Datasheet

PIC18F1220/1320
DS39605F-page 46 © 2007 Microchip Technology Inc.
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
program memory address embedded into the instruc-
tion. 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 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
represents the number of single-word instructions that
the PC will be offset by. Section 20.0 “Instruction Set
Summary” provides further details of the instruction
set.
FIGURE 5-5: INSTRUCTIONS IN PROGRAM MEMORY
5.7.1 TWO-WORD INSTRUCTIONS
PIC18F1220/1320 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 20.0 “Instruction Set
Summary” for further details of the instruction set.
EXAMPLE 5-3: TWO-WORD INSTRUCTIONS
Word Address
LSB = 1 LSB = 0 ↓
Program Memory
Byte Locations
→
000000h
000002h
000004h
000006h
Instruction 1:
MOVLW 055h
0Fh 55h 000008h
Instruction 2:
GOTO 000006h
EFh 03h 00000Ah
F0h 00h 00000Ch
Instruction 3:
MOVFF 123h, 456h
C1h 23h 00000Eh
F4h 56h 000010h
000012h
000014h
CASE 1:
Object Code Source Code
0110 0110 0000 0000 TSTFSZ REG1 ; is RAM location 0?
1100 0001 0010 0011 MOVFF REG1, REG2 ; No, skip this word
1111 0100 0101 0110 ; Execute this word as a NOP
0010 0100 0000 0000 ADDWF REG3 ; continue code
CASE 2:
Object Code Source Code
0110 0110 0000 0000 TSTFSZ REG1 ; is RAM location 0?
1100 0001 0010 0011 MOVFF REG1, REG2 ; Yes, execute this word
1111 0100 0101 0110 ; 2nd word of instruction
0010 0100 0000 0000 ADDWF REG3 ; continue code