PIC16C55X(A)
4.3.2
STACK
4.3
PCL and PCLATH
The PIC16C55X(A) family has an 8 level deep x 13-bit
wide hardware stack (Figure 4-1, Figure 4-2 and
Figure 4-3). The stack space is not part of either pro-
gram or data space and the stack pointer is not read-
able 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 program counter (PC) is 13-bits wide. The low byte
comes from the PCL register, which is a readable and
writable register.The high bits (PC<12:8>) are not directly
readable or writable and come from PCLATH. On any
reset, the PC is cleared. Figure 4-10 shows the two
situations for the loading of the PC.The upper example in
the figure shows how the PC is loaded on a write to PCL
(PCLATH<4:0> → PCH).The lower example in the figure
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 4-10: 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
8
PCLATH<4:0>
PCLATH
5
Note 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 vectoring to an interrupt
address.
ALU result
PCH
12 11 10
PCL
8
7
0
GOTO, CALL
PC
PCLATH<4:3>
PCLATH
11
2
Opcode <10:0>
4.3.1
COMPUTED GOTO
A computed GOTO is 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
application note “Implementing a Table Read" (AN556).
1997 Microchip Technology Inc.
Preliminary
DS40143B-page 21
MICROCHIP [ MICROCHIP ]
