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PIC18F8722 FAMILY
DS39646B-page 68
Preliminary
2004 Microchip Technology Inc.
5.1.3.4
Stack Full and Underflow Resets
Device Resets on stack overflow and stack underflow
conditions are enabled by setting the STVREN bit in
Configuration Register 4L. When STVREN is set, a full
or underflow will set the appropriate STKFUL or
STKUNF bit and then cause a device Reset. When
STVREN is cleared, a full or underflow condition will set
the appropriate STKFUL or STKUNF bit, but not cause
a device Reset. The STKFUL or STKUNF bits are
cleared by the user software or a Power-on Reset.
5.1.4
FAST REGISTER STACK
A fast register stack is provided for the Status, WREG
and BSR registers, to provide a “fast return” option for
interrupts. The stack for each register is only one level
deep and is neither readable nor writable. It is loaded
with the current value of the corresponding register
when the processor vectors for an interrupt. All inter-
rupt sources will push values into the stack registers.
The values in the registers are then loaded back into
their associated registers if the
RETFIE, FAST
instruction is used to return from the interrupt.
If both low and high priority interrupts are enabled, the
stack registers cannot be used reliably to return from
low priority interrupts. If a high priority interrupt occurs
while servicing a low priority interrupt, the stack register
values stored by the low priority interrupt will be
overwritten. In these cases, users must save the key
registers in software during a low priority interrupt.
If interrupt priority is not used, all interrupts may use the
fast register stack for returns from interrupt. If no inter-
rupts 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
CALL
label
,
FAST
instruction
must be executed to save the STATUS, WREG and
BSR registers to the fast register stack. A
RETURN
,
FAST
instruction is then executed to restore
these registers from the fast register stack.
Example 5-1 shows a source code example that uses
the fast register stack during a subroutine call and return.
EXAMPLE 5-1:
FAST REGISTER STACK
CODE EXAMPLE
;STATUS, WREG, BSR
;SAVED IN FAST REGISTER
;STACK
5.1.5
LOOK-UP TABLES IN PROGRAM
MEMORY
There may be programming situations that require the
creation of data structures, or look-up tables, in
program memory. For PIC18 devices, look-up tables
can be implemented in two ways:
Computed
GOTO
Table Reads
5.1.5.1
A computed
GOTO
is accomplished by adding an offset
to the program counter. An example is shown in
Example 5-2.
A look-up table can be formed with an
ADDWF PCL
instruction and a group of
RETLW nn
instructions. The W
register is loaded with an offset into the table before exe-
cuting a call to that table. The first instruction of the called
routine is the
ADDWF
PCL
instruction. The next instruction
executed will be one of the
RETLW nn
instructions that
returns the value ‘
nn
’ to the calling function.
The offset value (in WREG) specifies the number of
bytes that the program counter should advance and
should be multiples of 2 (LSb =
0
).
In this method, only one data byte may be stored in
each instruction location and room on the return
address stack is required.
Computed GOTO
EXAMPLE 5-2:
COMPUTED GOTO USING AN OFFSET VALUE
Note:
The “
ADDWF PCL
” instruction does not
update the PCLATH and PCLATU registers.
A read operation on PCL must be performed
to update PCLATH and PCLATU.
CALL SUB1, FAST
SUB1
RETURN, FAST
;RESTORE VALUES SAVED
;IN FAST REGISTER STACK
MAIN:
ORG
MOVLW
CALL
0x0000
0x00
TABLE
…
ORG
MOVF
RLNCF
ADDWF
RETLW
RETLW
RETLW
RETLW
RETLW
END
0x8000
PCL, F
W, W
PCL
‘A’
‘B’
‘C’
‘D’
‘E’
TABLE
; A simple read of PCL will update PCLATH, PCLATU
; Multiply by 2 to get correct offset in table
; Add the modified offset to force jump into table