Datasheet

 2001-2013 Microchip Technology Inc. DS39598F-page 23
PIC16F818/819
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 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
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).
FIGURE 2-5: LOADING OF PC IN
DIFFERENT SITUATIONS
2.3.1 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 AN556, “Implementing a Table Read”
(DS00556).
2.3.2 STACK
The PIC16F818/819 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.
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).
2.4 Indirect Addressing: INDF and
FSR Registers
The INDF register is not a physical register. Addressing
INDF actually addresses the register whose address is
contained in the FSR register (FSR is a pointer). This is
indirect addressing.
EXAMPLE 2-1: INDIRECT ADDRESSING
Reading INDF itself indirectly (FSR = 0) will produce
00h. Writing to the INDF register indirectly results in a
no operation (although status bits may be affected).
A simple program to clear RAM locations, 20h-2Fh,
using indirect addressing is shown in Example 2-2.
EXAMPLE 2-2: HOW TO CLEAR RAM
USING INDIRECT
ADDRESSING
An effective 9-bit address is obtained by concatenating
the 8-bit FSR register and the IRP bit (Status<7>) as
shown in Figure 2-6.
PC
12 8 7 0
5
PCLATH<4:0>
PCLATH
Instruction with
ALU
GOTO,CALL
Opcode <10:0>
8
PC
12 11 10 0
11
PCLATH<4:3>
PCH PCL
87
2
PCLATH
PCH PCL
PCL as
Destination
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.
• Register file 05 contains the value 10h
• Register file 06 contains the value 0Ah
• Load the value 05 into the FSR register
• A read of the INDF register will return the value
of 10h
• Increment the value of the FSR register by one
(FSR = 06)
• A read of the INDF register now will return the
value of 0Ah
MOVLW 0x20 ;initialize pointer
MOVWF FSR ;to RAM
NEXT CLRF INDF ;clear INDF register
INCF FSR ;inc pointer
BTFSS FSR, 4 ;all done?
GOTO NEXT ;NO, clear next
CONTINUE
: ;YES, continue