Datasheet

 2003-2013 Microchip Technology Inc. DS39609C-page 45
PIC18F6520/8520/6620/8620/6720/8720
4.6 Instruction Flow/Pipelining
An “Instruction Cycle” consists of four Q cycles (Q1,
Q2, Q3 and Q4). The instruction fetch and execute are
pipelined, such that fetch takes one instruction cycle,
while decode and execute takes another instruction
cycle. However, due to the pipelining, each instruction
effectively executes in one cycle. If an instruction
causes the program counter to change (e.g., GOTO),
then two cycles are required to complete the instruction
(Example 4-2).
A fetch cycle begins with the program counter (PC)
incrementing in Q1.
In the execution cycle, the fetched instruction is latched
into the “Instruction Register” (IR) in cycle Q1. This
instruction is then decoded and executed during the
Q2, Q3 and Q4 cycles. Data memory is read during Q2
(operand read) and written during Q4 (destination
write).
EXAMPLE 4-2: INSTRUCTION PIPELINE FLOW
4.7 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 4-5 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 4.4 “PCL,
PCLATH and PCLATU”).
The CALL and GOTO instructions have an 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 4-5 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 24.0 “Instruction Set
Summary” provides further details of the instruction
set.
FIGURE 4-5: INSTRUCTIONS IN PROGRAM MEMORY
All instructions are single cycle, except for any program branches. These take two cycles since the fetch instruction is “flushed”
from the pipeline, while the new instruction is being fetched and then executed.
TCY0TCY1TCY2TCY3TCY4TCY5
1. MOVLW 55h
Fetch 1 Execute 1
2. MOVWF PORTB
Fetch 2 Execute 2
3. BRA SUB_1
Fetch 3 Execute 3
4. BSF PORTA, BIT3 (Forced NOP)
Fetch 4 Flush (NOP)
5. Instruction @ address SUB_1
Fetch SUB_1 Execute SUB_1
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