PIC18F2220/2320/4220/4320
The CALLand GOTOinstructions 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 24.0 “Instruction Set
Summary” provides further details of the instruction
set.
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”).
FIGURE 5-5:
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
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 opera-
tion. A program example that demonstrates this con-
cept is shown in Example 5-3. Refer to Section 24.0
“Instruction Set Summary” for further details of the
instruction set.
5.7.1
TWO-WORD INSTRUCTIONS
PIC18F2X20/4X20 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 decoded as a 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 5-3:
CASE 1:
TWO-WORD INSTRUCTIONS
Object Code
Source Code
0110 0110 0000 0000 TSTFSZ
REG1
; is RAM location 0?
; No, skip this word
; Execute this word as a NOP
; continue code
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
MOVFF
REG1, REG2
ADDWF
REG3
CASE 2:
Object Code
Source Code
0110 0110 0000 0000
1100 0001 0010 0011
1111 0100 0101 0110
0010 0100 0000 0000
TSTFSZ
MOVFF
REG1
; is RAM location 0?
; Yes, execute this word
; 2nd word of instruction
; continue code
REG1, REG2
REG3
ADDWF
DS39599C-page 58
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
