PIC18F2585/2680/4585/4680
DS39625B-page 64
Preliminary
2004 Microchip Technology Inc.
5.1.2.4
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.
5.1.3
FAST REGISTER STACK
A fast register stack is provided for the Status, WREG
and BSR registers, to provide a “fast return” option for
interrupts. Each stack 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 interrupt 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 regis-
ters 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
5.1.4
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
5.1.4.1
Computed GOTO
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
executing 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.
EXAMPLE 5-2:
COMPUTED GOTO USING
AN OFFSET VALUE
OFFSET, W
TABLE
5.1.4.2
A better method of storing data in program memory
allows two bytes of data to be stored in each instruction
location.
Look-up table data may be stored two bytes per
program word by using table reads and writes. The
Table Pointer (TBLPTR) register specifies the byte
address and the Table Latch (TABLAT) register
contains the data that is read from or written to program
memory. Data is transferred to or from program
memory one byte at a time.
Table read and table write operations are discussed
further in
Section 6.1 “Table Reads and Table
Writes”
.
Table Reads and Table Writes
CALL
SUB1, FAST
SUB1
RETURN, FAST
;RESTORE VALUES SAVED
;IN FAST REGISTER STACK
MOVF
CALL
nn00h
ADDWF
RETLW
RETLW
RETLW
.
.
.
ORG
TABLE
PCL
nnh
nnh
nnh