Datasheet

PIC18F2480/2580/4480/4580
DS39637D-page 72 © 2009 Microchip Technology Inc.
6.2.3 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). To maintain alignment
with instruction boundaries, the PC increments in steps
of 2 and the LSB will always read ‘0’ (see Section 6.1.1
“Program Counter”).
Figure 6-4 shows an example of how instruction words
are stored in the program memory.
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 6-4 shows how the
instruction, GOTO 0006h, 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 26.0 “Instruction Set Summary”
provides further details of the instruction set.
FIGURE 6-4: INSTRUCTIONS IN PROGRAM MEMORY
6.2.4 TWO-WORD INSTRUCTIONS
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
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 6-4
shows how this works.
EXAMPLE 6-4: 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 0006h
EFh 03h 00000Ah
F0h 00h 00000Ch
Instruction 3:
MOVFF 123h, 456h
C1h 23h 00000Eh
F4h 56h 000010h
000012h
000014h
Note: See Section 6.5 “Program Memory and
the Extended Instruction Set” for infor-
mation on two-word instructions in the
extended instruction set.
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