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flat assembler > Non-x86 architectures > raspberry pi, arduino

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edfed



Joined: 20 Feb 2006
Posts: 4171
Location: 2018
hello, i am trying to use assembly langage on rpi and arduino compatible boards (arm, avr, esp, stm) in order to understand their architectures.

it is very powerfull to use assembly langage to get the deep understanding of an architecture. then, i plan to do some stuff inside them, using fasm/fasmg of course.

and as i not really active here these times, i have two choices:

asking everybody.
searching in the exponential number of topics and search results.

and then, create the topic that will reference every pertinent ressources about this subject. cause i like a lot to create things.

[fasmarm] rpi lower16 upper16
Raspberry Pi
[AVR] fasmg assembles with error(s)

_________________
Smile fool iem.design
Post 25 Oct 2018, 10:47
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16125
Location: Hyperborea
Hi edfed.

What are you planning?
Post 25 Oct 2018, 22:13
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edfed



Joined: 20 Feb 2006
Posts: 4171
Location: 2018
hi revolution.

i am planning to program avr boards using assembly cause i meet lot of limitations using the "c++" arduino ide.

and i want to code avr in asm, just because it helps a lot to understand the core itself.

about the current projects:

for the raspberry pi, i need to just have a starfield on the hdmi screen at startup and use gpio to change some stuff.

on arduino, it is just in order to get more knowledge on the avr architecture, and be able to bypass the arduino ide and tools to upload binaries Smile

as arduino boards are seen as serial ports, it is very possible to code something in fasm to get it working directlly, and be able to do self modifiable code, in order to change functions and data at runtime, without flashing the entire binary.
Post 29 Oct 2018, 12:19
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16125
Location: Hyperborea
There is a good chance you can get help with the rPi code on this board.
Post 30 Oct 2018, 19:07
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edfed



Joined: 20 Feb 2006
Posts: 4171
Location: 2018
hurrah, i got the rasp working using ssh on my pc.

a friend installed it for me, and i got a cpp project running, with code edition with netbeans on the pc Crying or Very sad

let say, to display one pixel, i have approximatelly 4G raspbian on the SD, and 1.5G of tools on the PC...Embarassed here, assembler takes all it's sense.

it is very ugly, the rasp is using raspbian jessie distro on a 4G SD, and i don't have many more room for files cause linux... is linux, very frustrating when it comes to play with access rights, paths, and so on....

the startup is very slow, i get a 1 minute console scrolling, very annoying.
and linux file system is really not my taste.

now thenext goal is to get the minimal set of files on the sd to fit my needs, and after, run some fasmarm code.

see you in 2034 then Laughing
Post 17 Nov 2018, 13:24
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bzt



Joined: 09 Nov 2018
Posts: 17
Hi,

I've never used fasm with arduino, but I can help with Raspberry Pi. You create a raw AArch64 binary (not ELF) and save it on the first (fat) partition on the SD card as "kernel8.img". If you've an AArch32 binary, name it as "kernel7.img". (Where the number designates ARMv8 for AArch64 and ARMv7 for AArch32 respectively). You'll also need three firmware files (bootcode.bin, start.elf, fixup.dat) on that fat partition, from here. That's all, forget all the other rasbian files and partitions.

What's common: on a RaspberryPi3 board, your code will be executed in paralell by all 4 cores at once (in contrast to x86, where you have to send an interrupt from BP to start APs). You can distinguish by reading a system register (mpidr_el1). You have to set up a stack and clear the bss yourself. For both modes, you can use the Mailbox interface (an MMIO area) to communicate with the board (like reading board number, setting screen resolution and require framebuffer address, etc.). In both cases your code will be running at hypervisor level (EL2), unless you set "kernel_old=1" in the config.txt, which will load your code at address 0 and executes it at EL3 (!). Therefore you should write a trampoline code which switches to supervisor (EL1) or userspace (EL0). I think EL1 is what you're after at first.

What's different: kernel7.img is loaded at 0x8000 (note 3 zeros), and you'll get some system table pointers in registers. Kernel8.img is loaded at 0x80000 (note 4 zeros) and you can't rely on the registers (linux uses a different method in 64 bit mode to pass dtb). So it's better to use a Mailbox call to get the command line for example, as that works for both AArch32 and AArch64.

An example minimal C trampoline code is here. Throw rocks on me, but I've used GNU toolchain for that. Thankfully there are lots of pure fasmarm examples as well, I suggest to take a look at
Peter Lemon's tutorials,
which includes examples in AArch32 and AArch64 for all models (Rpi1, Rpi2, Rpi3). The main difference among board models is that 2 and 3 uses a different MMIO address than the original Pi, and only Rpi3 supports AArch64. Not a starfield, but Peter's graphic mode hello world example is here, illustrating core and Mailbox handling as well as writing to the framebuffer:
https://github.com/PeterLemon/RaspberryPi/blob/master/HelloWorld/CPU/kernel8.asm#L22

Oh, and very important. Don't believe the BCM2835/6/7 documentation! They are full of typos and bugs, always look for an errata!

Cheers,
bzt
Post 17 Nov 2018, 14:53
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pelaillo
Missing in inaction


Joined: 19 Jun 2003
Posts: 864
Location: Colombia
Post 22 Nov 2018, 12:50
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