PIC18F2331 Datasheet, PDF (64/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
5.7 Instructions in Program Memory
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). Figure 5-5 shows an
example of how instruction words are stored in the pro-
gram memory. To maintain alignment with instruction
boundaries, the PC increments in steps of 2 and the
LSB will always read ‘0’ (see Section 5.4 “PCL,
PCLATH and PCLATU”).
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 5-5 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 repre-
sents the number of single-word instructions that the
PC will be offset by. Section 23.0 “Instruction Set
Summary” provides further details of the instruction
set.
FIGURE 5-5:
INSTRUCTIONS IN PROGRAM MEMORY
Program Memory
Byte Locations →
LSB = 1
LSB = 0
Instruction 1: MOVLW
055h
0Fh
55h
Instruction 2: GOTO
000006h
EFh
03h
F0h
00h
Instruction 3: MOVFF
123h, 456h
C1h
23h
F4h
56h
Word Address
↓
000000h
000002h
000004h
000006h
000008h
00000Ah
00000Ch
00000Eh
000010h
000012h
000014h
5.7.1 TWO-WORD INSTRUCTIONS
PIC18F2331/2431/4331/4431 devices have four two-
word instructions: MOVFF, CALL, GOTO and LFSR. The
second word of these instructions has the 4 MSBs set
to ‘1’s and is decoded as a NOP instruction. The lower
12 bits of the second word contain 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 necessary when the two-word instruction is
preceded by a conditional instruction that results in a
skip operation. A program example that demonstrates
this concept is shown in Example 5-3. Refer to
Section 23.0 “Instruction Set Summary” for further
details of the instruction set.
EXAMPLE 5-3: TWO-WORD INSTRUCTIONS
CASE 1:
Object Code
0110 0110 0000 0000
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
Source Code
TSTFSZ REG1
; is RAM location 0?
MOVFF
REG1, REG2 ; No, skip this word
; Execute this word as a NOP
ADDWF
REG3
; continue code
CASE 2:
Object Code
0110 0110 0000 0000
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
Source Code
TSTFSZ REG1
; is RAM location 0?
MOVFF
REG1, REG2 ; Yes, execute this word
; 2nd word of instruction
ADDWF
REG3
; continue code
DS39616B-page 62
Preliminary
 2003 Microchip Technology Inc.