PIC18F2480 MICROCHIP [Microchip Technology], PIC18F2480 Datasheet - Page 68

• Manufacturer Part Number: PIC18F2480
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with ECAN Technology, 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

PIC18F2480/2580/4480/4580
5.2.3
The program memory is addressed in bytes. Instruc-
tions 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). To maintain alignment
with instruction boundaries, the PC increments in steps
of 2 and the LSB will always read ‘0’ (see Section 5.1.1
“Program Counter”).
Figure 5-4 shows an example of how instruction words
are stored in the program memory.
FIGURE 5-4:
5.2.4
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 use of ‘1111’ in the 4 MSbs of an instruction spec-
ifies a special form of NOP. If the instruction is executed
in proper sequence – immediately after the first word –
the data in the second word is accessed and used by
EXAMPLE 5-4:
DS39637A-page 66
CASE 1:
Object Code
CASE 2:
Object Code
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
INSTRUCTIONS IN PROGRAM
MEMORY
TWO-WORD INSTRUCTIONS
Instruction 1:
Instruction 2:
Instruction 3:
INSTRUCTIONS IN PROGRAM MEMORY
TWO-WORD INSTRUCTIONS
Source Code
TSTFSZ
MOVFF
Source Code
TSTFSZ
ADDWF
ADDWF
MOVFF
Program Memory
Byte Locations
MOVLW
GOTO
MOVFF
055h
0006h
123h, 456h
REG1
REG1, REG2 ; No, skip this word
REG3
REG1
REG1, REG2 ; Yes, execute this word
REG3
Preliminary
; is RAM location 0?
; Execute this word as a NOP
; continue code
; is RAM location 0?
; 2nd word of instruction
; continue code
LSB = 1
EFh
C1h
0Fh
F0h
F4h
The CALL and GOTO instructions have the absolute pro-
gram memory address embedded into the instruction.
Since instructions are always stored on word bound-
aries, 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 5-4 shows how the
instruction GOTO
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
number of single-word instructions that the PC will be
offset by. Section 25.0 “Instruction Set Summary”
provides further details of the instruction set.
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 condi-
tional instruction that changes the PC. Example 5-4
shows how this works.
Note:
LSB = 0
See Section 5.5 “Program Memory and
the Extended Instruction Set” for infor-
mation on two-word instructions in the
extended instruction set.
55h
03h
00h
23h
56h
0006h is encoded in the program
Word Address
 2004 Microchip Technology Inc.
00000Ah
00000Ch
00000Eh
000000h
000002h
000004h
000006h
000008h
000010h
000012h
000014h