Fixed address variable in C

Your linker and compiler don't know about that (without you telling it anything, of course). It's up to the designer of the ABI of your platform to specify they don't allocate objects at those addresses.

So, there is sometimes (the platform i worked on had that) a range in the virtual address space that is mapped directly to physical addresses and another range that can be used by user space processes to grow the stack or to allocate heap memory.

You can use the defsym option with GNU ld to allocate some symbol at a fixed address:

--defsym symbol=expression

Or if the expression is more complicated than simple arithmetic, use a custom linker script. That is the place where you can define regions of memory and tell the linker what regions should be given to what sections/objects. See here for an explanation. Though that is usually exactly the job of the writer of the tool-chain you use. They take the spec of the ABI and then write linker scripts and assembler/compiler back-ends that fulfill the requirements of your platform.

Incidentally, GCC has an attribute section that you can use to place your struct into a specific section. You could then tell the linker to place that section into the region where your registers live.

Registers regs __attribute__((section("REGS")));

A linker would typically use a linker script to determine where variables would be allocated. This is called the "data" section and of course should point to a RAM location. Therefore it is impossible for a variable to be allocated at an address not in RAM.

You can read more about linker scripts in GCC here.

Your linker handles the placement of data and variables. It knows about your target system through a linker script. The linker script defines regions in a memory layout such as .text (for constant data and code) and .bss (for your global variables and the heap), and also creates a correlation between a virtual and physical address (if one is needed). It is the job of the linker script's maintainer to make sure that the sections usable by the linker do not override your IO addresses.

When the embedded operating system loads the application into memory, it will load it in usually at some specified location, lets say 0x5000. All the local memory you are using will be relative to that address, that is, int x will be somewhere like 0x5000+code size+4... assuming this is a global variable. If it is a local variable, its located on the stack. When you reference 0x100, you are referencing system memory space, the same space the operating system is responsible for managing, and probably a very specific place that it monitors.

The linker won't place code at specific memory locations, it works in 'relative to where my program code is' memory space.

This breaks down a little bit when you get into virtual memory, but for embedded systems, this tends to hold true.

Cheers!

Getting the GCC toolchain to give you an image suitable for use directly on the hardware without an OS to load it is possible, but involves a couple of steps that aren't normally needed for normal programs.

1. You will almost certainly need to customize the C run time startup module. This is an assembly module (often named something like crt0.s) that is responsible initializing the initialized data, clearing the BSS, calling constructors for global objects if C++ modules with global objects are included, etc. Typical customizations include the need to setup your hardware to actually address the RAM (possibly including setting up the DRAM controller as well) so that there is a place to put data and stack. Some CPUs need to have these things done in a specific sequence: e.g. The ColdFire MCF5307 has one chip select that responds to every address after boot which eventually must be configured to cover just the area of the memory map planned for the attached chip.

2. Your hardware team (or you with another hat on, possibly) should have a memory map documenting what is at various addresses. ROM at 0x00000000, RAM at 0x10000000, device registers at 0xD0000000, etc. In some processors, the hardware team might only have connected a chip select from the CPU to a device, and leave it up to you to decide what address triggers that select pin.

3. GNU ld supports a very flexible linker script language that allows the various sections of the executable image to be placed in specific address spaces. For normal programming, you never see the linker script since a stock one is supplied by gcc that is tuned to your OS's assumptions for a normal application.

4. The output of the linker is in a relocatable format that is intended to be loaded into RAM by an OS. It probably has relocation fixups that need to be completed, and may even dynamically load some libraries. In a ROM system, dynamic loading is (usually) not supported, so you won't be doing that. But you still need a raw binary image (often in a HEX format suitable for a PROM programmer of some form), so you will need to use the objcopy utility from binutil to transform the linker output to a suitable format.

So, to answer the actual question you asked...

You use a linker script to specify the target addresses of each section of your program's image. In that script, you have several options for dealing with device registers, but all of them involve putting the text, data, bss stack, and heap segments in address ranges that avoid the hardware registers. There are also mechanisms available that can make sure that ld throws an error if you overfill your ROM or RAM, and you should use those as well.

Actually getting the device addresses into your C code can be done with #define as in your example, or by declaring a symbol directly in the linker script that is resolved to the base address of the registers, with a matching extern declaration in a C header file.

Although it is possible to use GCC's section attribute to define an instance of an uninitialized struct as being located in a specific section (such as FPGA_REGS), I have found that not to work well in real systems. It can create maintenance issues, and it becomes an expensive way to describe the full register map of the on-chip devices. If you use that technique, the linker script would then be responsible for mapping FPGA_REGS to its correct address.

In any case, you are going to need to get a good understanding of object file concepts such as "sections" (specifically the text, data, and bss sections at minimum), and may need to chase down details that bridge the gap between hardware and software such as the interrupt vector table, interrupt priorities, supervisor vs. user modes (or rings 0 to 3 on x86 variants) and the like.