This is a collection of baremetal examples specifically for the
raspberry pi zero.  Look at my other repositories for other raspberry
pi boards.

I have a raspberry pi repo at github (same place you found this repo)
that has been popular (relative to anything else I have put out there).
It started as soon as I got my first raspberry pi which was part of
the mad rush when they first came out for the general public.  For some
reason I assumed the design of both the board and chip would remain
somewhat static.  But that didnt happen.  First the boards then the
chips.  Granted the pi-zero changed once, but didnt affect anything
I was using.  And who knows simply writing this down may cause a
murphy's law thing and ruin this repo as well (they may change the
chip/board or get rid of it all together).  I do not dislike the other
chips or cards, but, the ARMv7 and ARMv8 cores independent of being
multicore, have added a plethera of security and priority features
that I struggle personally to absorb.  I have absorbed enough to get
through my normal mix of examples, but the ARMv6 has close ties to the
early ARM cores, and has enough features to allow Linux or other
operating systems to run, but does not have a myriad of different
possible ways of running it with a number of rules for each.  So with
the pi-zero I can both isolate the examples I have already written, and
perhaps refactor, and make some more.

As far as I am concerned baremetal means no operating system.  The
cloud computing community is trying to mis-use the word to mean fewer
operating systems, so I recommend you use the term "cloud baremetal"
for that incorrect use case.  Or use whatever term you want.  These
examples take the reference material with specific register addresses
in the ARM address space and talk to those register directly to make
things happen.  Or to create some simple libraries/functions that
are called by the main program, but there is no operating system to
take that direct access to the hardware away from you.  At the same
time I am taking control of the GNU compiler toolchain, mostly for
portability reasons (all different pre-builts and ways to build your
own that wouldnt have worked), but also as an educational tool for
the first baremetal stumbling block, just getting the thing to boot.

There is a very nice community of fellow baremetal developers at the
raspberry pi website in the forums under Programming -> baremetal.

https://www.raspberrypi.org

So click on forums at the top then under Programming the first forum
there is baremetal (alphabetically not because we are any more
important than others).

Lets dive in and then talk about what is going on.


---- building the sd card ----

Go to

https://github.com/raspberrypi

You DO NOT need to clone any of these repositories.

Click on the firmware repo
Then the boot directory

bootcode.bin file then click on View Raw to download (or right click
and save as depending on your browser).

then back up one level and download start.elf the same way.

Those are all the files we need from this repo, dont need kernel.img
or anything else.

You are going to need an sd card, doesnt need to be very big 1GB,
2GB, etc.  Can probalby only get 4GB, 8GB or larger these days, thos
are fine.

Format the sd card for FAT32, if not already, you wont need any
other files so you can clean up the root directory.

Copy bootcode.bin and start.elf to the root directory of the sdcard.
I have left a kernel.img file in the root directory of this repo
copy that file over as well, do not create a config.txt like you read
about on other sites, will cover that later.  Safely unmount your sd
card and place it in the pi-zero, plug a usb cable into the connector
nearest the corner.  The led nearest that corner should blink in a
heartbeat like manner, two blinks, pause, two blinks, pause...If this
doesnt work then you cant go any further, either you have done something
wrong or they have changed the firmware in a way that is now incompatible
with my example.  The heartbeat directory contains the source for this
kernel.img.

----

One of two things is going to happen from here on out.  For each example
or at least almost all of them.  You can copy the kernel.img file
from that example to the sd card, and put it back in the raspi and
power it on.  I call this the sd card dance.

1) power off raspi
2) remove sd card
3) insert sd card in reader
4) plug reader into computer
5) mount/wait
6) copy binary file to kernel.img
7) sync/wait
8) unmount
9) insert sd card in raspi
10) power raspi
11) repeat

And that is just part of the job if this is the programmable interface
you have been given then like myself and others you may have to repeat
this dance hundreds of times per application.

The first simplest alternative is to make or use a bootloader.  I have
one or a few and they are about as lean and mean and simple as it gets.
No features, I have no interest in hundreds of thousands of lines of
code in a bootloader, or a bootloader that pretty much is or needs
an operating system itself just to deal with all of its features.  My
bootloader will allow you to do the sd card dance one more time then
after that you can download your program over serial/uart into the
pi's memory and run it.  For each build you want to test you then
only need to:

1) power off raspi
2) power on raspi
3) download program
4) run program

Or if you add a reset button
(see the reset_switch images switches easy to come by, broke the legs
off one side twisted the others to match the holes)

1) reset raspi
2) download program
3) run program

There are two pins near the P1 header that say RUN next to them if you
short those pins together it will reset the PI.

Unfortunately for everyone the pi-zero did not come with the P1 headers
installed, so at some point you are going to want access to at least the
uart pins if not the whole header.  So either some soldering is required
or some push in friction based pins are needed.  Likewise a momentary
switch (normally open) to use as a reset button.

In order to use my bootloader you will need to gain access to those
uart pins, direct wired or header pins or some paperclips between boards
whatever.  This is actually not a compilicated thing you just need
the right tools (not expensive for ones that will work) and the
confidence to try.  I now have a collection of boards, almost all
FTDI based, but that is just due to their popularity, they are a
more expensive part than others.  Note RS-232 has no business in this
discussion, it is an electrical standard and will blow up your board,
you are looking for 3.3v serial or uart from usb on the host side.

Some examples

https://www.adafruit.com/product/954
https://www.sparkfun.com/products/9873
https://www.sparkfun.com/products/13263

At least one of which (I reserve the right to change this list at any
time) may also require soldering or cuts and jumpers to select the
desired voltage.  I bought a bunch from asia on ebay for like $2 each
or less with a 3.3v and 5v jumper you want 3.3v for the raspberry pi.

mysetup.png is a picture of my pi zero setup

A uart solution, along with an led and resistor to blink are your
two most important tools in your baremetal programmers toolbox.  Plus
jumper wires or some way to hook these things up.  Soldering is
eventually required, but you can sometimes just get someone to do
that for you, or buy it that way.  Note that the general rule is the
tx and rx are in reference to that board/product so the raspberry pi
tx pin is an output, and rx an input, so you want to hook the tx of the
uart to the rx of the raspi and the tx of the raspi to the rx of the
usb uart board/cable.

I tend to use FTDI breakout boards with male pins on them that came
with or I added.  And female to female jumper wires I bought in a 100
pack from Sparkfun.  Adafruit carries these things as well.

-----

You are going to need to go to the raspberrypi.org baremetal forum page
https://www.raspberrypi.org click on forums, under programming click on
baremetal.

or maybe this link works directly

https://www.raspberrypi.org/forums/viewforum.php?f=72

There is a sticky topic near the top called Bare Metal resources.
(is it bare metal with a space or baremetal without?  I have seen and
used both)

You definitely need the BCM2835-ARM-Peripherals document
And note the errata link there as well, there are lots of errors in the
document (which is true for any vendors documentation).

Either there or just google raspberry pi pinout to see the P1 header
pins.

The ones we care about initially are when you are holding the board
such that you are looking down on the top of the board (the side
the bulk of the components are on) and the dual row header is on the
right, the top right pin is pin1 the top left pin is pin 1 and they
alternate like that.  The right edge of the board is thus 2,4,6,8
and so on.

2  outer corner
4
6  ground
8  TX out
10 RX in

Wire these up to your usb uart solution tx to rx, rx to tx.  Figure
out a dumb terminal program like minicom on linux or teraterm on windows
or a myriad of others (putty works).  And then you can install my
bootloader on your sd card, and have a lot of fun without needing to
remove the sd card again.  When ready to deploy your application then
copy your binary file to the board with the filename kernel.img and
power on.  Note that I am initializing some peripherals so if you rely
on those and didnt initialize them yourself then you have more work
to do.

See the bootloader10 directory for more information on using this
bootloader.  A binary bootloader.img has been left in the base
directory of this repo.  Copy this file to kernel.img on the sd card
in order to use this binary.

-----

Start with the blinker programs like blinker01.  Blinker01 covers
specific detail on how the bootstrap works to get the C programs up
and running, linker script and other build topics.

The raspberry pi zero has an led tied to GPIO pin 47 so these programs
will blink that led using various methods to determine the time
period of the blinks.  Various timers in the system and/or different
ways to use the timers.

These examples are built to be placed and run from the address 0x8000
which is NOT where the ARM boots.  What we believe happens is

-- some on chip (p)rom containing gpu code runs on the gpu to find
the bootcode.bin file and load it (bootcode.bin is a gpu program not ARM)

-- bootcode.bin initializes DRAM and other things and looks for and
loads start.elf (another gpu program)

--  start.elf contains the main gpu firmware that manages video for us
and other things.  It then searches for kernel.img and loads it into
the ARM and runs it.

Note there are now several supported file names other than kernel.img
that have subtle differences in what they do, this allows you to
have one sd card you can move around I guess across your collection
of different raspberry pi cards.  Dont know...We will use kernel.img
here.  You can modify what happens by using a file named config.txt
but I recommend against that for these examples.  You can/will make it
so these cant run if you start to mess around there.  Other than early
firmare images that were replaced before the masses had access to the
raspberry pi, for the pi-zero and older pi1 cards using kernel.img
it places the binary at 0x8000.  The arm boots from address 0x00000000
there is some code which we can examine if we want, that is placed
at 0x00000000 that is meant to prep the ARM to run linux, doesnt
hurt us, and then branches to 0x8000.  So all of these programs are
built to run at 0x8000, you are welcome to venture off from there if
you want.

I will copy or replace the bssdata directory/readme that I wrote for
the other repository.  The very short story is linker scripts dont
port and are an artform to themselves.  You might not yet realize
that a program does not enter at main(), there is a bootstrap that
happens before that.  Something has to zero your uniniitalized
variables and initialize the others:

unsigned int x;   //assumed to be 0 when main() runs
unsigned int y=5; //assumed to be 5 when main() runs

This takes code and more important linker script magic to get it all
somewhat automatically built by the tools, I dont support that
directly although the longer explanation in bssdata will show some
ways to make that work since we are running ram only programs here.
On a microcontroller where baremetal is very common your program
and information about global variables (x starts off as zero and y
a five above) needs to be in non-volatile memory: flash/rom.  And then
copied to ram or some ram zeroed.  Generally the four minimal things
a bootstrap does

1) set the stack pointer
2) zero .bss
3) copy .data
4) call main()

I skip 2 and 3.

The other thing I do which you may or may not like is I abstract the
load/store access in PUT32/GET32 type functions:

void PUT32 ( unsigned int address, unsigned int data);
unsigned int GET32 ( unsigned int address );

Like we see here

    ra=GET32(GPFSEL4);
    ra&=~(7<<21);
    ra|=1<<21;
    PUT32(GPFSEL4,ra);

I have many many years of experience on various platforms at various
levels and inside and connected to logic simulators for processors
and/or peripherals.  This abstraction has countless benefits, but
yes it does have a performance hit, but you can always inline your
way out of that and not modify the PUT32() calls.  But without the
abstraction it is a signifcant amount of work to get the features you
gave up back.  Take it or leave it, your choice, this is how I do it
and what you will see here.

Since I do spend so much time day job and for fun evenings deep in
the bowels of the chip or system.  The datasheet on one side of the
screen and the code on the other, bits and addresses are directly
visible, layers of hidden header files drive me nuts, cant see it
cant debug it, sometimes have to compile and disassemble just to
see what is going on.  In turn this drives other folks nuts, make
your own repository with your own methods.

This code is not meant to be a library or that kind of solution in
any way, it is instead a reference design, or an example.  Instead of
just taking as is, you being to replace a little at a time or the
whole thing at once.  Meant to be as easy to read as possible in that
it is usually brute force, very few files so everything is very visible,
you can look at the manual and the code on the screen at the same time
and connect all of the dots.  Then re-write it in whatever language or
style you prefer, as simple or as complicated as you care to make it.

I used to use newlib to primarily get at printf(), but then it became
difficult to impossible to build for a while there, now it builds
again but I realized I dont need any of the C library calls, the very
rare occasion (memcpy, memset perhaps) I just implement my own.  I
still rely heavily on uart output for debugging and seeing what is
going on, and have a very small function that prints hex numbers out
as you will see.

printf("Hello World\n");

Is actually one of the most complicated programs you can write, the
backend required to support that in a typical system is massive.  A
large amount of the C library, then there is all the system and video
stuff.  You will see how simple that can be avoided by using a uart
and a dumb terminal (the dumb terminal is itself now your computer
so very complicated as well, but boils down into a dumb task).

If you are not comfortable with binary/hexadecimal, you are not quite
ready for baremetal or these examples and manuals.  You can count to 16
with the four digits other than your thumb on one hand (two hands you
can count well past 256), practice, or practice writing out 0 to 15
(0x0 to 0xF, 0b0000 to 0b1111) with all three columns, binary, hex and
decimal

0000 0x0 0
0001 0x1 1
0010 0x2 2
0011 0x3 3
0100 0x4 4
0101 0x5 5
0110 0x6 6
0111 0x7 7
1000 0x8 8
1001 0x9 9
1010 0xA 10
1011 0xB 11
1100 0xC 12
1101 0xD 13
1110 0xE 14
1111 0xF 15

Just memorize it.  That is the easy part, shifting and masking come
next along with ANDing, ORing, XORing, NOTing...Basic skills you will
need for writing values to or reading values from registers in
peripherals.

Also get a calculator that you can convert between hex and decimal with
two button presses (shift + hex, shift + decimal) or use a soft one
on your computer or smart phone or get crafty with python or perl or
bash commands on the terminal.

We are not going to even talk about floating point here.

I have used JTAG on these boards, and may venture into that as I have
working examples in the other repository.  I use it constantly at my
day job for running programs, but for most of my raspi work I just use
my own bootloader on boards that I added a reset button.  The GPU is
the processor in control on this chip and its JTAG is exposed, the ARM
jtag is buried and uses shared pins, so you have to have a small program
that internally connects those pins to the outside just to use JTAG
to get at the on chip debugger in the ARM.

-----

After the blinker programs look at the uart programs.  Then wonder
about to see what else I might have shown.

-----

As far as the basic peripherals go this chip is very easy to program,
now as it often happens the documentation is lacking, most of baremetal
work is reading docs and figuring things out, the writing of code is
the easy part, in the noise.  Fortunately as mentioned above there
is a page that contains errata found by the community.  Microcontrollers
which this is not are far more concerned about power consumption
and there are additinal layers of work you have to do, they often
need more features in the peripherals as that is kinda why you are
using the thing, where here the peripherals are not why you are using
the thing, this is meant to be a general purpose computer, not a
tiny widgit you use to display the time on a clock radio.

-----

The BCM2835 ARM peripheral document mentioned above, which you will
need along with the ARMv6 Architectural Reference Manual and the
ARM1176JZFS Technical Reference Manual from ARM's website
(Actually the ARMv6 is covered in the ARMv5 Architectural Reference
Manual which was just the ARM ARM and then things got too complicated
to keep in one manual, just like this additional repo, so they
made new manuals and ARMv4 through ARMv6 are in the ARMv5 manual
for the most part).
http://infocenter.arm.com
You might have to give up an email address.  Anyway, the document shows
addresses for things in the form 0x7Exxxxxx, but there is an important
drawing that shows how that GPU/system address connects to the ARM
address space at 0x20xxxxxx.  Now they werent quite thinking this
through as 0x20000000 is 512MB, granted MMU magic can work around that
but with the pi2 they changed where the peripherals are in the ARM
address space to 0x3Fxxxxxx giving a larger linear address space for
memory.  The pi zero uses the BCM2835 and the gpu firmware and/or
hardware, but I assume it is a soft setting, uses 0x20xxxxxx as the
base address for all of the peripherals.

The total advertised amount of memory (DRAM) for a raspberry pi is
shared between the ARM and GPU, you might be able to do some config.txt
thing and take it all (and lose graphics/video/display) or move
the dividing line around a bit.  All of these examples are tiny in
the KBytes so I dont have to worry about this but may eventually
depending on your applications you write.

----- toolchain -----

See the TOOLCHAIN file for information on where to get a toolchain to
use for building these programs.

----- assembly language -----

At time small amounts of assembly language are required for baremetal
work.  With practice you can often write simple C functions, compile
and disassemble to discover instructions of interest.  I have provided
all the assembly you will need to get started.  You are welcome to
learn more and I recommend it.  As mentioned above one of the big
jobs in baremetal is reading and interpreting documentation, another is
mastering the toolchain as your program has to conform to the hardware
the chip the processor, not some operating system as the compiler is
setup to do by default.  A major trap you will fall into is simply not
booting or crashing so early you cant "see" what happened.  So some
level of understanding of both the tools and assembly language and
machine code at times will separate you from the pack and allow you
to debug through these problems.  A big reason for this repo and these
examples is to get folks past this point, it is scary to dive into
this kind of programming and the number of ways you can fail is massive
first and foremost loading your program and resetting or powering on
and have nothing happen and absolutely no idea how to debug it.  There
is no way to know the sheer number of folks that have given up at
that point and never returned to this, a large number of them could have
been great contributors no doubt.  Some may go off and preach that you
should never work at this level...

A simple example of what I am talking about is the first time you setup
a new build system for yourself and a linker script you should check to
see that it is placing things where want them.   We need the entry point
which I call _start because I think gnu requires that but perhaps not
either way it has to be the first instruction, and I have successfully
done that.

Disassembly of section .text:

00008000 <_start>:
    8000:   e3a0d902    mov sp, #32768  ; 0x8000
    8004:   eb000005    bl  8020 <notmain>

00008008 <hang>:
    8008:   eafffffe    b   8008 <hang>

0000800c <PUT32>:
    800c:   e5801000    str r1, [r0]
    8010:   e12fff1e    bx  lr

00008014 <GET32>:
    8014:   e5900000    ldr r0, [r0]
    8018:   e12fff1e    bx  lr

0000801c <dummy>:
    801c:   e12fff1e    bx  lr

00008020 <notmain>:
    8020:   e92d4070    push    {r4, r5, r6, lr}
    8024:   e59f00ac    ldr r0, [pc, #172]  ; 80d8 <notmain+0xb8>
    8028:   ebfffff9    bl  8014 <GET32>
    802c:   e3c0160e    bic r1, r0, #14680064   ; 0xe00000
    8030:   e3811602    orr r1, r1, #2097152    ; 0x200000

But another thing that is not obvious here is the branch link (bl)
to main shows 0x8020 as the address, which is correct, but for this
instruction set that address is not embedded in the instruction, the
instruction says branch link to 5+2 instructions ahead of where I am
now.  I could load and run this code at a different address and at
least that part would work, and sometimes you need to do this.  This
is called position independent code (PIC) and you sometimes need
to know how to do this.

But for now, these examples are tested, I have provided all the
assembly language you need.

This is not the absolute minimim linker script that one could make
for gnu ld, but pretty close.

MEMORY
{
    ram : ORIGIN = 0x8000, LENGTH = 0x10000
}

SECTIONS
{
    .text : { *(.text*) } > ram
    .bss : { *(.bss*) } > ram
}

Again see the bssdata directory on a much longer discussion.  The linker
script here is its own programming language, and linkers are toolchain
specific so I dont like to rely heavily on the linker script and
connnections between it and the main code, so I dont, I do the minimum
to get it to build for the address spaces I want and place the code
in the right order so it boots.  The LENGTH value is just an arbitrary
size I chose, ideally the amount of memory you want to allow for the
program/data in this case, with some space left for the stack if you
put your stack above this (see blinker01).  The skeleton for this code
is highly portable and I dont want to be constantly changing this
size, so I pick small numbers that fit most places and only increase
when it fails to build, for that one application.

My prefernce is to call the linker directly rather than have gcc do it
indirectly (gcc the program is for the most part just a program that
calls other programs, the compile process takes many steps and a number
of separate programs it launches, including the assembler at the end as
it generates assembly language then callse the assembler use -save-temps
to see the intermediate files, the linker is one of those called unless
you use -c or -S).  And then feed it a linker script, can do this from
the gcc command line.  If/when you run into gcc library issues (need
to do a divide for example) the linker for whatever reason does not
know where it was launched from or want to use that information, gcc
does and will use that information to find the gcc library archives in
relative paths, so at times using gcc with -Xlinker is needed or at
least makes for less work.  I tend to wait for it to break rather than
use gcc all the time.