PIC18F8722-ESLASHPTQTP datasheet(72/446 Pages) MICROCHIP

72 / 446 page

PIC18F8722 FAMILY

DS39646C-page 70

5.2.3

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 5.1.2

“Program Counter”).

Figure 5-5 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 boundar-

ies, 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

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 5-5:

INSTRUCTIONS IN PROGRAM MEMORY

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

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