PIC18F2220/2320/4220/4320
5.2.2
RETURN STACK POINTER
(STKPTR)
5.2
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, RETLWor a RETFIEinstruction. PCLATU
and PCLATH are not affected by any of the RETURNor
CALLinstructions.
The STKPTR register (Register 5-1) 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 for return stack maintenance.
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.
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 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 writ-
able through the top-of-stack Special File 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.
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.
When the stack has been popped enough times to
unload the stack, the next pop will return a value of zero
to the PC and sets the STKUNF bit, while the stack
pointer remains at zero. The STKUNF bit will remain
set until cleared by software or a POR occurs.
5.2.1
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 5-3). 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.
Note:
Returning a value of zero to the PC on an
underflow has the effect of vectoring the
program to the Reset vector, where the
stack conditions can be verified and
appropriate actions can be taken. This is
not the same as a Reset, as the contents
of the SFRs are not affected.
The user must disable the global interrupt enable bits
while accessing the stack to prevent inadvertent stack
corruption.
FIGURE 5-3:
RETURN ADDRESS STACK AND ASSOCIATED REGISTERS
Return Address Stack
11111
11110
11101
STKPTR<4:0>
TOSU
00h
TOSH
1Ah
TOSL
34h
00010
00011
001A34h 00010
000D58h 00001
00000
Top-of-Stack
DS39599C-page 54
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
