PIC18F1220ESLASHSOSQTP datasheet(45/308 Pages) MICROCHIP

45 / 308 page

DS39605F-page 43

PIC18F1220/1320

STKPTR REGISTER

5.2.3

PUSH AND POP INSTRUCTIONS

Since the Top-of-Stack (TOS) is readable and writable,

the ability to push values onto the stack and pull values

off the stack, without disturbing normal program execu-

tion, is a desirable option. To push the current PC value

onto the stack, a PUSH instruction can be executed.

This will increment the Stack Pointer and load the cur-

rent PC value onto the stack. TOSU, TOSH and TOSL

can then be modified to place data or a return address

on the stack.

The ability to pull the TOS value off of the stack and

replace it with the value that was previously pushed

onto the stack, without disturbing normal execution, is

achieved by using the POP instruction. The POP instruc-

tion discards the current TOS by decrementing the

Stack Pointer. The previous value pushed onto the

stack then becomes the TOS value.

5.2.4

STACK FULL/UNDERFLOW RESETS

These Resets are enabled by programming the STVR

bit in Configuration Register 4L. When the STVR bit is

cleared, a full or underflow condition will set the appro-

priate STKFUL or STKUNF bit, but not cause a device

Reset. When the STVR bit is set, a full or underflow

condition will set the appropriate STKFUL or STKUNF

bit and then cause a device Reset. The STKFUL or

STKUNF bits are cleared by the user software or a

Power-on Reset.

R/C-0

U-0

R/W-0

STKFUL

STKUNF

—

SP4

SP3

SP2

SP1

SP0

bit 7

bit 0

bit 7(1)

STKFUL: Stack Full Flag bit

= Stack became full or overflowed

= Stack has not become full or overflowed

bit 6(1)

STKUNF: Stack Underflow Flag bit

= Stack underflow occurred

= Stack underflow did not occur

bit 5

Unimplemented: Read as ‘0’

bit 4-0

SP4:SP0: Stack Pointer Location bits

Note 1: Bit 7 and bit 6 are cleared by user software or by a POR.

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented

C = Clearable only bit

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

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