Datasheet

PIC16(L)F1512/3
DS41624B-page 30 Preliminary  2012 Microchip Technology Inc.
3.3 PCL and PCLATH
The Program Counter (PC) is 15 bits wide. The low byte
comes from the PCL register, which is a readable and
writable register. The high byte (PC<14:8>) is not directly
readable or writable and comes from PCLATH. On any
Reset, the PC is cleared. Figure 3-4 shows the five
situations for the loading of the PC.
FIGURE 3-4: LOADING OF PC IN
DIFFERENT SITUATIONS
3.3.1 MODIFYING PCL
Executing any instruction with the PCL register as the
destination simultaneously causes the Program
Counter PC<14:8> bits (PCH) to be replaced by the
contents of the PCLATH register. This allows the entire
contents of the PC to be changed by writing the desired
upper seven bits to the PCLATH register. When the
lower eight bits are written to the PCL register, all 15
bits of the PC will change to the values contained in the
PCLATH register and those being written to the PCL
register.
3.3.2 COMPUTED GOTO
A computed GOTO is 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 AN556, “Implementing a Table Read” (DS00556).
3.3.3 COMPUTED FUNCTION CALLS
A computed function CALL allows programs to maintain
tables of functions and provide another way to execute
state machines or look-up tables. When performing a
table read using a computed function CALL, care
should be exercised if the table location crosses a PCL
memory boundary (each 256-byte block).
If using the CALL instruction, the PCH<2:0> and PCL
registers are loaded with the operand of the CALL
instruction. PCH<6:3> is loaded with PCLATH<6:3>.
The CALLW instruction enables computed calls by
combining PCLATH and W to form the destination
address. A computed CALLW is accomplished by
loading the W register with the desired address and
executing CALLW. The PCL register is loaded with the
value of W and PCH is loaded with PCLATH.
3.3.4 BRANCHING
The branching instructions add an offset to the PC.
This allows relocatable code and code that crosses
page boundaries. There are two forms of branching,
BRW and BRA. The PC will have incremented to fetch
the next instruction in both cases. When using either
branching instruction, a PCL memory boundary may be
crossed.
If using BRW, load the W register with the desired
unsigned address and execute BRW. The entire PC will
be loaded with the address PC + 1 + W.
If using BRA, the entire PC will be loaded with PC + 1 +,
the signed value of the operand of the BRA instruction.
PCLPCH
0
14
PC
06
7
ALU Result
8
PCLATH
PCLPCH
0
14
PC
06
4
OPCODE <10:0>
11
PCLATH
PCLPCH
0
14
PC
06
7
W
8
PCLATH
Instruction with
PCL as
Destination
GOTO, CALL
CALLW
PCL
PCH
0
14
PC
PC + W
15
BRW
PCLPCH
0
14
PC
PC + OPCODE <8:0>
15
BRA