WRITING A PROGRAM
We have made it very easy for you to write programs by starting with a set of experiments to show what can be done and how to create the lines of code.
All programs are written on a template in NotePad2 and the template contains the background information needed by the assembler (MPASM) to produce a .hex file for the chip. The assembler is a program that takes your .asm (assembly code) as written on the template, and converts it to a .hex file for "burning" into the chip.
We use the term "burn" to distinguish it from the word "program" as the original chips (not PIC chips) were "burnt" via a high voltage into an EPROM and erased with ultraviolet light. When we say "burn," you know what we are doing.
All you have to do is change some of the values in the code on the template to extend or shorten the time of the flash of a LED or the sound delivered to a piezo.
The layout of the program has already been provided on the template and the microcontroller starts at the first line of code at location 00 (address 00) and executes each line at a rate of one million lines (instructions) per second. If the instruction says GOTO, CALL, RETURN or Skip, the instruction takes 2 microseconds.
This means your program will be fully executed in a fraction of a second if the code did not contain lines that told the micro to go back and repeat previous instructions a number of times.
To create a blinking LED that lasts one second, the micro will have to repeat and carry out one million instructions per second and this is mainly done in routines called delay routines.
The micro starts at the first set of instructions called SetUp then goes to Main.
Main is where you write your program and when you want the micro to perform a particular job, feat or task, you create a routine called a sub-routine and place it in the sub routine section.
This allows to you call the sub-routine again, (at a different time) and not have to re-write the instructions.
The same applies to delay sub-routines.
You can create a number of timing (delay) sub-routines and CALL  them at any time.
This means a million microseconds of microcontroller-time can be laid out in a few lines of code - taking 1 second.
To further assist you in writing programs, we have produced a LIBRARY OF ROUTINES to answer every requirement.
And on top of this we have provided many projects that can can be copied.
This will allow you to copy-and-paste most of your sub-routines and just change a few values to suit the new requirement.
We have also provided a list of the 56 instructions that are needed to write a program.
These instructions are called mnemonics (a word to assist memory recall) in which each letter represents a word to make a simple sentence such as decfsz = decrement the file and jump over (skip) the next instruction if the result is not zero.
There is always a learning curve whenever you start a new field of endeavour and programming is no different.
It's a challenging field, no matter how you look at it and only by making the introduction as smooth as possible, will we be able to introduce beginners to the field.

This is the list of ALL THE INSTRUCTIONS

20 COMMON INSTRUCTIONS
Here are the most-common instructions:

We will name a file "red"  This is actually file 20h (the first available file in a group of 64 files) that you can use and name with any wording that suits the job it will do. 
Each file-name you create will be placed in a list at the beginning of the program and allocated a file in the micro (starting at 20h) by using the "equ" statement. This means "red" equates to (corresponds-to)  file 20h. 

In the following instructions, the word clear means: "make zero = "0"
The word set means: "make = "1"

The in/out port of the micro is file 05 but you can also call it "gpio" - general-purpose input/output.
To send data to this port, the individual lines are made HIGH = 1 or LOW =0.
bcf    gpio,0   - the lowest in/out line of the input/output port will go LOW
bsf    gpio,0   - the lowest in/out line of the input/output port will go HIGH  
to read data on an input, the line is tested:
btfss    gpio,0   - the lowest in/out line of the input/output port is tested to see if it is "1"
btfsc    gpio,0   - the lowest in/out line of the input/output port is tested to see if it is "0"
bcf   red,0     - clear the lowest bit in file "red"   
bsf   red,1     - set the second bit in file "red"    bsf     red,6   - set the 6th bit in file "red."  
btfsc red,2  - test the third bit in file "red" and if it is "0" jump over the next instruction and execute the following instruction.
example:
btfsc red,2
jump over this instruction  -  this instruction will normally send the micro to another location.
execute this instruction
btfss red,2 - test the third bit in file "red" and if it is "1" jump over the next instruction and execute the following instruction.
btfss    gpio,0   - the lowest in/out line of the input/output port is tested to see if it is "1"
call    delay1     - call a sub-routine  When the sub-routine has been fully executed, the micro goes back to the original location and executes the next instruction.
clrf     - clear a file
clrw   - clear w
decfsz  red,1  - decrement the "red" file and skip the next instruction if the result is zero.
goto     counter   - goto a sub-routine. When the counter sub-routine has been executed, the last instruction must tell the micro where to go.
goto   $+3  - the micro advance down the program 3 instructions (see below)
goto   $-1  - the micro goes to the previous instruction (see below)
movlw   literal   load w with a value from 00 to FFh.
movf      red,w     - copy the contents of "red" into w
movwf       gpio    - copy the contents of w to the output port. Only bits: 0,1,2,4,5 will go HIGH
movwf     red        - copy the contents of w in "red"
nop     - No Operation.  Waste 1uS of time - do nothing - used in delays to create extra time.
retlw   - return from a sub-routine (you can also place a value in w and it will return with this value)
trisio   - this file makes the in/out lines INPUT or OUTPUT. Each bit in trisio corresponds to gpio,0 gpio,1 gpio,2 gpio,3 gpio,4 and gpio,5   If a bit in trisio is "0" the corresponding line is output. If a bit in trisio is "1" the corresponding line in INPUT. 
xorwf    red,w  - exclusive-OR w with file "red" and place the result in w. With this instruction "red" is not changed.
  

WHERE DO YOU FIND THE RESULT OF AN INSTRUCTION?
Every instruction has a result (an answer) and the result can be found in the file contained in the instruction or in the "working register." The "working register" is the register that transfer the result form one file to another file. It is the only register that you can put a value into. A value or "number" is called a LITERAL. Any file can be incremented or decremented but only the "working register" can accept a NEW number.
The working register is identified as "w" or "0" when creating an instruction and it is loaded with 5 by the following instruction:
movlw     .5   this means: move literal (number) decimal 5 into the working register. The result of this instruction is w = 5   - the working register holds the value "5"
 
There are a group of files (empty envelopes) starting at file 20h that you can use for your routines. These files can hold a value from 00 to FF (00 to .255) and each file is given a name so you can remember it during the writing of a program.
Names such as: count, loop, show, red, top, display, time1, time2, time3 etc are suitable.

To put the value .5 into "red"

movlw      .5
movwf      red     means: move w  into file "red"  (actually COPY 5 into red as w still contains 5 and can be copied into another file such as "top").

file "red" can be incremented and the result can be placed in the file or in "w"

incf       red,w     the result is placed in the "w" BUT the file "red" does NOT contain the new value. 
incf       red,f      the result is placed in the file "red" and "w" is not touched.   That's why nearly all instructions will use ,f so the result is placed in the file (as this is what you normally want to do).

The result of an instruction is contained WITHIN the same line as the instruction.
The result will be in the working register (w) or the file being operated on, in the above instructions.

Now we come to more instructions:
These are more-complex to understand.
To compare two files to see if they are the same, we load one of the files into w and xor it with the other file:

first file is called "red"
second file is called "blue"

movf     red    - this loads  "red" into w
xorwf    blue,w   - this will produce the result  0000  0000 in w if the two files are the same. 

or 
xorwf    blue,f   - will produce the result  0000  0000 in the "blue" file if the two files are the same.

suppose red  = 1010 1010
blue              = 1010 1010
the xor operation will produce:
                       0000 0000   because there was no incidence where only one of the bits was different to the other. The answer is "1" for 1,0  or 0,1. the other two results are "0."
But we still do not know if the files are the same. We have to do a "test:"

the next instruction to place in your program tests the zero bit in the OPTION REGISTER.
The Option Register has a number of results, every time you perform a Boolean operation.
One of the results is the ZERO FLAG. This flag is bit2 in the Option_Reg and when the result of an operation is zero, the flag goes "1" - in other words it is SET.  
The OPTION register is register 03
btfss   03,z      When the result of an operation is 0000 0000 the zero bit is SET. In other words it "turns ON" to let you know the answer is ALL ZEROS. You can also write:
btfss   option_reg,z and the assembler will assemble the program correctly.


THE DIFFERENCE BETWEEN CALL AND GOTO
Both CALL and GOTO send the micro to the same location as specified by the name of the sub-routine included in the instruction.
But CALL comes back after executing the instructions in the sub-routine and executes the next instruction in the program.
This means you can advance down the program and CALL the same sub-routine again.
At the end of the sub-routine you must add:  retlw  00. 
This means the sub-routine is ONLY set-up for a CALL.
If you use the instruction goto counter  the micro will not know what to do when it reaches the final instruction retlw  00.  The micro will use the return value it finds on the "stack" and this brilliant piece of programming can be used IF YOU KNOW WHAT YOU ARE DOING.
The stack holds up to 8 values and this means you can go to a sub-routine by using a CALL instruction and the sub-routine can have a CALL instruction as one of the instructions and the next sub-routine can also have a CALL instruction and this sub-routine can also have a CALL instruction and this can continue for 8 CALL instructions.  The micro must come back via the 7th, 6th, 5th, 4th, 3rd, 2nd, 1st called sub-routine and finally return to the main program.  

ADVANCING UP AND DOWN THE PROGRAM
The micro normally advances down a program, executing one instruction at a time.
The location of the micro is held in a counter called the PROGRAM COUNTER (PC)
You can advance down a program by using the instruction:
goto   $+3   or goto   $+.12 
It works like this:
the current location is $  
If you use   goto  $  the micro will read the instruction, automatically increment the Program Counter and then decrement the counter to remain on the same address.
In other words the micro will not advance and it will get "stuck" on the same instruction.
This a HALT instruction - used for testing a program.
goto   $-1  sends the micro to the previous instruction and this produces a "loop" and the micro will get "stuck ion a loop."
goto   $+1 - advances the micro to the next location but this instruction takes 2uS and is used to increase a DELAY TIME.
nop   - this advances the micro down the program to the next instruction and wastes 1uS.
goto  $+2   - this advances the micro down the program by jumping over 1 instruction.
goto  $+3   - this advances the micro down the program by jumping over 2 instructions.
goto  $-3   - this sends the micro UP the program by jumping over 2 instructions.

CREATING A LOOP
To get something done a number of times, a loop is created. It may be to waste time, illuminate a LED or produce a tone.
The micro advances down the program at one million instructions per second and if we make an output pin HIGH to turn on a LED, we need to waste some time so the illumination of the LED can be seen. This involves a "do-nothing" sub-routine called a delay.
For a 1-second delay we need about one-million instructions as each instruction takes 1uS to execute.
Any instruction with a requirement to go anywhere other than a simple advancement, takes 2uS.
We start a delay routine by selecting a file and calling it delayA.
This file will have nothing in it and if we select the instruction decfsz we will be able to decrement it 256 times and it will have zero contents.
Here is the reason:  The file starts with zero. The instruction immediately decrements the file and it "rolls-over" so that all the bits are "1."  This gives the file a value of 0FFh or decimal 255.
When the file is decremented the second time it will contain 0FEh or 254.
This can be repeated until the file contains 1 and the instruction will decrement it to zero. The same instruction now invokes the "skip if zero" portion of the instruction and the next line of code is jumped-over. Thus we have created 256 loops.
The next part of designing the delay is working out the size of each loop.
If you tell the micro to go up the program 6 locations, the loop will contain 7 instructions and the time for each loop will be 10uS.
This is executed 256 times producing a total of 2560uS = 2.56mS.     

goto     $+1   = 2uS
goto     $+1   = 2uS
goto     $+1   = 2uS
goto     $+1   = 2uS
nop             = 1 uS
decfsz  delayA,1 = 1uS
goto     $-6     = 2uS

to increase the delay-time we can go to the routine above 256 times to produce a total delay of
2560 x 256 = 655,360uS  = 0.65 seconds, by adding the following instructions:

decfsz  delayB,1  = 1uS
goto     $-8     = 2uS

The file being decremented MUST be different to the first delay file, that why it is named delayB.
The two instructions above add 256 x 3  plus 256uS for the time when decfsz  delay,A is zero = 768uS + 256, making a total of 656,384uS.   
The timing for simple delays doesn't have to be worked out accurately and you can consider the delay equal to half-a-second.
As you can see, there is an inner loop and an outer loop. This is called a "NESTED LOOP."
 

INPUTTING AND OUTPUTTING
To detect the action of a push-button, ANY of the pins must be set to INPUT. This is done in the INPUT/OUTPUT file called the "trisio" file. This file is hidden inside the micro and can be accessed by the following instruction:
bsf      rpo

The pin you want to be an INPUT corresponds to a particular bit in the trisio file and is given the value "1" - to correspond to INPUT.
At the same time, you can make all the other pins OUTPUT.
This is done by the following to instructions

movlw       b'00000001'
movwf       trisio

finally we have to move away from the hidden file and back to the area where you place your program, with the following instruction:

bcf   rp0

The voltage on the switch connected to the input line must go from HIGH to LOW or LOW to HIGH when the switch is pressed.
The file connected to the pins is called the gpio file or gpio register.
The following instruction will test bit0 to see if it is HIGH:

btfss   gpio,0


It works like this:
btfss   gpio,0
the micro will go HERE if the switch is pressed
the micro will go HERE if the switch is NOT pressed.

To turn ON an output (In other words "make a pin that is already defined as an OUTPUT-PIN via the trisio register"  - HIGH) to illuminate a LED, for example, the corresponding bit in the gpio register is SET:

bsf    gpio,1    (the ,1 represents the second lowest pin.  ,0 represents the lowest pin, and this is pin 7 on the chip - GP0.     ,5 represents the highest pin - GP5 and this is pin 2 on the chip.)
Don't forget: Pin 4 is ONLY an INPUT.

To turn ON more than one pin at a time, a number of pins can be made HIGH by loading the required bits into w and then moving the value to the gpio register. It is the "1's" in the binary value that make each pin HIGH. (Don't get confused with "1's" in the trisio register turning a pin into an input).
This will turn on 3 pins:

movlw      'b00001110'
movwf      gpio

Only the 6 lowest bits correspond to the in-out pins being HIGH/LOW. The two highest bits are NOT USED. (This also applies to the trisio register being IN/OUT - the two highest bits do not control any IN/OUT feature).

INCREMENTING AND DECREMENTING
Incrementing and decrementing a file is nearly always done in a DELAY routine to "waste time."
When you choose a file to increment or decrement, it may be zero or it may contain some bits that are "set."
To load a file with a particular value, such as .30  (decimal thirty), the value is called a LITERAL and is loaded into w:

movlw      .30

It is then passed (actually copied) into tempA

movwf    tempA

By using the decrement function, the file will be decremented to zero. If you keep decrementing, the file will go to .255 then .254 and keep decrementing and looping.
However the instruction decfsz (decrement file and skip  - jump over the next instruction - if the file is zero) detects when the file is zero and you can go to another part of the program and perform another task.
We have already explained the concept of advancing UP THE PROGRAM and if you add a command to "go up the program" such as:   goto  $-3  the micro will loop and decrement tempA, on each pass.
Eventually tempA will be zero (after exactly thirty loops) and the instruction decfsz will jump over the next instruction and "get out of the loop."
You can use the instruction incfsz but you need to work out the number of loops between the value loaded into the file and the value .256  (because .256 will be 000)

If you need to increment a file, use the following instruction:

incf,   tempA,1    - the ,1 is needed to signify the result of incrementing will appear in tempA.

the instruction:   tempA,0 will put the result in w   BUT tempA will NOT be INCREMENTED.

For instance: decfsz  tempA,0    will NEVER "get out of the loop" (covered above)  because  tempA will never be decremented.

You MUST use: decfsz  tempA,1

You can also use the instruction: decf,   tempA,1 and check the zero flag in the STATUS file, but the instruction  decfsz  tempA,1 is much easier to use.

If you want to increment or decrement a file use:  incf   tempA,1   or decf  tempA,1
 

IMPLEMENTING A TABLE - MAKING A TABLE - CREATING A TABLE
If you want a list of numbers (called data) you need to add them to the program in a TABLE.
This table can be as long as 255 entries (values) but it MUST be in the first 256 locations of your program.
This means the first location (000) must contain: goto Start to send the micro past the first page (called page0).
On the second location you can start the table.

The first instruction in a table adds a value to the PROGRAM COUNTER to send the micro down the table to the required value.

this instruction is: addwf   02,1  

File 02 is the Program Counter and ,1 indicates the result of addition will be in the PC.
Before going to a table, the w register must be loaded with a value. This is the value to be added to the PC.  If the value is zero, the first piece of data in the table will be placed in the w register.
Each item of data is placed on a separate line with an instruction in hex or decimal or binary - or any mixture:

    retlw     06h
    retlw     .25
    retlw    b'00001110'
   

the instruction means: return with the following value in w.

Here's your program:
 
xxxxxx
movlw    03
call        table1
movwf   counter
xxxxxx


table1   addwf   02,1  
             retlw     06h
             retlw     .25
             retlw    b'00001110'
             retlw     .55

Here's what will happen:
The micro will advance to the instruction movlw    03 and place 3 in the w register.
The micro will create a CALL condition in which it will remember the location of the next instruction in the program (so it can return to you program).
The micro will go to (via a CALL condition) table1 and increment the program counter.
This will make the micro advance down the table to retlw  .55  (because the first table-data is advancement zero.   
At location  retlw   .55 the w register will be loaded with the value decimal 55 and the micro will return to your program to movwf   counter.
The micro will place the value decimal 55 into the file: counter.

The reason why the table is placed in the first page (page0) of the program is because addwf cannot compute a value higher than 8 bits. This means the table cannot go over the boundary between page0 and page1 or the boundary between page1 and page2 or the other boundaries.
However the table can be at the beginning of page1 and extend to the end of page1 or inside page3 or page4.
To make sure the table remains in its intended location, the instruction before table2 is:

             org     0x0100     - this instruction doe NOT take up a location in your program
table2   addwf   02,1  
             retlw     07h
             retlw     .35
             retlw    b'01001110'
             retlw     .75

Make sure your instructions in page0 do not go over page0 boundary or they will clash with the beginning of table2.
You can have any number of tables in any page, but make sure none go over the page boundary.
The PIC12F629 has 4 pages: page0, page1, page2, page3. Each page is 256 lines of code or table-values. (255 table values is best).

BOOLEAN OPERATIONS
In the list of PIC INSTRUCTIONS you will find instructions with the words: AND, OR, (more accurately called Inclusive-OR) and  XOR.
These are called Boolean Instructions or Operations and are very powerful because they operate on all the bits in a file at the same time.
BUT YOU MUST KNOW WHAT YOU ARE DOING - because the result may not be what you expect and you continue the program with the wrong result.

The AND operation is not addition. There are no "carry-over" bits. Two files are compared and if both files contain "1"  the result is "1"
Example:

File A     0000 0011
File B     1100 1110
Result:   0000 0010

The XOR is a very powerful operation.  It means exclusive-OR and this means the result is "1" ONLY if one of the files is "1".   It can also be written: EX-OR or X-OR in some text books.
For example:

File A     0000 0011
File B     1100 1110
Result:   1100 1101

Note:     xxxx xx1x
             xxxx xx1x
result     xxxx xx0x      The result is "0" because BOTH files are "1"

Here is the result for the OR operation:

File A     0000 0011
File B     1100 1110
Result:   1100 1111   The OR operation cannot detect when BOTH files are "1"

That's why we use the XOR function. 

The XOR function can change a bit with an unknown value to the opposite value. This is handy to change an output HIGH to an output LOW (to flash a LED or toggle a bit to produce a sound).
You will find the instructions in the Library of Routines under "Toggle a bit."
You can toggle up to 8 bits with 1 instruction. Make sure to copy the instructions to get the correct outcome.  

P1
P3 - writing your own program