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2010-2012 Microchip Technology Inc.

DS39977F-page 103

PIC18F66K80 FAMILY

6.1.2

PROGRAM COUNTER

The Program Counter (PC) specifies the address of the

instruction to fetch for execution. The PC is 21 bits wide

and 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 and 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

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 6.1.5.1 “Computed GOTO”).

The PC addresses bytes in the program memory. To

prevent the PC from becoming misaligned with word

instructions, the Least Significant bit (LSb) of PCL is

fixed to a value of ‘0’. The PC increments by two 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.

6.1.3

RETURN ADDRESS STACK

The return address stack enables execution of any

combination of up to 31 program calls and interrupts.

The PC is pushed onto the stack when a CALL or

RCALL

instruction is executed or an interrupt is

Acknowledged. The PC value is pulled off the stack on

a RETURN, RETLW or a RETFIE instruction. The value

is also pulled off the stack on ADDULNK and SUBULNK

instructions if the extended instruction set is enabled.

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 (TOS) 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, has overflowed or has underflowed.

6.1.3.1

Top-of-Stack Access

Only the top of the return address stack is readable and

writable. A set of three registers, TOSU:TOSH:TOSL,

holds the contents of the stack location pointed to by

the STKPTR register (Figure 6-3). This allows users to

implement a software stack, if necessary. After a CALL,

RCALL

or interrupt (or ADDULNK and SUBULNK instruc-

tions, if the extended instruction set is enabled), 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

TOSU:TOSH:TOSL and do a return.

While accessing the stack, users must disable the

Global Interrupt Enable bits to prevent inadvertent

stack corruption.

FIGURE 6-3:

RETURN ADDRESS STACK AND ASSOCIATED REGISTERS

00011

001A34h

11111

11110

11101

00010

00001

00000

00010

Return Address Stack<20:0>

Top-of-Stack

000D58h

TOSL

TOSH

TOSU

34h

1Ah

00h

STKPTR<4:0>

Top-of-Stack Registers

Stack Pointer

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