 1:
Instruction 2:
0Fh
EFh
F0h
C1h
F4h
55h
03h
00h
23h
56h
MOVLW
GOTO
055h
000006h
Instruction 3:
MOVFF
123h, 456h
EXAMPLE 6-4:
CASE 1:
TWO-WORD INSTRUCTIONS
Source Code
Object Code
0110 0110 0000 0000 TSTFSZ
REG1
; is RAM location 0?
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
CASE 2:
MOVFF
REG1, REG2 ; No, skip this word
; Execute this word as a NOP
; continue code
ADDWF
REG3
REG1
Object Code
Source Code
0110 0110 0000 0000
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
TSTFSZ
MOVFF
; is RAM location 0?
REG1, REG2 ; Yes, execute this word
; 2nd word of instruction
ADDWF
REG3
; continue code
DS39616D-page 66
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
