PIC18F2331/2431/4331/4431
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 the Top-of-Stack SFRs. Status bits
indicate if the stack is full, has overflowed or underflowed.
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 contained in three 8-bit registers. The low byte,
known as the PCL register, is both readable and writ-
able. The high byte (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 the PCU register
and contains the bits, PC<20:16>. This register is also
not directly readable or writable. Updates to the PCU
register are performed through the PCLATU register.
6.1.2.1
Top-of-Stack Access
The top of the stack is readable and writable. Three
register locations, TOSU, TOSH and TOSL, hold 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,
RCALLor interrupt, the software can read the pushed
value by reading the TOSU, TOSH and TOSL registers.
These values can be placed on a user-defined software
stack. At return time, the software can replace the
TOSU, TOSH and TOSL and do a return.
The contents of PCLATH and PCLATU are transferred
to the program counter by any operation that writes to
the 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 user must disable the global interrupt enable bits
while accessing the stack to prevent inadvertent stack
corruption.
The PC addresses bytes in the program memory. To
prevent the PC from becoming misaligned with word
instructions, the Least Significant bit of the PCL is fixed
to a value of ‘0‘. The PC increments by two to address
sequential instructions in the program memory.
6.1.2.2
Return Stack Pointer (STKPTR)
The STKPTR register (Register 6-1) contains the Stack
Pointer value, the STKFUL (Stack Full) status bit and
the STKUNF (Stack Underflow) status bits. The value
of the Stack Pointer can be 0 through 31. The Stack
Pointer increments before values are pushed onto the
stack and decrements after values are popped off the
stack. At Reset, the Stack Pointer value will be zero.
The user may read and write the Stack Pointer value.
This feature can be used by a Real-Time Operating
System (RTOS) for return stack maintenance.
The CALL, RCALL, GOTOand program branch instruc-
tions 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
(Program Counter) is pushed onto the stack when a
CALLor RCALLinstruction 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
RETURNor CALLinstructions.
After the PC is pushed onto the stack 31 times (without
popping any values off the stack), the STKFUL bit is
set. The STKFUL bit is cleared by software or by a
POR.
The action that takes place when the stack becomes
full depends on the state of the STVREN (Stack Over-
flow Reset Enable) Configuration bit. (Refer to
Section 23.1 “Configuration Bits” for a description of
the device Configuration bits.) If STVREN is set
(default), the 31st push will push the (PC + 2) value
onto the stack, set the STKFUL bit and reset the
device. The STKFUL bit will remain set and the Stack
Pointer will be set to zero.
The stack operates as a 31-word by 21-bit RAM and a
5-bit Stack Pointer, with the Stack Pointer initialized to
00000b after all Resets. There is no RAM associated
with Stack Pointer, 00000b. This is only a Reset value.
During a CALLtype instruction, causing a push onto the
stack, the Stack Pointer is first incremented and the
RAM location pointed to by the Stack Pointer is written
with the contents of the PC (already pointing to the
instruction following the CALL). During a RETURNtype
instruction, causing a pop from the stack, the contents
of the RAM location pointed to by the STKPTR are
transferred to the PC and then the Stack Pointer is
decremented.
If STVREN is cleared, the STKFUL bit will be set on the
31st push and the Stack Pointer will increment to 31.
Any additional pushes will not overwrite the 31st push
and STKPTR will remain at 31.
DS39616D-page 62
2010 Microchip Technology Inc.
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