PIC18LC658-I/CL View Datasheet(PDF) - Microchip Technology
Part Name
Description
Manufacturer
PIC18LC658-I/CL Datasheet PDF : 366 Pages
| |||
PIC18CXX8
4.2.3 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 a return address on the
stack.
The POP instruction discards the current TOS by decre-
menting the stack pointer. The previous value pushed
onto the stack then becomes the TOS value.
4.2.4 STACK FULL/UNDERFLOW RESETS
These RESETs are enabled by programming the
STVREN configuration bit. When the STVREN bit is
disabled, a full or underflow condition will set the appro-
priate STKFUL or STKUNF bit, but not cause a device
RESET. When the STVREN bit is enabled, a full or
underflow will set the appropriate STKFUL or STKUNF
bit and then cause a device RESET. The STKFUL or
STKUNF bits are only cleared by the user software or
a POR.
4.3 Fast Register Stack
A “fast return” option is available for interrupts and
calls. A fast register stack is provided for the STATUS,
WREG and BSR registers and is only one layer in
depth. The stack is not readable or writable and is
loaded with the current value of the corresponding reg-
ister when the processor vectors for an interrupt. The
values in the fast register stack are then loaded back
into the working registers if the fast return instruc-
tion is used to return from the interrupt.
A low or high priority interrupt source will push values
into the stack registers. If both low and high priority
interrupts are enabled, the stack registers cannot be
used reliably for low priority interrupts. If a high priority
interrupt occurs while servicing a low priority interrupt,
the stack register values stored by the low priority inter-
rupt will be overwritten.
If high priority interrupts are not disabled during low pri-
ority interrupts, users must save the key registers in
software during a low priority interrupt.
If no interrupts are used, the fast register stack can be
used to restore the STATUS, WREG and BSR registers
at the end of a subroutine call. To use the fast register
stack for a subroutine call, a fast call instruction
must be executed.
Example 4-1 shows a source code example that uses
the fast register stack.
EXAMPLE 4-1: FAST REGISTER STACK
CODE EXAMPLE
CALL SUB1, FAST
•
•
;STATUS, WREG, BSR
;SAVED IN FAST REGISTER
;STACK
SUB1 •
•
•
RETURN FAST
;RESTORE VALUES SAVED
;IN FAST REGISTER STACK
DS30475A-page 44
Advanced Information
2000 Microchip Technology Inc.
Share Link: 