4th December 2007

18F4550 and Assembly Language Overview

In this post, I attempt to give a brief overview of the PIC18F4550 microcontroller, its instructions and their assembly language implementation. Please consult section 26.1 in the 18F4550 datasheet for a complete list and explanation of the instructions.

"A microcontroller (also MCU or ┬ÁC) is a computer-on-a-chip. It is a type of microprocessor emphasizing self-sufficiency and cost-effectiveness, in contrast to a general-purpose microprocessor (the kind used in a PC). In addition to all arithmetic and logic elements of a general purpose microprocessor, the microcontroller usually also integrates additional elements such as read-only and read-write memory, and input/output interfaces."

Couldn’t have said it better myself. The PIC18F4550 microcontroller does indeed have read only and read-write memory. The former, flash ROM, can actually be erased and reprogrammed during development. However, during code execution, this memory is used as read-only. The latter includes registers (and probably other things – suggestions are welcome).

The microcontroller has 35(!) I/O pins. These pins are arranged into ports A,B,C,D and E. These pins are usable for a combination of inputs, outputs, analog-to-digital or digital-to-analog conversion and PWM. Additionally, you can use these pins for serial communication, USB communication, or to trigger interrupts.

But the sales pitch ends here. You now need to consider that you are responsible for ensuring that everything on that chip functions exactly the way you want it to. Microcontrollers do not make mistakes. People do.

It is a good idea to visualize the process of programming a PIC microcontroller:

I am confident that after setting your eyes on this visual feast, you are completely ready to take on the PIC assembly monster. But, just in case you’re not feeling completely ready, I will describe the concepts using words. The analogy is that programming a PIC is similar to having to move objects in and out of a storage space with many cubbyholes using only one hand. In such a scenario, you can do one of several things:

  1. Pick up an object with your hand
  2. Place an object in hand into a cubbyhole
  3. Grab an object out of a cubbyhole with your hand
  4. Drop the object from your hand

Similarly, a PIC consists of a single working register W, a register file with various readable/writable registers (the general term for one of these file registers is F), and various data you can read and write. The registers in the register file alter the microcontroller’s operation depending on the values stored in them. You can do one of several things:

1)Write a number to W
2)Place the number you have in W into some register in F
3)Retrieve a number from some register in F and place it into W
4)Clear the number from W

Unfortunately, this is where the cubbyhole example starts to become a bit useless. For example, you can also cheat and just move the value from one register to another without using your working register (directly from one cubbyhole to another without using your hand). And that’s 5 instructions down, 70 to go. Addition, subtraction, etc..

It is important to keep in mind that while the 18F4550 is a 16-bit device (instructions are 16 bits < l o n g >), registers consist of only 8 bits. Yes, your hand only has 8 fingers.

There are three types of instructions (18F4550 datasheet used as reference).

  • Byte-oriented: These instructions involve operations on all 8 bits of a register. These include moving W (the working register) to a different register, adding W and a different register and incrementing a register. For example, a byte-oriented instruction could be used to set all the pins in PORT D to HIGH at the same time.
  • Bit-oriented: There are 5 of these instructions. They perform operations on single bits within a register. This is often useful with I/O, such as when a desired output needs to be set high, low, or toggled on a single pin.
  • Control: These instructions do not directly influence registers, but perform other actions to direct program flow. These include branches (conditionals), stack operations, a reset command and NOP, an instruction which does nothing.

Please do refer to the datasheet (again) and look over the instructions. Most of them are fairly simple to understand.

As an example, I will explain the Add instruction ADDWF.

The datasheet says:

ADDWF F,D,A | Add WREG (W in the above description) and F | 1 Cycle | 0010 01da ffff ffff | C, DC, Z, OV, N

The ADDWF instruction takes 3 arguments – F, D and A (though A is optional and will be ignored for now). As the description says, F is the register you would like to add to W. D determines the destination (D=0 to store in W, d=1 to store in F).

The 1 in the cycles column means that the instruction takes 1 cycle. When an instruction says 1 (2 or 3), it means that the number of cycles it takes depends on the result of the instruction (they are all conditional instructions). To borrow an explanation from Prince on the Microchip forums, you can see it the following way: The instruction itself takes 1 cycle. 2 cycles will be taken if the conditional instruction ends up skipping the following instruction. The same conditional instruction may take 3 cycles if there is a call/goto instruction following it and being skipped, since call/goto are effectively 2 instructions long. A skipped instruction is similar to executing a NOP instruction. To summarize, while the conditional instruction takes only 1 cycle, any instructions it skips still end up using cycles.

The next column gives the the machine code of the instruction in hexadecimal form. I cannot think of a use for this field for the average programmer.

The last column provides a list of the affected status bits, which compose a status register. These bits are set/cleared after the given instructions to provide more information about the result. For example the OV status bit can be checked following an addition operation to determine if an overflow has occurred.

You should now be feeling confident about starting your first assembly program. Go on, then…

This entry was posted on Tuesday, December 4th, 2007 at 3:14 pm and is filed under 18F4550 Assembly Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

There are currently 10 responses to “18F4550 and Assembly Language Overview”

Why not let us know what you think by adding your own comment! Your opinion is as valid as anyone elses, so come on... let us know what you think.

  1. 1 On January 25th, 2008, zaid said:

    hey comon dude…the first part wis the pictures were already cool until u said what the data sheet said…please in your own word for a pic beginners like me!!we wouldnt be searching all over the net if we knew how to read the datasheet….huhu…

  2. 2 On January 31st, 2008, Andrey said:

    I would definitely like to make these tutorials more beginner friendly. Please visit the forum (http://forum.pic18f.com) and I will gladly help you with any questions. Your questions would definitely help me provide more clear information in these tutorials.

  3. 3 On February 4th, 2008, zaid said:

    ok cool!

  4. 4 On August 4th, 2008, Rick said:

    I love this kind of tutorial- absolutely excellent!! Can you make examples for all the features :-) One question- are there any other destination options.. If the only options are W or F that’s fine, but it doesn’t say. I am a total beginner with this chip. Can you send this value directly to a port, to the flash ram, the static ram, an external ram?
    I’m really looking to figure out how to get USB data out (1000 XY points) the fastest way possible. Streaming parallel port looks like about a tie with the serial port for speed.
    Examples would be so easy if they had simplest code possible, absolutely the minimum number of lines needed, and a schematic if needed. I used to program with the Atmel 89C51 till my programmer broke. Here’s the smallest code to blink 8 leds connected to port1. This is so simple.

    .org H’100
    start: inc A (increment A)
    mov P1,A (mov to port1, A)
    sjmp start (short jump)

    if you want so slow this so you can see the leds blink,
    .org H’100
    start: inc A ;increment A
    mov P1,A ;mov to port1, A

    Delay: Djnz R0,Delay ;Dec jmp if not 0
    Djnz R1,Delay ;delay= FF*FF= 65535. It’s perfect.

    sjmp start (short jump)

    Is there an example anywhere like this for the 4550?
    Thanks, your example was a tremendous help for understanding that add command, the W,F registers and the ones that I don’t even want to know about.

  5. 5 On August 17th, 2009, bimbo said:


    Whats the diffrence between this 44 pin QFN pic?


  6. 6 On January 19th, 2010, stb said:

    great tutorial, very useful for beginners, thanks..

  7. 7 On February 24th, 2010, zarlimoe said:

    I want to learn about 18 f series in C language.

  8. 8 On July 26th, 2010, jomar rondina said:

    i want ta learn about diagonal code in assembly language.

  9. 9 On August 23rd, 2010, luis fernandes said:

    Once thanks for your time a patience to do those intructiosn to let others learn. I’am an absolute beginner, and was fascinated when I read a brazilian book about the pic16f648. Meanwhile i would like to modify a program inside a card, I bought. It could do lots of thinks, but unfourtunatly is sending back all captured/processed data, thru the USB port. I need to learn how to create a similar program inside the 18f4553, but to answer just when I poll it with some query. I don’t know if I can do it in assembler, I don’t know c code, and I cannot find the instruction set of the 18F4553, or 18fxxxx for instance. Can you point me to a place to find them?
    thanks for your time
    luis fernandes

  10. 10 On January 3rd, 2012, Jared P said:

    Great analogies for demonstrating how the PIC microcontroller works. The one question that I have regarding the 18F4550 is how many (general purpose) registers can you have? Are they also separated into different banks like in the 16F917? Thanks, keep up the great content!

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