PIC16F913/914/916/917/946
2.3
PCL and PCLATH
Note 1: There are no Status bits to indicate stack
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 2-6
shows the two situations for the loading of the PC. The
upper example in Figure 2-6 shows how the PC is
loaded on a write to PCL (PCLATH<4:0> → PCH).
The lower example in Figure 2-6 shows how the PC is
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, RETLW and RETFIE instruc-
tions or the vectoring to an interrupt
address.
loaded during
(PCLATH<4:3> → PCH).
a
CALL or GOTO instruction
2.4
Program Memory Paging
All PIC16F91X/946 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 CALLor GOTOinstruction,
the upper 2 bits of the address are provided by
PCLATH<4:3>. When doing a CALL or GOTO instruc-
tion, 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 CALLinstruction
(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).
FIGURE 2-6:
LOADING OF PC IN
DIFFERENT SITUATIONS
PCH
PCL
Instruction with
12
8
7
0
PCL as
Destination
PC
8
PCLATH<4:0>
PCLATH
5
ALU Result
PCH
12 11 10
PC
PCL
8
7
0
GOTO, CALL
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
GOTOinstructions.
PCLATH<4:3>
PCLATH
11
2
OPCODE<10:0>
2.3.1
COMPUTED GOTO
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 (if interrupts are used).
A computed GOTOis accomplished by adding an offset
to the program counter (ADDWF PCL). When perform-
ing a table read using a computed GOTOmethod, 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).
EXAMPLE 2-1:
CALL OF A SUBROUTINE
IN PAGE 1 FROM PAGE 0
ORG 500h
BCF PCLATH,4
BSF PCLATH,3 ;Select page 1
;(800h-FFFh)
CALL SUB1_P1 ;Call subroutine in
:
2.3.2
STACK
The PIC16F91X/946 family has an 8-level x 13-bit wide
hardware stack (see Figures 2-1 and 2-2). 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, RETLWor a RETFIE
instruction execution. PCLATH is not affected by a
PUSH or POP operation.
;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)
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).
DS41250F-page 40
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
