SE9 datasheet(49/478 Pages) MICROCHIP

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DS41414D-page 49

PIC16(L)F1946/1947

3.5

Stack

All devices have a 16-level x 15-bit wide hardware

stack (refer to Figures 3-4 and 3-5). The stack space is

not part of either program or data space. The PC is

PUSHed onto the stack when CALL or CALLW instruc-

tions are executed or an interrupt causes a branch. The

stack is POPed in the event of a RETURN, RETLW or a

RETFIE instruction execution. PCLATH is not affected

by a PUSH or POP operation.

The stack operates as a circular buffer if the STVREN

bit is programmed to ‘0‘ (Configuration Words). This

means that after the stack has been PUSHed sixteen

times, the seventeenth PUSH overwrites the value that

was stored from the first PUSH. The eighteenth PUSH

overwrites the second PUSH (and so on). The

STKOVF and STKUNF flag bits will be set on an Over-

flow/Underflow, regardless of whether the Reset is

enabled.

3.5.1

ACCESSING THE STACK

The stack is available through the TOSH, TOSL and

STKPTR registers. STKPTR is the current value of the

Stack Pointer. TOSH:TOSL register pair points to the

TOP of the stack. Both registers are read/writable. TOS

is split into TOSH and TOSL due to the 15-bit size of the

PC. To access the stack, adjust the value of STKPTR,

which will position TOSH:TOSL, then read/write to

TOSH:TOSL. STKPTR is 5 bits to allow detection of

overflow and underflow.

During normal program operation, CALL, CALLW and

Interrupts will increment STKPTR while RETLW,

RETURN, and RETFIE will decrement STKPTR. At any

time STKPTR can be inspected to see how much stack

is left. The STKPTR always points at the currently used

place on the stack. Therefore, a CALL or CALLW will

increment the STKPTR and then write the PC, and a

return will unload the PC and then decrement the STK-

PTR.

Reference Figure through Figure for examples of

accessing the stack.

FIGURE 3-5:

ACCESSING THE STACK EXAMPLE 1

Note 1: There are no instructions/mnemonics

called PUSH or POP. These are actions

that occur from the execution of the

CALL, CALLW, RETURN, RETLW and

RETFIE instructions or the vectoring to

an interrupt address.

Note:

Care should be taken when modifying the

STKPTR while interrupts are enabled.

0x0F

0x0E

0x0D

0x0C

0x0B

0x0A

0x09

0x08

0x07

0x06

0x05

0x04

0x03

0x02

0x01

0x00

0x0000

STKPTR = 0x1F

Initial Stack Configuration:

After Reset, the stack is empty. The

empty stack is initialized so the Stack

Pointer is pointing at 0x1F. If the Stack

Overflow/Underflow Reset is enabled, the

TOSH/TOSL registers will return ‘0’. If

the Stack Overflow/Underflow Reset is

disabled, the TOSH/TOSL registers will

return the contents of stack address 0x0F.

0x1F

STKPTR = 0x1F

Stack Reset Disabled

(STVREN = 0)

Stack Reset Enabled

(STVREN = 1)

TOSH:TOSL

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