Datasheet

© 2008 Microchip Technology Inc. DS41319B-page 19
PIC12F519
4.6 Program Counter
As a program instruction is executed, the Program
Counter (PC) will contain the address of the next
program instruction to be executed. The PC value is
increased by one every instruction cycle, unless an
instruction changes the PC.
For a GOTO instruction, bits <8:0> of the PC are pro-
vided by the GOTO instruction word. The Program
Counter (PCL) is mapped to PC<7:0>. Bit 5 of the STA-
TUS register provides page information to bit 9 of the
PC (Figure 4-3).
For a CALL instruction, or any instruction where the
PCL is the destination, bits <7:0> of the PC again are
provided by the instruction word. However, PC<8>
does not come from the instruction word, but is always
cleared (Figure 4-3).
Instructions where the PCL is the destination, or modify
PCL instructions, include MOVWF PCL, ADDWF PCL
and BSF PCL,5.
FIGURE 4-3: LOADING OF PC
BRANCH INSTRUCTIONS
4.6.1 EFFECTS OF RESET
The PC is set upon a Reset, which means that the PC
addresses the last location in the last page (i.e., the
oscillator calibration instruction). After executing
MOVLW XX, the PC will roll over to location 00h and
begin executing user code.
The STATUS register page preselect bits are cleared
upon a Reset, which means that page 0 is pre-selected.
Therefore, upon a Reset, a GOTO instruction will
automatically cause the program to jump to page 0 until
the value of the page bits is altered.
4.7 Stack
The PIC12F519 device has a two-deep, 12-bit wide
hardware PUSH/POP stack.
A CALL instruction will PUSH the current value of Stack
1 into Stack 2 and then PUSH the current PC value,
incremented by one, into Stack Level 1. If more than two
sequential CALLs are executed, only the most recent two
return addresses are stored.
A RETLW instruction will POP the contents of Stack
Level 1 into the PC and then copy Stack Level 2
contents into Stack Level 1. If more than two sequential
RETLWs are executed, the stack will be filled with the
address previously stored in Stack Level 2. Note that
the W register will be loaded with the literal value
specified in the instruction. This is particularly useful for
the implementation of data look-up tables within the
program memory.
Note: Because PC<8> is cleared in the CALL
instruction or any modify PCL instruction,
all subroutine calls or computed jumps are
limited to the first 256 locations of any
program memory page (512 words long).
PA0
Status
PC
87 0
PCL
910
Instruction Word
70
GOTO Instruction
CALL or Modify PCL Instruction
PA0
Status
PC
87 0
PCL
910
Instruction Word
70
Reset to ‘0’
Note 1: There are no Status bits to indicate stack
overflows or stack underflow conditions.
2: There are no instruction mnemonics
called PUSH or POP. These are actions
that occur from the execution of the CALL
and RETLW instructions.