Datasheet

PIC18F2221/2321/4221/4321 FAMILY
DS39689F-page 60 © 2009 Microchip Technology Inc.
6.1.1 PROGRAM COUNTER
The Program Counter (PC) specifies the address of the
instruction to fetch for execution. The PC is 21 bits wide
and is contained in three separate 8-bit registers. The
low byte, known as the PCL register, is both readable
and writable. The high byte, or PCH register, contains
the PC<15:8> bits; it is not directly readable or writable.
Updates to the PCH register are performed through the
PCLATH register. The upper byte is called PCU. This
register contains the PC<20:16> bits; it is also not
directly readable or writable. Updates to the PCU
register are performed through the PCLATU register.
The contents of PCLATH and PCLATU are transferred
to the program counter by any operation that writes
PCL. Similarly, the upper two bytes of the program
counter are transferred to PCLATH and PCLATU by an
operation that reads PCL. This is useful for computed
offsets to the PC (see Section 6.1.4.1 “Computed
GOTO”).
The PC addresses bytes in the program memory. To
prevent the PC from becoming misaligned with word
instructions, the Least Significant bit of PCL is fixed to
a value of ‘0’. The PC increments by 2 to address
sequential instructions in the program memory.
The CALL, RCALL, GOTO and program branch
instructions write to the program counter directly. For
these instructions, the contents of PCLATH and
PCLATU are not transferred to the program counter.
6.1.2 RETURN ADDRESS STACK
The return address stack allows any combination of up
to 31 program calls and interrupts to occur. The PC is
pushed onto the stack when a CALL or RCALL instruc-
tion is executed or an interrupt is Acknowledged. The
PC value is pulled off the stack on a RETURN, RETLW
or a RETFIE instruction. PCLATU and PCLATH are not
affected by any of the RETURN or CALL instructions.
The stack operates as a 31-word by 21-bit RAM and a
5-bit Stack Pointer, STKPTR. The stack space is not
part of either program or data space. The Stack Pointer
is readable and writable and the address on the top of
the stack is readable and writable through the Top-of-
Stack Special Function Registers. Data can also be
pushed to, or popped from the stack, using these
registers.
A CALL type instruction causes a push onto the stack;
the Stack Pointer is first incremented and the location
pointed to by the Stack Pointer is written with the
contents of the PC (already pointing to the instruction
following the CALL). A RETURN type instruction causes
a pop from the stack; the contents of the location
pointed to by the STKPTR are transferred to the PC
and then the Stack Pointer is decremented.
The Stack Pointer is initialized to ‘00000’ after all
Resets. There is no RAM associated with the location
corresponding to a Stack Pointer value of ‘00000’; this
is only a Reset value. Status bits indicate if the stack is
full or has overflowed or has underflowed.
6.1.2.1 Top-of-Stack Access
Only the top of the return address stack (TOS) is
readable and writable. A set of three registers,
TOSU:TOSH:TOSL, hold the contents of the stack
location pointed to by the STKPTR register (Figure 6-2).
This allows users to implement a software stack if
necessary. After a CALL, RCALL or interrupt, the
software can read the pushed value by reading the
TOSU:TOSH:TOSL registers. These values can be
placed on a user-defined software stack. At return time,
the software can return these values to
TOSU:TOSH:TOSL and do a return.
The user must disable the global interrupt enable bits
while accessing the stack to prevent inadvertent stack
corruption.
FIGURE 6-2: RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
00011
001A34h
11111
11110
11101
00010
00001
00000
00010
Return Address Stack <20:0>
To p- of -Sta ck
000D58h
TOSLTOSHTOSU
34h1Ah00h
STKPTR<4:0>
Top-of-Stack Registers Stack Pointer