PIC12F683
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).
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 2-3 shows the
two situations for the loading of the PC. The upper
example in Figure 2-3 shows how the PC is loaded on a
write to PCL (PCLATH<4:0> → PCH). The lower exam-
ple in Figure 2-3 shows how the PC is loaded during a
CALLor GOTOinstruction (PCLATH<4:3> → PCH).
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, RETLW and RETFIE
instructions or the vectoring to an
interrupt address.
FIGURE 2-3:
LOADING OF PC IN
DIFFERENT SITUATIONS
2.4
Indirect Addressing, INDF and
FSR Registers
PCH
PCL
Instruction with
PCL as
12
8
7
0
Destination
PC
The INDF register is not a physical register. Addressing
the INDF register will cause indirect addressing.
8
PCLATH<4:0>
PCLATH
5
ALU Result
Indirect addressing is possible by using the INDF
register. Any instruction using the INDF register
actually accesses data pointed to by the File Select
Register (FSR). Reading INDF itself indirectly will
produce 00h. Writing to the INDF register indirectly
results in a no operation (although Status bits may be
affected). An effective 9-bit address is obtained by
concatenating the 8-bit FSR register and the IRP bit
(Status<7>), as shown in Figure 2-4.
PCH
12 11 10
PC
PCL
8
7
0
GOTO, CALL
PCLATH<4:3>
PCLATH
11
2
OPCODE<10:0>
A simple program to clear RAM location 20h-2Fh using
indirect addressing is shown in Example 2-1.
2.3.1
COMPUTED GOTO
EXAMPLE 2-1:
INDIRECT ADDRESSING
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).
MOVLW
MOVWF
CLRF
INCF
BTFSS
GOTO
0x20
FSR
INDF
FSR
FSR,4
NEXT
;initialize pointer
;to RAM
;clear INDF register
;inc pointer
;all done?
;no clear next
;yes continue
NEXT
CONTINUE
2.3.2
STACK
The PIC12F683 family has an 8-level x 13-bit wide
hardware stack (see Figure 2-1). 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 CALLinstruction is executed or
an interrupt causes a branch. The stack is POPed in
the event of a RETURN,RETLWor a RETFIEinstruction
execution. PCLATH is not affected by a PUSH or POP
operation.
2004 Microchip Technology Inc.
Preliminary
DS41211B-page 17
MICROCHIP [ MICROCHIP ]
