Datasheet

PIC18(L)F1XK22
DS41365E-page 30 2009-2011 Microchip Technology Inc.
3.2.3 INSTRUCTIONS IN PROGRAM
MEMORY
The program memory is addressed in bytes.
Instructions are stored as either two bytes or four bytes
in program memory. The Least Significant Byte (LSB)
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 3.1.1 “Program Counter”).
Figure 3-4 shows an example of how instruction words
are stored in the program memory.
The CALL and GOTO instructions have the absolute
program memory address embedded into the
instruction. 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 3-4 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 23.0 “Instruction Set Summary”
provides further details of the instruction set.
FIGURE 3-4: INSTRUCTIONS IN PROGRAM MEMORY
3.2.4 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 instruction always has
1111’ as its four Most Significant bits (MSb); the other
12 bits are literal data, usually a data memory address.
The use of1111’ in the 4 MSbs of an instruction
specifies 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 conditional instruction that changes the
PC. Example 3-4 shows how this works.
EXAMPLE 3-4: 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 3.6 “PIC18 Instruction
Execution and the Extended Instruc-
tion Set” for information 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