PIC12F629/675
2.3.2
STACK
2.3
PCL and PCLATH
The PIC12F629/675 family has an 8-level deep 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 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.
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
example in Figure 2-3 shows how the PC is loaded
during a CALL or GOTO instruction (PCLATH<4:3>
PCH).
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).
FIGURE 2-3:
LOADING OF PC IN
DIFFERENT SITUATIONS
PCH
PCL
Note 1: There are no Status bits to indicate Stack
Overflow or Stack Underflow conditions.
12
8
7
0
Instruction with
PCL as
Destination
PC
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.
8
PCLATH<4:0>
PCLATH
5
ALU result
PCH
12 11 10
PC
PCL
8
7
0
GOTO, CALL
PCLATH<4:3>
PCLATH
11
2
Opcode <10:0>
2.3.1
COMPUTED GOTO
A computed GOTOis accomplished by adding an offset
to the PC (ADDWF PCL). When performing 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, “Implementing
a Table Read”
(AN556).
2010 Microchip Technology Inc.
DS41190G-page 19
MICROCHIP [ MICROCHIP ]
