This discussion explains the instructions for the PIC12F629 and PIC16F628.
(most of this discussion has been taken from PCB Heaven but the English has been corrected plus many mistakes have been corrected). The PIC chips we are using have just 35 instructions to carry out all the features of the chip.

All instructions have the same format. First is the name of the instruction, such as movlw for “move” or btfss for “bit test.” Then the instruction itself. It can define a register (f), such as movwf test1 where test1 is register 22h or a bit (b), such as btfss test1,2 a literal, (a literal is another word for a “number,” such as movlw d35 where 35 is put into the “working register” of the micro. The number can also be called a “constant” or a “piece of data.” Or the instruction can define a place to go to in a program. This place will be identified by a “label” such as goto testB. Or the instruction can have a destination (d), such as goto $+4 where the $ symbol identifies the present address and +4 tells the micro to advance 4 instructions down the program. The instruction $-6 tells the micro to go UP the program 6 instructions.

File Address (f).

A file is also known as a Register. All PICs have a number of files or registers that can be used during the execution of a program to hold pieces of data. The files for the PICs we are using start at 20h to 5Fh for the PIC12F629 and 20h to 7Fh (plus 128 extra) for the PIC16F628. A file address is a field that indicates the absolute position of a register within the RAM. This field could be filled with either a number (e.g. 0x05h for PortA or 0x06h for PortB). Files start at 20h.
You can work with a file by using its address, such as 20h via an instruction such as movf 20h,0 or btfss 20h,1 or decfsz 20h,1
Or you can define each file you will using in your program, at the start. Such as:

green    equ   20h
DTMF   equ   21h
test1    equ   22h

You then produce instructions such as: movf green,0 or btfss green,1 or decfsz green,1 and the compiler will associate green with file 20h during the conversion of your .asm (your hand-produced program in NotePad) to .hex file that will be used to “burn” (program) the chip.

Bit Address (b)

Each byte is composed of bits. For the 8-bit PICs, each byte is composed of 8 bits. When a field requires a bit address, it means a number from 0 to 7 must be placed in the instruction. 0 corresponds to the Least Significant Bit (LSB) and it is the most right placed bit within the byte. 7 corresponds to the Most Significant Bit (MSB) and it is the most left placed bit within the byte.

Literal, constant, data or label (k)

This field is generally a number. This number may be from a previous-defined constant, or piece of data, or from a label within the code. When we talk about a literal we mean a number. This is generally between 00 and 255. A constant is also a number. A piece of data is also a number. A label is generally the name of a sub-routine and identifies the start of the sub-routine.

Destination (d)

After an instruction is executed, the result can be placed in the original file or it can be placed in W (the working Register) and the original file will not be altered.

The destination can be either 0 or 1, where 0 represents the Working register and 1 represents the file. It is used in instructions that will return a byte as a result of an operation. The destination field will define where this byte will be stored. If this field is 0, the byte will be stored in the working register (W). If the destination is 1, the byte will be stored in the register where the operation was performed. You can also write ’w’ for 0 or ’f’ for the file.
Typical examples to show the destination of the result of an operation:

movf    green,0   The value stored in green will be copied to the working register.
movf    green,1   The value stored in green will be taken out and then moved back to the green file.
                  The zero flag in the Status file will be SET if the file is EMPTY.
decfsz  green,1   The green file will be decremented and the result will be placed in the file.
                  The micro will jump over the next instruction if the file is EMPTY.

Loading Instructions

There is no direct way to place a number directly into a file. It must first be loaded into w (using the movlw instruction: movlw d55) then moved to the file using movlw, such as movwf green.

Binary

8-bit PIC microcontrollers can load binary values up to 8-bits. If you try to load values larger than 8-bits, only the first 8 bits will be loaded and the rest will be omitted.

• b’10101010’

The ’b’ tells the compiler that a binary number follows. The quotes are necessary. Just like the decimal human-orientated base, if a binary number needs less than 8-bits to be represented, all zeros on the left side of the number can be omitted. For example, these two are the same:

• b’00011110’
• b’11110’

Decimal

The decimal base is human-orientated. It is easier to understand 255 than b’11111111’.

• d’255’

The letter ‘d’ before the number tells the compiler that a decimal number follows. The quotes are necessary.

Hexadecimal

This is another useful base that you may need to know. It is most useful because number representations such as memory positions use this base. To represent a hexadecimal number, you use the follow form:

• 0xa2

Unlike before, the value does not require a letter before the number. Instead, 0x is used and no quotes enclose the number. The 8-bit system can handle hexadecimal numbers up to 0x0ff. Larger numbers will act as previously mentioned.

Flow Instructions

These instructions affect the normal flow of the program. Usually, the program flows line by line. This means that when an instruction on line 10 is executed, the next instruction will be executed. But some instructions change the flow. A GOTO for example may send the program from line 10 to line 100.

Mathematical Instructions

Logic Functions

Logic Functions Table

The following table shows the result of the above functions between two bits.

Bit Orientated Instructions

Byte Orientated Instructions

The byte orientated instructions are instructions to alter a complete byte of a file:

Instruction	Description
rlf file,d	Rotate Left through carry. This instruction will shift all bits of the file one position left. The MSB bit will be sent to the Carry, and the Carry bit will be sent back to the register’s LSB. If d is 0 the result is stored in the W register, if d is 1 the result is stored back to the file.
rrf file,d	Rotate Right through carry. This instruction will shift all bits of the file one position right. The LSB bit will be sent to the Carry, and the Carry bit will be sent back to the register’s MSB. If d is 0 the result is stored in the W register, if d is 1 the result is stored back to the file.
incf file,d	The contents of the file are increased by 1. If d is 0 the result is stored in the W register, if d is 1 the result is stored back to the file.
decf file,d	The contents of the file are decreased by 1. If d is 0 the result is stored in the W register, if d is 1 the result is stored back to the file.
incfsz file,d	The contents of the file are increased by 1. If d is 0 the result is stored in the W register, if d is 1 the result is stored back to the file. If the result is ‘0’, the next instruction is jumped over and the following instruction is executed. The layout of instructions are as follows: incfsz fileC,1 goto $-1 goto show The micro will execute incfsz fileC,1 and move to goto $-1 then go to incfsz fileC,1 and increment to goto $-1 etc etc etc. But when fileC contains “ff” the next increment will make it “00” and the micro will jump to goto show.
decfsz file,d	The contents of the file are decreased by 1. If d is 0 the result is stored in the W register, if d is 1 the result is stored back to the file. If the result is ‘0’, the next instruction is jumped over and the following instruction is executed. The layout of instructions are as follows: incfsz fileC,1 goto $-1 goto show The micro will execute incfsz fileC,1 and move to goto $-1 then go to incfsz fileC,1 and increment to goto $-1 etc etc etc. But when fileC contains “ff” the next increment will make it “00” and the micro will jump to goto show.
clrf file	The file is cleared. The Zero Status is always SET: * Z - Zero
clrw	The working register W is cleared. The Zero Status is always SET: * Z - Zero

Miscellaneous Instructions

The Status Register

The Status register contains the arithmetic status of the Arithmetic Logic Unit, the Reset status of the PIC and the Bank Selection bits, direct and indirect. The bits of the Status register as as follows:

Bit 0 - C: Carry/-Borrow

This bit is Readable and Writable and after a reset it’s value will be unknown. If an instruction executed has an effect on this bit, then this bit becomes automatically and temporarily Read Only. The Carry bit watches if a carry-out from the MSB bit occurs. The values it can get are:

• 0: No carry-out from the MSB of the result occurred
• 1: A carry-out from the MSB of the result occurred

Bit 1 - DC: Digit Carry/-Borrow

This bit is Readable and Writable and after a reset it’s value will be unknown. If an instruction executed has an effect on this bit, then this bit becomes automatically and temporarily Read Only. The Digit Carry bit watches if a carry out from the 4th lower order of the byte is occurred. The values it can get are:

• 0: No carry-out from 4th lower order of the byte occurred
• 1: A carry-out from the 4th lower order of the byte occurred

Bit 2 - Z: Zero

This bit is Readable and Writable and after a reset it’s value will be unknown. If an instruction executed has an effect on this bit, then this bit becomes automatically and temporarily Read Only. The Zero bit watches if the result of an arithmetic or logic operation was zero. The values it can get are:

• 0: The result of an arithmetic or logic operation was not zero
• 1: The result of an arithmetic or logic operation was zero

Bit 3 - -PD: Power Down

This bit is Read Only and after a reset it will get the value 1. The Power Down bit watches if the ‘SLEEP’ instruction was executed. The values it can get are:

• 0: The ‘SLEEP’ instruction was executed
• 1: Power Up or the ‘CLRWDT’ instruction was executed

Bit 4 - -TO: Time Out

This bit is Read Only and after a reset it will get the value 1. The Power Down bit watches if a Watchdog timer time out occurred. The values it can get are:

• 0: A Watchdog timer time out occurred
• 1: Power Up, ‘SLEEP’ or ‘CLRWDT’ instruction was executed

Bit 6:5 - RP<1:0>: Register Bank Selection

Those bits are Readable and Writable and after a reset it will all get the value 0. Their task is to select the active Register Bank in use. The values it can get are:

• 00: Bank 0 is selected
• 01: Bank 1 is selected
• 10: Bank 2 is selected
• 11: Bank 3 is selected

Bit 7 - IRP: Register Bank Selection used with indirect addressing

During the indeterminate pages of this book, the indirect addressing shall be discussed and explained. The indirect addressing is a way of accessing registers (General Purpose or Special Function Registers) without directly accessing their memory position, but throughout an indirect addressing register. When this register is used, the current bank selected is different than the indirect current bank selected. The values it can get are:

• 0: Indirect addressing current bank selected is Bank 0 and Bank 1
• 1: Indirect addressing current bank selected is Bank 2 and Bank 3

<!-- //TODO FIXME -->

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The Option_Reg Register

The Option_Reg register is a Readable and Writable register that is used to control some modules of the PIC. The bits of the Option_Reg are as follows:

Bit <2:0> - PS<2:0>: Prescaler rate selection

Those bits are Readable and Writable and after a reset they will get the value ‘1’. They are used to set the Prescaler rate. The values it can get are:

Bit 3 - PSA: Prescaler Assignment

This bit is Readable and Writable and after a reset it will get the value 1. This bit is used to assign the prescaler to to the Watchdog timer or the Timer 0 module. The values it can get are:

• 0: The prescaler is assigned to the Timer 0 module (TMR0)
• 1: The prescaler is assigned to the Watchdog timer (WDT)

Bit 4 - TOSE: Timer 0 Source Edge Select

This bit is Readable and Writable and after a reset it will get the value 1. It is used to select the RA4/TOCKI pin clock edge (High to Low or Low to High) on which the Timer 0 will count. The values it can get are:

• 0: Increment on Low to High
• 1: Increment on High to Low

Bit 5 - T0CS: Timer 0 Clock Source Select

This bit is Readable and Writable and after a reset it will get the value 1. This bit will define the Timer 0 module clock source. It can be either the RA4/TOCKI pin or the Internal Instruction Cycle Clock (CLKO). The values it can get are:

• 0: Timer 0 clock source is the Internal instruction cycle clock (CLKO)
• 1: Timer 0 clock source is the RA4/TOCKI pin

Bit 6 - INTEDG: RB0/INT pin Interrupt Edge Select

This bit is Readable and Writable and after a reset it will get the value 1. By altering this bit, you can select the RB0/INT pin pulse edge that the RB0/INT interrupt will occur. The values it can get are:

• 0: The RB0/INT interrupt will occur on the falling edge of the RB0/INT pin
• 1: The RB0/INT interrupt will occur on the rising edge of the RB0/INT pin

Bit 7 - -RBPU: PORTB Pull-up Enable

This bit is Readable and Writable and after a reset it will get the value 1. The RB ports have an internal programmable pull-up resistor to minimize the use of external pull-up resistors when needed. This bit will enable or disable those resistors. The values it can get are:

• 0: The RB pull-up resistors are enabled
• 1: The RB pull-up resistors are disabled

<!-- TODO FIXME -->
var HINTS_ITEMS = \[wrap2('<a target="_self" href="http://pcbheaven.com/wikipages/Reading_Part_Values">Reading Part Values</a><br /><a target="_self" href="http://pcbheaven.com/wikipages/Connecting_Resistors">Connecting Resistors</a><br /><a target="_self" href="http://pcbheaven.com/wikipages/Ohms_Law">Ohm Law</a><br /><a target="_self" href="http://pcbheaven.com/wikipages/theresistor">The Resistor</a>'),wrap2('<a target="_self" href="http://pcbheaven.com/wikipages/The_PS2_protocol">The PS2 protocol</a>'),wrap2('<a target="_self" href="http://pcbheaven.com/wikipages/Dimmer_Theory">Dimmer Theory</a>'),wrap2('<a target="_self" href="http://pcbheaven.com/wikipages/555_Theory">555 Theory</a><br /><a target="_self" href="http://pcbheaven.com/wikipages/555_Circuits">555 Circuits</a>')\]; var myHint = new THints (HINTS_ITEMS, HINTS_CFG);

• Reading Part Values
• Connecting Resistors
• Ohm Law
• The Resistor

The PS2 protocol

Dimmer Theory

555 Theory
555 Circuits

The TRIS and PORT registers

The PIC12F629 has 8 pins with 5 I/O pins and one input-only pin.
The PIC16F628 has 18 pins with 15 I/O pins one input-only pin.

These pins are called PORT PINS.
The pins of the PIC12F629 are called General Purpose In Out pins - abbreviated to GPIO.
And the pins of the PIC16F628 are separated into two sets of 8 pins, called portA and portB.
Each pin can be changed to an input or an output at any time during the running of a program and this is done by a file called TRIS. The name of the file for the PIC12F629 is called TRISIO and for the PIC16F628, it is called TRISA and TRISB.
When a bit in the TRIS register is “1” the corresponding bit in the in/out file becomes an INPUT. When a bit in the TRIS file is “0” the corresponding bit in the in/out file becomes an OUTPUT.
When a bit in the TRISIO register is “1” the corresponding bit in the in/out file (this is called GPIO) becomes an INPUT. When a bit in the TRISIO file is “0” the corresponding bit in the in/out file (this is called GPIO) becomes an OUTPUT.

CONFIGURING TRISIO

For the PIC12F629 the TRIS file is called TRISIO and only the 6 lower bits are used.
The TRISIO file is in bank1 (most of the other files are in bank0). To get to bank1, the following instructions are needed:

bsf   status, rpo     ;Bank1
movlw   b'xx000000'   ;(however GP3 is input-only and will not become an output)
movwf   trisio        ;Write to TRISIO register
bcf   status, rpo     ;Bank0

CONFIGURING GPIO

GPIO is the actual port (consisting of 6 lines) in a PIC12F629, that drive LEDs (up to 25mA per line) or can be configured as inputs.
If the lines have been configured as outputs, via TRISIO, they can now be made either HIGH or LOW by defining the bits in GPIO, thus:

  movlw   'xx110000'
  movwf   gpio    (however GP3 is input-only and will not become an output)

CONFIGURING TRISA

The PIC16F628 has 2 ports, PORTA and PORTB.
Suppose you set TRISA to: b’11111111’ PORTA will be all inputs.
And if you set TRISA to: b’00000000’ PORTA will be all outputs.
The TRISA file is in bank1 (most of the other files are in bank0). To get to bank1, the following instructions are needed:

bsf   status, rpo     ;Bank1
movlw   b'00000000'   ;PORTA will be all outputs.   (however RA5 is input only and will not become an output)
movwf   trisA           ;Write to TRISA register
bcf   status, rpo     ;Bank0

CONFIGURING PORTA

You can now make each line of PORTA either HIGH or LOW by defining the bits, thus:

  movlw   b'11110000'
  movwf   portA

The 4 lower outputs will be LOW and the 4 upper lines will be HIGH. (however RA5 is input only and will not go HIGH).