Step. 02 – Bare Metal Programming in C Pt. Valvers. In this part of the tutorial we’ll look deeper into the linker and the C- Library so that we end up with a working C- Library link. Exciting stuff, huh?! Let’s look further into what the compiler and linker are doing in order to create our bare- metal executable. The C- Runtime (different to the C- Library!) is currently missing from our code.
In a lot of embedded systems the C- Runtime is essential, or else things break instantly.The most notable thing that’s instantly visible in most embedded systems is that static variables are not initialised.This is why in our previous example, we were working without pre- initialised variables.Instead, we initialise the variable in the code at the start of main from a pre- processor define.Github. The code for the tutorials is now on Github. Toshiba E Studio 200 Drivers Download Windows 7 there.
You can either browse the code, checkout the code, fork, branch, or download as a zip from Gib. Hub. Understanding the C- Runtime Startup. Let’s modify and use a pre- initialised variable instead: part- 2/armc- 0. GPIO Register set */. GPIO_BASE. /** Simple loop variable */.
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Thanks for this. Tried setting it up as per your suggestions but it crashes out during the ct-ng build with [DEBUG] Trying ‘ftp://gcc.gnu.org/pub/gcc/infrastructure. Software development and applications programmin on Linux. YoLinux: Linux Information Portal includes informative tutorials and links to many Linux sites. The YoLinux. Biopython Tutorial and Cookbook Jeff Chang, Brad Chapman, Iddo Friedberg, Thomas Hamelryck, Michiel de Hoon, Peter Cock, Tiago Antao, Eric Talevich, Bartek Wilczy&#.
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Main function - we'll never return from here */. Write 1 to the GPIO1. Function Select 1 GPIO. GPIO1. 6 as an output */. LED_GPFSEL] |= (1 < < LED_GPFBIT). Never exit as there is no OS to exit to!
Set the LED GPIO pin low ( Turn OK LED on for original Pi, and off. LED_GPCLR] = (1 < < LED_GPIO_BIT). Set the LED GPIO pin high ( Turn OK LED off for original Pi, and on. LED_GPSET] = (1 < < LED_GPIO_BIT).
Compile it (using the build. The size of the binary image is now 3.
The code, when written to the SDCARD still works – this isn’t really expected without a working C- Runtime in place to initialise the variable gpio before calling main!? In fact, this embedded system is different to a lot because we’re loading an entire binary image into RAM and then executing from RAM. The majority of systems have a non- volatile memory section (Flash/ROM) where the executable code resides, and a volatile memory section (RAM) where the variable data resides. Variables exist in RAM, everything has a position in RAM.
When we compile for a target that executes from an image in Flash and uses RAM for variables, we need a copy of the initial values for the variables from Flash so that every time the system is started the variables can be initialised to their initial value, and we need the code in Flash to copy these values into the variables before main() is called. This is one of the jobs of the C- Runtime code (CRT). This is a code object that is normally linked in automagically by your tool- chain. This is usually not the only object to get linked to your code behind your back – usually the Interrupt Vector Table gets linked in too, and a Linker Script tells the linker how to organise these additional pieces of code in your memory layout. Normally of course, this happens without you knowing.
In general, you’ll select your processor or embedded system on the command line and the appropriate linker script and C- Runtime is chosen for you and linked in. I urge you to go and look at your arm- none- gcc- eabi install now to see some of these files. Look under the arm- none- eabi sub- directory and then under the lib sub- directory. The C- Runtime code is a binary object file and is called crt. C Library for info is an archive of object files called libc.
Under the ldscripts subdirectory you’ll find the standard linker scripts. It’s just worth a look to know they’re there. GCC uses a thing called specs files too, which allow specifying system settings so that you can create a machine specification that allows you to target a machine easily.
You can select a custom specs file with a command line option for GCC, otherwise gcc uses it’s built- in specs. GNU tool- chain, but they provide an excellent way of supplying machine- specific compilation settings. For the embedded engineer they’re worth knowing about!
So, now we’ve got two questions, why does our code work – because the initialisation isn’t present in the C- Runtime? The code works without any initialisation because the variables exist in the same memory space as the code. The bootloading process results in the raspberry- pi kernel being loaded into RAM in order to be executed, the GPU bootloader runs before the ARM processor we’re targeting runs, and loads the kernel. Because of this, the variables position within the binary image becomes their variable memory location. The image is loaded by the boot- loader at address 0x. So the bootloader has essentially done a taskt that the C- Runtime would normally do, copy the initial values of initialised variables from non- volatile memory to volatile memory.
Cool. Look at the code produced closer with a disassembler. You’ve already got a disassembler! It comes with the toolchain; Weclome to the world of objdump (or in our case arm- non- eabi- objdump). We diassemble the elf file because then objdump knows what processor the binary was built for. It also then has knowledge of the different code sections too.
There’s a disassemble. You’ll get a kernel. RPI2) code: Disassembly of section . Disassembly of section .
The first line loads r. Program Counter(PC) + 1. At this address, there’s the value 0x. Remember that this file is loaded at. Virtually all ARM devices are the same in this respect, so thelinker is expecting this to happen. At the base address 0x. RPI1, for the RPI2 this will be 0x.
F2. 00. 00. 0. So this is why the codeworks without any explicit loading or initialisation, but let’s look at exactly what’s going onand find out why it works like this. The value at the end of our executable image can be viewed by using hex editor: The next line however loads r. PC + 1. 20. At thisaddress, there’s the value 0x. This is outside of our binary image size. This variable valueis going to be undefined.
This is the tim variable, which is indeed not initialised with anythinguntil we start using it in a for loop on line 7. If you know the C- Standard, then you’ll knowthis is actually incorrect behaviour, but we’ll get to that in a bit!)Let’s do a sanity check and make sure we’re right: \arm- tutorial- rpi\part- 2\armc- 0. A __bss_end__. 0. A __bss_start. 0. A __bss_start__. 0.
D __data_start. 0. A __end__. 0. 00.
A _bss_end__. 0. 00. A _edata. 0. 00. 10. A _end. 0. 00. 80. N _stack. U _start. D gpio. 0. 00. 08. T main. Yep! We’re talking the same as nm.
That’s good at least! Simulation. If you’re running on Linux you get the added bonus of having GDB built with a simulator for ARM. Therefore, at a basic level, you can load and step through code on the simulator. Below, I stepthrough the armc- 0. It’s great to be able to do this whenyou’re trying to understand exactly what’s going on: brian@brian- PH6.
UD3- B3 ~ $ arm- none- eabi- gdb. GNU gdb (7. 6. 5. Copyright (C) 2. 01. Free Software Foundation, Inc. License GPLv. 3+: GNU GPL version 3 or later < http: //gnu. This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying". This GDB was configured as "- -host=x.
Type "show configuration" for configuration details. For bug reporting instructions, please see. Find the GDB manual and other documentation resources online at. For help, type "help". Type "apropos word" to search for commands related to "word".
Connected to the simulator. Loading section . Loading section . Start address 0x.
Transfer rate: 1. Reading symbols from kernel. Breakpoint 1 at 0x. Starting program: /home/brian/kernel. Breakpoint 1, main () at armc- 0. No such file or directory. Dump of assembler code for function main.
End of assembler dump. Now let’s do an experiment. There’s a reason I decoded the value 0x. GPIO Register set */.
GPIO_BASE. /** Simple loop variable */. Main function - we'll never return from here */. Write 1 to the GPIO1.
Function Select 1 GPIO. GPIO1. 6 as an output */. LED_GPFSEL] |= (1 < < LED_GPFBIT). Never exit as there is no OS to exit to! Set the LED GPIO pin low ( Turn OK LED on for original Pi, and off. LED_GPCLR] = (1 < < LED_GPIO_BIT). Set the LED GPIO pin high ( Turn OK LED off for original Pi, and on.
LED_GPSET] = (1 < < LED_GPIO_BIT). Set the LED GPIO pin low ( Turn OK LED on for original Pi, and off.
LED_GPCLR] = (1 < < LED_GPIO_BIT).When we look at thiswith nm we see that the __data_start section shifts by the same amount as the code we’ve added: C: \Users\Brian\Documents\Git. Windows Update Does Not Install . Hub\arm- tutorial- rpi\part- 2\armc- 0.A __bss_end__. 0.A __bss_start. 0.A __bss_start__. 0.