PIC16F87XA
2.3
PCL and PCLATH
Note 1: There are no status bits to indicate stack
overflow or stack underflow conditions.
The program counter (PC) is 13 bits wide. The low byte
comes from the PCL register, which is a readable and
writable register. The upper bits (PC<12:8>) are not
readable, but are indirectly writable through the
PCLATH register. On any RESET, the upper bits of the
PC will be cleared. Figure 2-5 shows the two situations
for the loading of the PC. The upper example in the fig-
ure shows how the PC is loaded on a write to PCL
(PCLATH<4:0> → PCH). The lower example in the fig-
ure shows how the PC is loaded during a CALLor GOTO
instruction (PCLATH<4:3> → PCH).
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
instructions, or the vectoring to an
interrupt address.
2.4
Program Memory Paging
All PIC16F87XA 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 popped
off the stack. Therefore, manipulation of the
PCLATH<4:3> bits is not required for the return instruc-
tions (which POPs the address from the stack).
FIGURE 2-5:
LOADING OF PC IN
DIFFERENT SITUATIONS
PCH
PCL
12
8
7
0
Instruction with
PCL as
Destination
PC
8
PCLATH<4:0>
PCLATH
5
ALU
PCH
12 11 10
PC
PCL
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.
8
7
0
GOTO,CALL
PCLATH<4:3>
PCLATH
11
2
Opcode <10:0>
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).
2.3.1
COMPUTED GOTO
A computed GOTOis accomplished by adding an offset
to the program counter (ADDWF PCL). When doing 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
EXAMPLE 2-1:
CALL OF A SUBROUTINE
IN PAGE 1 FROM PAGE 0
ORG 0x500
BCF PCLATH,4
BSF PCLATH,3 ;Select page 1
;(800h-FFFh)
CALL SUB1_P1 ;Call subroutine in
application note, “Implementing
(AN556).
a Table Read”
:
:
;page 1 (800h-FFFh)
;page 1 (800h-FFFh)
2.3.2
STACK
The PIC16F87XA family has an 8-level deep x 13-bit
wide hardware stack. 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.
ORG 0x900
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).
DS39582A-page 28
AdvanceInformation
2001 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
