PIC18F2331/2431/4331/4431
Example 6-1 shows a source code example that uses
the Fast Register Stack during a subroutine call and
return.
6.1.2.3
PUSHand POPInstructions
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
current PC value onto the stack. TOSU, TOSH and
TOSL can then be modified to place data or a return
address on the stack.
EXAMPLE 6-1:
FAST REGISTER STACK
CODE EXAMPLE
CALL SUB1, FAST
;STATUS, WREG, BSR
;SAVED IN FAST REGISTER
;STACK
The PUSHinstruction places the current PC value onto
the stack. This increments the Stack Pointer and loads
the current PC value onto the stack. The POPinstruc-
tion discards the current TOS by decrementing the
Stack Pointer. The previous value pushed onto the
stack then becomes the TOS value.
SUB1
RETURN FAST
;RESTORE VALUES SAVED
;IN FAST REGISTER STACK
6.1.4
LOOK-UP TABLES IN PROGRAM
MEMORY
6.1.2.4
Stack Full/Underflow Resets
These Resets are enabled by programming the
STVREN bit in Configuration Register 4L. When the
STVREN bit is cleared, a full or underflow condition will
set the appropriate STKFUL or STKUNF bit, but not
cause a device Reset. When the STVREN 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.
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 two ways:
• Computed GOTO
• Table Reads
6.1.4.1
Computed GOTO
A computed GOTOis accomplished by adding an offset
to the program counter. An example is shown in
Example 6-2.
6.1.3
FAST REGISTER STACK
A Fast Register Stack is provided for the STATUS,
WREG and BSR registers, to provide a “fast return”
option for interrupts. 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
interrupt sources will push values into the stack
registers.
A look-up table can be formed with an ADDWF PCL
instruction and a group of RETLWnn 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 PCLinstruction. The next
instruction executed will be one of the RETLW nn
instructions that returns the value “nn” to the calling
function.
The values in the registers are then loaded back into
their associated registers if the RETFIE, FASTinstruc-
tion 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 inter-
rupts. 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.
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 can be stored in
each instruction location and room on the return
address stack is required.
EXAMPLE 6-2:
COMPUTED GOTOUSING
AN OFFSET VALUE
If interrupt priority is not used, all interrupts may use the
Fast Register Stack for returns from 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 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.
MOVFW
CALL
OFFSET
TABLE
ORG
TABLE
0xnn00
ADDWF
RETLW
RETLW
RETLW
PCL
0xnn
0xnn
0xnn
.
.
.
DS39616D-page 64
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
