Microchip Technology AC244045 Data Sheet
PIC16F72X/PIC16LF72X
DS41341E-page 30
© 2009 Microchip Technology Inc.
2.3
PCL and PCLATH
The Program Counter (PC) is 13 bits wide. The low
byte comes from the PCL register, which is a readable
and writable register. The high byte (PC<12:8>) is not
directly readable or writable and comes from
PCLATH. On any Reset, the PC is cleared. Figure 2-7
shows the two situations for the loading of the PC. The
upper example in Figure 2-7 shows how the PC is
loaded on a write to PCL (PCLATH<4:0>
byte comes from the PCL register, which is a readable
and writable register. The high byte (PC<12:8>) is not
directly readable or writable and comes from
PCLATH. On any Reset, the PC is cleared. Figure 2-7
shows the two situations for the loading of the PC. The
upper example in Figure 2-7 shows how the PC is
loaded on a write to PCL (PCLATH<4:0>
→ PCH).
The lower example in Figure 2-7 shows how the PC is
loaded during a CALL or GOTO instruction
(PCLATH<4:3>
loaded during a CALL or GOTO instruction
(PCLATH<4:3>
→ PCH).
FIGURE 2-7:
LOADING OF PC IN
DIFFERENT SITUATIONS
DIFFERENT SITUATIONS
2.3.1
COMPUTED GOTO
A computed GOTO is accomplished by adding an offset
to the program counter (ADDWF PCL). When perform-
ing a table read using a computed GOTO method, care
should be exercised if the table location crosses a PCL
memory boundary (each 256-byte block). Refer to the
Application Note AN556, “Implementing a Table Read”
(DS00556).
to the program counter (ADDWF PCL). When perform-
ing a table read using a computed GOTO method, care
should be exercised if the table location crosses a PCL
memory boundary (each 256-byte block). Refer to the
Application Note AN556, “Implementing a Table Read”
(DS00556).
2.3.2
STACK
All devices have an 8-level x 13-bit wide hardware
stack (refer to Figures 2-1 and 2-3). The stack space is
not part of either program or data space and the Stack
Pointer is not readable or writable. The PC is PUSHed
onto the stack when a CALL instruction is executed or
an interrupt causes a branch. The stack is POPed in
the event of a RETURN, RETLW or a RETFIE instruction
execution. PCLATH is not affected by a PUSH or POP
operation.
stack (refer to Figures 2-1 and 2-3). The stack space is
not part of either program or data space and the Stack
Pointer is not readable or writable. The PC is PUSHed
onto the stack when a CALL instruction is executed or
an interrupt causes a branch. The stack is POPed in
the event of a RETURN, RETLW or a RETFIE instruction
execution. PCLATH is not affected by a PUSH or POP
operation.
The stack operates as a circular buffer. This means that
after the stack has been PUSHed eight times, the ninth
PUSH overwrites the value that was stored from the
first PUSH. The tenth PUSH overwrites the second
PUSH (and so on).
after the stack has been PUSHed eight times, the ninth
PUSH overwrites the value that was stored from the
first PUSH. The tenth PUSH overwrites the second
PUSH (and so on).
2.4
Program Memory Paging
All devices are capable of addressing a continuous 8K
word block of program memory. The CALL and GOTO
instructions provide only 11 bits of address to allow
branching within any 2K program memory page. When
doing a CALL or GOTO instruction, the upper 2 bits of
the address are provided by PCLATH<4:3>. When
doing a CALL or GOTO instruction, the user must ensure
that the page select bits are programmed so that the
desired program memory page is addressed. If a return
from a CALL instruction (or interrupt) is executed, the
entire 13-bit PC is POPed off the stack. Therefore,
manipulation of the PCLATH<4:3> bits is not required
for the RETURN instructions (which POPs the address
from the stack).
word block of program memory. The CALL and GOTO
instructions provide only 11 bits of address to allow
branching within any 2K program memory page. When
doing a CALL or GOTO instruction, the upper 2 bits of
the address are provided by PCLATH<4:3>. When
doing a CALL or GOTO instruction, the user must ensure
that the page select bits are programmed so that the
desired program memory page is addressed. If a return
from a CALL instruction (or interrupt) is executed, the
entire 13-bit PC is POPed off the stack. Therefore,
manipulation of the PCLATH<4:3> bits is not required
for the RETURN instructions (which POPs the address
from the stack).
Example 2-1 shows the calling of a subroutine in
page 1 of the program memory. This example assumes
that PCLATH is saved and restored by the Interrupt
Service Routine
page 1 of the program memory. This example assumes
that PCLATH is saved and restored by the Interrupt
Service Routine
(if interrupts are used).
EXAMPLE 2-1:
CALL OF A SUBROUTINE
IN PAGE 1 FROM PAGE 0
IN PAGE 1 FROM PAGE 0
PC
12
8
7
0
5
PCLATH<4:0>
PCLATH
Instruction with
ALU Result
GOTO
, CALL
OPCODE<10:0>
8
PC
12
11 10
0
11
PCLATH<4:3>
PCH
PCL
8
7
2
PCLATH
PCH
PCL
PCL as
Destination
Note 1: There are no Status bits to indicate stack
overflow or stack underflow conditions.
2: There are no instructions/mnemonics
called PUSH or POP. These are actions
that occur from the execution of the CALL,
RETURN
that occur from the execution of the CALL,
RETURN
, RETLW and RETFIE instruc-
tions or the vectoring to an interrupt
address.
address.
Note:
The contents of the PCLATH register are
unchanged after a RETURN or RETFIE
instruction is executed. The user must
rewrite the contents of the PCLATH regis-
ter for any subsequent subroutine calls or
GOTO
unchanged after a RETURN or RETFIE
instruction is executed. The user must
rewrite the contents of the PCLATH regis-
ter for any subsequent subroutine calls or
GOTO
instructions.
ORG 500h
PAGESEL SUB_P1
;Select page 1
;(800h-FFFh)
CALL
SUB1_P1 ;Call subroutine in
:
;page 1 (800h-FFFh)
:
ORG
900h
;page 1 (800h-FFFh)
SUB1_P1
:
;called subroutine
;page 1 (800h-FFFh)
:
RETURN
;return to
;Call subroutine
;in page 0
;(000h-7FFh)