PIC16LF18854 Datasheet, PDF (84/668 Pages) Microchip Technology – C Compiler Optimized RISC Architecture

PIC16(L)F18856/76
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 3-3 shows the five
situations for the loading of the PC.
FIGURE 3-3:
LOADING OF PC IN
DIFFERENT SITUATIONS
Rev. 10-000042A
7/30/2013
14 PCH
PC
67
0
PCLATH
PCL
8
0 _Instruction
with PCL as
Destination
ALU result
14 PCH
PC
64
PCLATH
PCL 0
0 ¹¹
OPCODE <10:0>
GOTO,
CALL
14 PCH
PC
67
0
PCLATH
PCL 0
CALLW
8
W
14 PCH
PC
PCL 0
BRW
15
PC + W
14 PCH
PC
PCL 0
BRA
15
PC + OPCODE <8:0>
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 program counter to be changed by writ-
ing the desired upper seven bits to the PCLATH regis-
ter. When the lower eight bits are written to the PCL
register, all 15 bits of the program counter 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 program counter (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 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.
DS40001824A-page 84
Preliminary
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