Another way to swap the contents of two files without using a temporary file:

SWAP THE CONTENTS OF A FILE WITH W  
Example: File 3C = 81h   W = 47h
               after operation:   File 3C = 47h   W = 81h
 

XORWF 3C,1 XORWF 3C,0  XORWF 3C,1

Switch (button, key) see also Poll
There is no difference between "Switch," "Button" and "Key."  They all refer to a push button that has momentary action. It can be an individual push button, one of a number of buttons or in a matrix of buttons such as a keypad. 
With all button instructions, there are three important things to remember. They are mechanical devices and their action is very slow in "microprocessor-time." They produce a lot of pulses when pushed and also when released (this is called switch-noise or switch-bounce). In any button routine, you must also prevent a button-press being registered more than once. 
Many sub-routines already have a switch or button feature included in them.  Select an existing sub-routine or use the following. 
The most important part of adding a switch or button routine is creating the debounce feature. 
This is to prevent "double-counting" when the button is pushed or released. Try slow-pressing and slow-release to see if the program produces a false result. If the program "double-counts," you will have to increase the debounce-time. 
Debounce can take up a lot of computer-time. Debounce is a delay routine that "masks" (hides - or disallows) the time when the button is pressed or released, so that only one pulse is detected. Instead of producing a separate debounce routines, you may be able to use the time taken to execute other sub-routines. In other words, the program looks before and after the execution of another routine and if the button is still pressed, the micro detects it as a "button-press." 
Finally, you need to detect the first press of a button and prevent the program operating on the button during the second pass of the program. 
The basis of detecting a button-press consists of 6 separate items that must be placed in the following order:
There are two flags. Bit0 in file 1F is the Debounce Flag and Bit1 in file 1F is the Button Pressed flag. 
The microprocessor executes Main and CALLs Sw. If Sw detects a key-press, two flags are SET. The first is the Button Pressed flag and the second is the Debounce flag.
The micro returns to Main and tests the Button Pressed flag to see if it is SET. If is is SET, the micro goes to a sub-routine such as CALL Increment, where a value can be incremented. The Button Pressed flag is then cleared and the micro CALLs Sw. If the switch is still pressed, the micro will return. The program is looking for the button to be released. When the button is released, the Sw sub-routine is ready to detect another button-push.

Table see also Output a Table Value and CALL Table
Suppose you need to turn on LEDs to produce the pips on the side of a dice (die). This can be done via a table.

In a sub-routine, a value between 1 and 6 is created and put into "w" and Table is called thus:

       movlw    03h      ;any value 1 - 6 is put into w
       call        Table    ; the micro will go to the table and return with 5Bh
       movwf    temp1  ; 5Bh will be put into the temporary file

To prevent the micro jumping beyond the end of the table, the instruction:

      andlw     06h        is used when the table contains 6 elements.

The layout for a Table is shown below:

	
cycle  
 
	movlw    30h
	xorwf    gpio,1    ;to toggle GP4 GP5
	movlw    .7
	movwf    loops
	call     delay
	call     delay
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	decfsz   loops,1
	goto     $-6
	call     delay
	call     delay
	movlw    20h
	xorwf    gpio,1    ;to toggle GP5
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	movlw    .7
	movwf    loops
	call     delay
	call     delay
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	decfsz   loops,1
	goto     $-6
	call     delay
	movlw    20h
	xorwf    gpio,1    ;to toggle GP5
	call     delay
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	movlw    .7
	movwf    loops
	call     delay
	call     delay
	call     delay
	movlw    30h
	xorwf    gpio,1    ;to toggle GP4 GP5
	decfsz   loops,1
	goto     $-6
	movlw    .7
	movwf    loops
	call     delay
	call     delay
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	decfsz   loops,1
	goto     $-6
	call     delay
	call     delay
	movlw    20h
	xorwf    gpio,1    ;to toggle GP5
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	movlw    .7
	movwf    loops
	call     delay
	call     delay
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	decfsz   loops,1
	goto     $-6
	call     delay
	movlw    20h
	xorwf    gpio,1    ;to toggle GP5
	call     delay
	call     delay
	movlw    10h
	xorwf    gpio,1    ;to toggle GP4
	movlw    .7
	movwf    loops
	call     delay
	call     delay
	call     delay
	movlw    30h
	xorwf    gpio,1    ;to toggle GP4 GP5
	decfsz   loops,1
	goto     $-6
	goto     cycle


delay    ;7246uS

	movlw    .8
	movwf    fileA
	movlw    .225
	movwf    fileB
	nop
	decfsz   fileB,1
	goto      $-2
	decfsz   fileA,1
	goto      $-4
	retlw    00

TRISIO also known as "tris"
The file that determines the in/out state of each line (pin) for the in/out Port is the TRISIO file. 
It is located in Bank1 of the microcontroller.  It has the value 85 for PIC12F629 to control the GPIO lines. For a PIC16F628 it is 85h for PortA and 86h for PortB.  For the PIC10F200 series, the tris register is not given an address number and is only accessed by the mnemonic "trisgpio"
To access the tris register. 
 GPIO has GP0, GP1, GP2, GP3, GP4 and GP5.  Eg: MOVLW 0FFh will only affect the 6 input lines of GPIO.  Bits 6, and 7 of a value will not have any effect.
There are two ways to get to the TRISIO file. It can be accessed directly by adding the following two instructions directly into your program at any location:
Note: We have found the instruction: movwf   tris, DOES NOT WORK for some micros. Use the instructions BSF 03,5 and BCF 03,5

Writing to TRIS Register:

   movlw       b'00000000'
   tris           gpio

The following has been tested in World's Simplest Program. You can bit-set the TRISIO file:

bsf      status, rp0 bcf      TRISIO,4 bcf      status, rp0 ;bank 1 ;make GP4 output ;bank 0

Or set any/all the bits via the w register:
 

bsf        status, rp0 movlw   b'11101111' movwf   TRISIO bcf        status, rp0 ;bank 1 ;bank 0

MOVLW 01h MOVWF TRISIO  or movlw    01h tris        gpio ;Put 01 into W ;Make GP0 an input and all others output ;this instruction has been tested and works for 12F629

The following instructions apply to the other lines: GP3 is input ONLY

PortA:   MOVLW 02h MOVWF TRISIO MOVLW 04h MOVWF TRISIO MOVLW 08h MOVWF TRISIO MOVLW 10h MOVWF TRISIO MOVLW 20h MOVWF TRISIO MOVLW 3Fh MOVWF TRISIO ;Put 0000 0010 into W ;Make GP1 an input and all others output ;Put 0000 0100 into W ;Make GP2 an input and all others output ;Put 0000 1000 into W ;Make GP3 an input and all others output ;Put 0001 0000 into W ;Make GP4 an input and all others output ;Put 0010 0000 into W ;Make GP5 an input and all others output ;Put 0011 1111 into W ;Will make all input! (except GP3)

The TRISIO file can also be accessed by inserting BSF 03,5  BCF 03,5 instructions into your program.
The BSF 05,3 instruction takes you to Bank1 where the TRISIO file is located. When in Bank1, the TRISIO file is file 05. See: SetUp for details of the instructions to be inserted between BSF 03,5 and BCF 03,5.
 

BSF 03,5 movlw   b'00000011' movwf   tris BCF 03,5 ;Go to Bank 1 ;Go to Bank 0 - the program memory area.

Wait  see also Stop.
When testing a program, you may need to know if the microcontroller has advanced to a certain part of the program. 
Insert the instruction: GOTO Wait.
Insert the following instructions. They should include an output to a LED etc so you can prove the micro has entered the loop. 

Wait NOP MOVLW 07h MOVWF GPIO GOTO Wait ;Hold the micro in a loop ;output

Watchdog Timer (WDT)
The PIC12F629 has an inbuilt timer that increments all the time. It has a normal time-out period of 18mS (18,000 instruction-cycles). You cannot turn the timer on or off via the program. It is turned on (or off) during the burning of the program by setting WDT in the __config file. 
To turn the watchdog timer off:    & _WDT_OFF
To turn the watchdog timer on:    & _WDT_ON
If it is active (turned ON) the micro will go to 000 (reset) after 18,000uS.  
This action has a purpose. 
If a program is badly designed, it may "run off the end of the program" and start executing "junk" commands. A glitch may cause the micro to go to an incorrect part of a program to cause it to "Freeze." To prevent this from occurring, the WDT takes the micro back to the beginning and starts the program over again. 
To prevent the WDT constantly resetting a program, the WDT must be constantly reset via the instruction CLRWDT. This gives the programmer another 18,000 instructions before having to reset the WDT. 
To keep WDT constantly reset, CLRWDT must be placed inside delays. 
Normal programming (experimenting etc) is carried out with WDT OFF. 
If 18mS is insufficient a prescaler can be added to increase the WDT time by 2, 4, 8, 16, 32, 64 or 128. The maximum time is 18mS x 128 = 2.4sec  

Weak Pull-Up Resistors
The PIC12F629 has inbuilt weak pull-up resistors (47k) for each of the 8 input lines. These resistors are ONLY enabled when any line is INPUT.
To enable this feature:

            bcf         OPTION_REG,7    ; enable weak pull-ups (Option_Reg address:  81h) (bank1)
            bsf         WPU, 5                ; enable wpu on GPIO 5 only (WPU address:  95h) (bank1)

No weak pull-up available on GP3.

XOR - XOR detects a MATCH! 
XOR is called exclusive OR. 
See also article:  The XOR Trick - for more details on this instruction.
It means the only time the output of a gate is HIGH is when ONE (and ONLY one) of the inputs is HIGH. 
When two numbers are XORed together, the result is "1" when one (and only one) of the numbers is "1." [The OR function is called inclusive OR see IOR]. 

To find out if two numbers are the same, they are XORed together. Since each binary digit will be the same (i.e. either a 0 or 1) the result will be 0000 0000. The result will set the zero flag in the status (03) file and by testing bit 2 (the Z flag) you can skip when SET.
You also have to decide where you want the result to be placed. If you don't want the value of the file to be altered, the result should be put into W.

Example:  To see if file 2E holds the value 3C. (file 2E does contain the value 3C)

MOVLW 3C  XORWF 2E,0 ;Put 3C in W ;XOR 3C with file 2E and put the result into W.

The micro will XOR file 2E with the value 3C. 
We know file 2E contains the value 3C, so the operation is: 
W:  0011 1100 
1E: 0011 1100 
      0000 0000  - this answer will be put into W

In this example, 3C is put into file 2E and the XOR operation is carried out:

MOVLW 3C MOVWF 2E XORLW 3C  BTFSS 03,2  GOTO clear GOTO SET ;Put 3C in W :Put 3C into file 2E ;XOR 3C with file 2E and put the result into W. ;Test the Zero flag

The zero flag will be set. (The zero flag is SET to show the answer is zero) i.e: a MATCH! and the program will go to the line GOTO SET. 

Zero Flag
The zero flag is found in the STATUS register (file 03).  It is bit 2.  When the result of an operations is zero, the zero flag is SET. In other words it is  = 1.  

Zero a file 
A file can be made zero in two ways. To zero a file is the same as clear a file.  
The instruction CLRF 1A makes the contents zero. 

CLRF 2A ;Make file 2A zero.

You can load 00 into W and move W to a file:

MOVLW 00 MOVWF 2A ;Put 00 into W ;Make file 2A zero.

Zero in a file (test for) 
To see if a file is zero, move it in and out of itself. The Z flag will be affected. If it is SET, the file contains 00h.

MOVF 2A,1 BTFSS 03,2 ___________ ___________ ;Move 2A in and out of 2A ;Test the zero flag ;Micro goes here if 2A is NOT zero ;Micro goes here if 2A is zero.

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