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How does raspberry pi OS work?

Thu Jun 19, 2014 5:56 am

I'm following Baking Pi – Operating Systems Development of Cambridge http://www.cl.cam.ac.uk/projects/raspbe ... orials/os/
That tutorial is specific, but i have a questions:

They provide a OS Template, and guide us to create files in source folder (ex: main.s .....). But they don't explain about another file (ex: kernel.ld, makefile .....). and how they work ?
Can anyone explain to me ?

Thank you very much. :)

tufty
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Re: How does raspberry pi OS work?

Thu Jun 19, 2014 11:10 am

The makefile is a set of rules used by the command-line program `make`, telling it how to run other programs in order to convert the source code you create when following the tutorial into object code and, finally, a linked kernel that you can run on the Pi. That's the 30 second explanation, the full explanation would be book-sized. Believe me, you really don't want or need to know anything much more than this - I've been using make for getting on for 30 years, and I still have to refer to the manual *every* *damn* *time* I write a makefile.

(More or less) The final step that make will invoke when using the rules in that makefile is known as "linking". This is a conversion from compiled / assembled code into a final executable. This is done using a program known as a "linker" (unsurprisingly enough); as input, it takes a bunch of compiled code and a "map" telling it where to put what in the final executable. Now, if you were building an executable for your desktop machine (or, indeed, a linux executable on the Pi), the default linker instructions would be sufficient. However, you'll be building something that isn't a "normal" executable, and certain things (notably the "reset" handler, the first thing the Pi runs when it starts your kernel) have to be in very specific places in th final executable so they end up in the right places in memory when you try to boot your kernel. That's what kernel.ld is.

Neither are required on the Pi once you've built your kernel - they are only required in order to build it.

jran1216
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 4:24 pm

Im a bit stuck aswell.

With the numerical system, I can speculate why you would logical shift left the 1, this must be how bits work, 1s or zeroes. But with the second part of the numerical bit where you have to assign another 1 and then logical shift it to 16 places to ignite the 16th pin how does the system know how many sets of 3 bits to move along seeming as the assembly code has only assigned to ready the 16th pin. Does it just know?

Thanks

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rpdom
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 5:22 pm

Can you post the snippet of code that you don't understand and we can break it down into simple bits for you.

jran1216
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 6:36 pm

mov r1,#1
lsl r1,#16
This bit. Simply put is this because its to do with the pins and if so how does the board know. Its all to do with shifting numbers to the correct place isnt it not the other way round?
str r1,[r0,#40]

and also is this right

mov r1,#1
This is just finding a place on the board to compute the correct number
lsl r1,#18
This is a cumulative way of finding the correct pin as it is to find the 6th pin within reason. That being that 4 bytes are assigned to each set of 10 pins and im guessing3x4 bits = a byte.
str r1,[r0,#4]
This is to maybe get it to the second set of 4 bytes. Seeming as we already treated bytes in a group of 3?

Ofcourse I could be completely wrong lol.

Thanks

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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 7:00 pm

jran1216 wrote:mov r1,#1
lsl r1,#16
This bit. Simply put is this because its to do with the pins and if so how does the board know. Its all to do with shifting numbers to the correct place isnt it not the other way round?
All this does is set bit 0 of r1 to 1, then it shifts that over to bit 16.
Bit 16 of the first word corresponds to the status of GPIO16 which is connected to the ACT LED.
str r1,[r0,#40]
Stores that value at the GPIO base address + 40 bytes, which is address GPCLR0 - GPIO Pin Output Clear 1. Writing to any bit in this word will set the corresponding GPIO output to 0V (The ACT LED is connected to +3.3v, so turns on when the GPIO is low and off when it is high).
and also is this right

mov r1,#1
This is just finding a place on the board to compute the correct number
lsl r1,#18
This is a cumulative way of finding the correct pin as it is to find the 6th pin within reason. That being that 4 bytes are assigned to each set of 10 pins and im guessing3x4 bits = a byte.
str r1,[r0,#4]
This is to maybe get it to the second set of 4 bytes. Seeming as we already treated bytes in a group of 3?
The locations GPIOBASE +0 to GPIOBASE+23 control the modes of the GPIOs. Each (4 bytes) word controls 10 GPIOs, using 3 bits per GPIO.
The second word controls GPIOs 10 to 19. By writing a 1 to bit 18 and leaving the rest as zero you are setting the mode of GPIO 10-15 and 17-19 as Input (000b) and GPIO 16 as Output (001b). This is a bit naughty, as it should really read the status of all those GPIOs and then alter the mode for GPIO 16, but for a bit of demo code it is OK.

Not a brilliant description, I'm afraid. I could do better.

The BCM2835 ARM Peripherals document describes the various memory locations and what they do in some details on Page 90 onwards. http://www.raspberrypi.org/wp-content/u ... herals.pdf

dwelch67
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 7:12 pm

The chip designers implemented the gpio in a certain way with certain features, this chip is quite simple BTW compared to others, pretty easy to get stuff up and running. The chip documentation which is pointed to by the anchored first topic in the bare metal forum and in a prior answer, describes which bits in which registers affect which gpio pins. There is more than one way to get the right value in the arm register the combinations of instructions were for just that to get a particular bit pattern in a cpu register then write that cpu register to a specific peripheral register (also within the chip but not within the arm processor) and writing to that peripheral (gpio) register causes the gpio to do certain things. The board designers chose to connect an led to a specific gpio pin, so from the board schematics we know which pin and which way makes the led light (I think a 0 on the gpio pin makes the led go on in this case which is not a-typical since you can sink more than you can source). So working backward from which gpio pin you have to enable that gpio pin as an output (usually being careful not to cause other gpio pins in the same peripheral config registers to do anything bad, in this case we are okay, a read-modify-write is sometimes the safest way to go), then output a 0 or a 1 on that line to make the led go on or off, etc.

Why did they do it this way or that in hardware, it was arbitrary that is how they chose to do it, why is the code setting a 1 then shifting it, it is arbitrary that is how that programmer chose to do it, there are other ways to get a 3 bit pattern in the right place in a register.

David

jran1216
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 8:30 pm

oh so could you say that the 3 bits in the association of pins are like empty sets. so basicly in each bit you have 000b. but really theres three bits governing the pin but there meaning is probably a way of relating to the rest of the board?

Like that did make sense theres really only 3 numbers to the bit but that number has to be displayed as a bit?
or i could quite simply say the shift was moving it to the 16th pin or bit right.

Dont worry if you dont want to reply. Super answer, both.

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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 8:33 pm

basicly there is no smaller umber than a bit?

dwelch67
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 9:03 pm

the docs show that bits 2:0 configure one of the gpio pins, that gives up to 8 possible configurations you could have for that pin, input, ouput, and whatever else they chose (open collector, alternate function, etc) I dont have the manual in front of me. bits 5:3 configure another pin as per the docs. Basically you just read the docs and you want to put the bit pattern in the right places for that peripheral register. The peripheral register has an address and the data value in the arm you do whatever you need to do to compute the register address and data value then you store the data value to that address and then the hardware is supposed to do what you told it, make some of those pins inputs, some outputs, or whatever.

Each chip vendor does this kind of thing their own way, some have one register just for output vs input one bit per gpio pin, and maybe another register for pull up one bit in the register per gpio pin. there are many ways to implement this and among all the processor vendors in the world over time they have come up with those many ways.

This is how this chip happens to work, so if the doc says for example to have the value in binary 100 in the bits 18:16 in some register then you can for example shift a 4 left 16 times or a 1 left 18 times. If you dont care about the other 29 bits in the register and/or zeros is okay for configuring all those other pins then you only need that one bit set, and when you write you will configure all of those pins at once. If you care about the other bits say you want to change only 5:3 you would want to

read the peripheral register
and with 0xFFFFFFC7
or with desired value<<3
write back to the peripheral register

dwelch67
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Re: How does raspberry pi OS work?

Tue Jul 01, 2014 9:06 pm

This is the bulk of bare metal programming. Reading the volumes of docs for each peripheral and each chip in the system you care about, looking at the schematics to see what is wired to what. And writing programs to read and write the bits per the docs, and understand that almost all docs have errors in them and you often have to hack your way through. In this case we have a wiki page for the Rasberry Pi where we keep track of the doc bugs we have found. And also the Raspberry Pi is real world in that the board had at least one rev change which can/does affect software. So from time to time your software has to somehow know the chip rev or board rev or whatever and have if-then-else code.

The baking pi example serves to get your feet wet, blinking an led is the printf("Hello World\n"); of bare metal programming.

David

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AndrewS
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Re: How does raspberry pi OS work?

Wed Jul 02, 2014 11:00 pm

dwelch67 wrote:In this case we have a wiki page for the Rasberry Pi where we keep track of the doc bugs we have found.
http://elinux.org/BCM2835_datasheet_errata

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Re: How does raspberry pi OS work?

Sat Jul 19, 2014 6:35 pm

Right OK thanks. Kinda understand that an operating system enables functions for operating and enables a program, or many to use certain parts efficiently. But another question that I can even begin to imagine. How on earth did Microsoft begin creating an initial operating system, if they didn't have a machine to assign numbers? Was that dos. Very clever stuff.

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Re: How does raspberry pi OS work?

Sun Jul 20, 2014 12:26 am

Microsoft history is readily available. they had a basic interpreter and a simulator to run it in and managed to get a fair amount of computer time which was a rare thing. Dropped out of college, moved to Albuquerque to be near MITS (in a trashy hotel across the street). Worked a deal to buy DOS for 50k or so, worked an even better deal to convince IBM to use it but ibm didnt own it microsoft did and microsoft could also sell dos. Everyone stole from Xerox PARC (apple, microsoft) and made a gui based os, and on and on (moved home to Seattle)...Microsoft started as a languages company though (as in compilers/interpreters BASIC), that basic we would call an ide today because you could edit and run programs in that command line environment (and save them to tape and retrieve them).

The good and bad of the ibm pc way was this idea of layers of bios then similar layers of dos on that, slowly going away but dos is one of the most successful embedded systems, some gas pumps, atms, cash registers, and various other machines were dos based for decades. Between the bios and dos you got basic services (disk access, video access, etc) or you could do your own thing, your choice. It is very tired now and I found it particularly annoying that the intel based board (Galileo?) that wants to be a beaglebone or raspberry pi, boots a bios like it is just a small pc. Once I saw that put it back in the box and it is a paperweight...

Microsoft had two factions the windows as in workgroups 3x folks which went up to windows 98 or maybe ME. Then the windows nt folks which is what we still see running. The NT folks were basically VMS folks that basically rewrote VMS.

So most of the microsoft story is not about inventing things but instead buying or stealing them from folks that had them, did they invent them? Sometimes, sometimes not...

Any operating system or embedded environment picks a starting point either you start bare metal and do a bootloader or perhaps you start with redboot or uboot, bios, efi, etc and you start your operating system on that as your low level. Ultimately having to build or get vendors or supporters to build drivers for the various peripherals (video, hard disk, keyboard, network, etc).

This bare metal forum is about the low end of that, starting with almost nothing, the gpu brings up ddr which is actually a fairly difficult thing, thank you broadcom, we get dumped into that the arm is running the on chip peripherals are super simple so there is no bringing those up really, the off chip are a little more work being usb based. The video on the raspberry pi is also done for us by the gpu, similar to the bios feel. The folks in this forum either want to or have written code that talks to the various peripherals to do various things. If this forum were not here and the documentation bugs and examples that have resulted, I would strongly suggest avoiding the raspberry pi, the bcm docs are pretty bad, much hacking is required to find the gaps. With that the thing is so easy to program I highly recommend it as a first time place to learn this kind of low level stuff or take it further and make your own operating system.

At and before that time you didnt write your programs on an editor and ran them right then you used punch cards, or pencil and paper and flipped switches representing bits to load memory (dec pdp macines for example or the altair as well). So programming you did by hand, then fed the computer, you had to load the bootloader with switches or whatever depending on the system, then that loader could read a paper tape or card reader, then that program was your operating system or main application or whatever and on and on. You didnt necessarily have terminal, maybe a printer, on the mainframes your program was loaded in, and when it was its turn to run it ran and you output stuff to a high speed printer, then later or the next day you went and picked up your output, figured out what the bug was, rearrange your punch cards, or make some more, and try again. google for the dec (digital equipment corporation) pdp 8 or pdp11 or other of the pdp family...as well as microsoft and ibm and the ibm pc, etc. intel 4004, 8008, 8080, etc. the zilog z80, the mos 6502. Back then you hand drew the chip transitors and metal interconnect layers by hand, drafting basically, then that was shrunk and built, the process is somewhat like a photographic process anyway. motorola 6800. CPM (an operating sysytem that ran on z80/8080 if I remember right which we see some of its legacy in dos).

Even if when we invent a new processor or operating system we have a wealth of tools, editors, compilers, etc from which we make other tools, so that the new invention can be rapidly prototyped and tested and eventually built.

rambling...sorry...

Someday in 20 years you will say "I remember when the raspberry pi came out, this is how things worked back then"...

David

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Re: How does raspberry pi OS work?

Sun Jul 20, 2014 7:41 pm

I think Microsoft was a bit cheeky holding back on the bare metal of their genius. Still it was the layers that made it so clever and would you say it developed computers as we see today? Simplicity. Thanks for your explanation. I will at least build something, hopefully quite a lot of things and that makes the raspberry pi so amazing. Shame about your tiny computer but isn't that what makes it more powerful? The shell. Yeah the architecture was something that baffled me and before I used to think operating systems could be built from electrical routes through the processor, and fluctuations stored by ram alongside components passed to store maths. Still baffles me. Suppose it poses the question, what could computers really be. Thanks again.

jran1216
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Re: How does raspberry pi OS work?

Sun Jul 20, 2014 8:54 pm

Because this is operating system related topic I will ask the question here. Now I know theres alot of subjects and information to read. Well up for that but if I'm reading, an idea's best to elaborate on. My idea is to create communication between devices and operating systems seem to be the best route. I have loads of ideas which I hope will make the operating system desirable. Now everyone likes a desktop and I myself like combining subjects. Ideas etc. Etc. I think its sad everyone has an idea but sometimes cant communicate it with physicalities, (what could the world be?). I myself can only coordinate and build or associate, alright that's one to combine. Now to cut a long story short, would a desktop that reshapes into a program be very processor hungry? My own second computer has stuck itself in software rendering mode which is annoying and I wouldn't like... well yeah. It just doesnt work. That being said if you can create a cheap computer, housing alright specs, in time it may work. Meaning only updating etc. Etc. Dont say its not possible, we see games do it all the time. Which I can make plenty of time if I want to make something.
If it is then I can atleast make universal programs like Microsoft so ideas can be created in other subjects. I think the two are close matched.
This is all behavioral right the sense I suppose is a larger document. One that pieces together.

tufty
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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 6:52 am

I'm kinda guessing english isn't your first language, am I right?

Please, try to explain what you mean by "a desktop that reshapes into a program".

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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 8:54 am

Second time thats been said. No english is my first. Dw I obv need to muddle through it on my own.

jamesh
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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 9:11 am

jran1216 wrote:Second time thats been said. No english is my first. Dw I obv need to muddle through it on my own.
Try reading through your posts after writing them - to the outside observer they are quiet confusing - hopefully a proof read will show places where more explanation of your ideas is necessary. Try looking from a readers POV.
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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 9:16 am

You should not need to muddle through I think you will find these replies a lot more helpful if you cut and paste into a word processor and just add some white space.

A lot of the replies from the geek end of the noob ubergeek spectrum forget how confusing a slab of words looks to a noob looking for answers.

I will read through this and see If I can decode some of this,

what jamesh said we were both posting at the same time I think his post was aimed at the writers.
Noob is not derogatory the noob is just the lower end of the noob--geek spectrum being a noob is just your first step towards being an uber-geek ;)

If you find a solution please post it in the wiki the forum dies too quick

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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 10:26 am

I thought it was a Turing test.

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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 2:23 pm

ame wrote:I thought it was a Turing test.
lol, me too!
Can't find the thread you want? Try googling : YourSearchHere site:raspberrypi.org

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Re: How does raspberry pi OS work?

Mon Jul 21, 2014 6:56 pm

I was avoiding mentioning it, but it read like the kind of crap spammers use to try and get round filters. Y'know, like this:
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Re: How does raspberry pi OS work?

Tue Jul 22, 2014 4:08 pm

Thats wicked humour, which is normally the best. Ok. You may think this makes a little sense and not a lot because purely my own thoughts are used, my apologies, but its still a rehash of information we communicate. (Really machines are used to create new ideas (that record data)) but sadly not computers... depends on the user. There is a lot of crap on a computer because the ideas of one is different to that of another. Now a screen is something that baffles me, and this ties into the user, it forms representations of things that we sense in reality, but the computer cant store that sense. Has there been any people that have tried?

Now we use perspectives from one moving reference getting a feel of what is happening around us. What if a computer could do that? It would need to merge from one reference to the next. Still keeping present information in ram causing it to be reintegrated back in a new scenario. Hence my question.

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Re: How does raspberry pi OS work?

Tue Jul 22, 2014 4:34 pm

Consider this. The universe is gradually moving to a decreasing entropy state (or is that increasing?). When you use a computer to sort an array of random numbers to a sorted array, does it's entropy decrease or increase? You have assigned more order to the universe, which is the opposite of the general view. Or, doe the heat dissipation of the system counter the decreasing/increasing entropy?
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