By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Note that I've added the volatile qualifier. This is always a good idea when reading or writing hardware registers, because it ensures that each access you perform in your C code actually shows up in the generated code. I like these better than direct use of pointers because:.

Sometimes often what the compiler thinks volatile means is not what the programmer intended and vice versa. It is good practice to always check the resulting machine code when the volatile keyword is used, in order to avoid surprises. For further references, see e. LCDCW1 is just an integer value. You need to cast it to integer or type you require pointer and then use it.

Learn more. Asked 10 years, 6 months ago. Active 10 years, 4 months ago. Viewed 23k times. The codes have problems, how to correct it? Dale Hagglund Active Oldest Votes. I like these better than direct use of pointers because: I think they read better in the code, identifying what I'm doing, reading a register, instead of focusing on how I'm doing it. Some registers require a multi-step process to read from the hardware. This is hidden easily in macros of this style, and the bulk of my code still refers to the registers I'm interested in, not the complicated way the hardware makes me touch them.

Finally, by using asm s, I know exactly how I'm accessing the register. Sometimes there are special instructions or address spaces needed to get to a register, which usually can't be generated by the C compiler. Even if you disagree with the motivation for using asm statements, I'd still suggest wrapping your register accesses in macros or inline functions like these. Dale Hagglund Dale Hagglund See Schedler's comment below for an interesting paper on how most compilers fail to implement volatile properly in various tricky cases.One of the key benefits of the C language, which is the reason it is so popular for embedded applications, is that it is a high-level, structured programming language, but has low-level capabilities.

The ability to write code that gets close to the hardware is essential and C provides this facility. This article looks at how C may be used to access registers in peripheral devices. Device registers The broad issue is quite straightforward. A peripheral device is likely to have a number of internal registers, which may be read from or written to by software.

These normally appear just like memory locations and can, for the most part, be treated in the same way. Typically a device register will have bit fields — groups of bits that contain or receive specific information.

Such fields may be single bits, groups of bits, or a whole word. There may also be bits that are unused — reading from them or writing to them normally has no effect.

For example, a serial interface UART might have an 8-bit register, with the bits used like this:. Memory addressing There are a few important matters to get right when accessing device registers from C.

The first is data type. Using traditional C data types, our example register would be declared unsigned char. If the devices registers are larger than 8 bits — perhaps 16 bits or 32 bits — endianity may be an issue; different CPUs locate most and least significant bytes differently.

Any attempt to break down such registers into 8-bit units is likely to go awry, for example.

how to write to a register in c

Normally, a compiler will detect multiple accesses to a variable and make the code more efficient by accessing the memory location once, working on the data in a CPU register, and then writing it back to memory later. With a device register, it is essential that the actual device is updated exactly when the code when requires it to be. Similarly, repeated read accesses must be honored. For example, there may be some code that looks like this:. How do I arrange for it to be mapped to the correct address?

Some embedded software development toolkits do make it fairly simple by providing a facility whereby a variable or a number of variables — a program section can be precisely located using the linker.

Although this is very neat and tidy, it renders the code somewhat toolchain-dependent, which is generally unwise. There is no standard way to achieve this result in the C language. The usual solution is to think in terms of pointers, which are effectively addresses.

how to write to a register in c

So, instead of creating a variable of the appropriate type, you need a pointer to that type. Then, accessing the device register is simply a matter of dereferencing the pointer.

So the above example may be re-written.

“register” keyword in C

There is just the question of making sure that the pointer does point to the device register.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

I'm writing system-level code for an embedded system without memory protection on an ARM Cortex-M1, compiling with gcc 4. So far, my code looks like this:. Is there any shorter way shorter in code, I mean that does not use a pointer?

I looking for a way to write the actual assignment code as short as this it would be okay if I had to use more defines :. Anything I tried so far ended up with gcc complaining that it could not assign something to the lvalue But what is the point of using a const pointer that cannot be null?

This is semantically why references were created for. This can be stuck straight in a header file, just like the C-style macro, but you have to use function call syntax to invoke it. You can go one further than Chris's answer if you want to make the hardware registers look like plain old variables:. It's a matter of taste which might be preferable.

I've worked in situations where the team wanted the registers to look like variables, and I've worked on code where the added dereference was considered 'hiding too much' so the macro for a register would be left as a pointer that had to be dereferenced explicitly as in Chris' answer.

I like to specify the actual control bits in a struct, then assign that to the control address. Something like:. Another option which I kinda like for embedded applications is to use the linker to define sections for your hardward devices and map your variable to those sections. This has the advantage that if you are targeting multiple devices, even from the same vendor such as TI, you will typically have to alter the linker files on a device by device basis.

Different devices in the same family have different amounts of internal direct mapped memory, and board to board you might have different amounts of ram as well and hardware at different locations. Here's an example from the GCC documentation:. Normally, the compiler places the objects it generates in sections like data and bss. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware.

The section attribute specifies that a variable or function lives in a particular section. For example, this small program uses several specific section names:.

Use the section attribute with global variables and not local variables, as shown in the example. Using the section attribute will change what section the variable goes into and may cause the linker to issue an error if an uninitialized variable has multiple definitions.

You can force a variable to be initialized with the -fno-common flag or the nocommon attribute. Learn more. Ask Question. Asked 10 years, 7 months ago. Active 2 years, 8 months ago. Viewed 20k times.The first example will be one of the simplest possible architectures, and it will consist of the following components:. In any assembly language numbers serve multiple uses. Aside from representing scalar values, they also represent register numbers and memory locations.

In this example I will follow the convention of using prefix characters to denote the role a number plays. After designing the VM, it is necessary to design the VM's instruction set. The instruction set is simply a reflection of the kinds of things we would like to do or allow others to do with this VM. Here are some things it should be possible to do with this VM:.

Let's keep it just that simple for now. After we have a working implementation we can go back and add more instructions. First let's write a short program using these three instructions to see what they might look like:. Note that in keeping with common convention I have chosen to place the destination registers first in the operand list.

how to write to a register in c

After the assembly language is created it is necessary to determine how to represent each instruction as a number. This establishes a one-to-one correspondence between each instruction in the assembly language and each instruction code in the set of instruction codes. Converting a program from assembly language to instruction codes is called assemblingand conversion from instruction codes back into assembly language is called disassembling.

First, to answer the last question, since there are only small numbers of instructions and registers in this VM it should not be very difficult to encode all operands in a single instruction word, even if for the sake of simplicity I were to use a bit instruction word. Thus, a bit number written in hexadecimal has 4 digits, giving us easy access to 4 information fields, each containing 16 variations and A-F.

Having made these decisions, let us now establish the encoding. Recall that we have 16 instruction numbers available. The halt instruction will be instruction 0, and there is an important reason for choosing 0 for this instruction. Since empty space in the computer's memory will most likely be filled with 0s, any run-away program will eventually encounter a 0 and attempt to execute this instruction, immediately halting the program.

The remaining two instructions can be assigned arbitrarily: the loadi instruction can be instruction 1, and the add instruction can be instruction 2. This is our current instruction encoding list:. Examining the first program instruction, we see that we must now encode the register and the immediate value:. There are three hexadecimal digits available for operands, so we will use the first of those three as the register number, and the second and third together as the immediate value.

Now we have just determined the complete encoding of the loadi instruction:. Thus, the complete bit hexadecimal instruction code for the first instruction is The second instruction is assembled in a similar way. The immediate value iswhich in hexadecimal is C8.Is this purely possible in C? I also asked before about the AVR libc license but that topic got unpublished after i made certain changes to it. The question was if it is free to use AVR libc for distribution, publishing, production.

If we open the header file for the ATmega8 you'll see that it's also just a bunch of defines for registers and such:. But you'll also have to take ists adress, cast it to some static const volatile pointer or somethig.

Write a small program, just doing e. Rebuild the project and then look for a file with the same name as your source but with file extension ". There's what the compiler sees after all pre-processing has been done. Ignore all lines beginning with a hash sign and you see that what the compiler proper will process is.

Creating a Virtual Machine/Register VM in C

As of January 15,Site fix-up work has begun! Now do your part and report any bugs or deficiencies here. Use the wrong words, communicate the wrong concept. JohanEkdahl wrote: Advice: Apart from the learning experience, don't rely on this style of coding.

But I bet OP wants make something like a struct with addr and data, that can be written. And then one clk more or less could easy be well spend compare to the flexibility and reduced size. Apart from the learning experience, what is the goal? The OP seems to have the idea that libraries are used for sfr access and that the code generated is non-optimal. As has been demonstrated, the compiler will generate native code as you would in assembler.

Kartman wrote: The OP seems to have the idea that OP only contributed the original post. I've re-read it several times, and I cannot tell what the aim really is. First, define "library". Is avr-libc a "library"? Why would a chip-include file and subordinate. Assuming that the OP means that they want to write C code without using ANY include files that they themselves haven't written, I see essentially two ways to do this:. Note that the. The various ioxxx. You can look at the copyrights; while they have similar "bsd-style" licensing language, the Atmel-provided files are Copyright by Atmel, while the community code has assorted individual names.Embedded programmers traditionally use C as their language of choice.

And why not? It's lean and efficient, and allows you to get as close to the metal as you want. Embedded programming is often seen as black magic by those not initiated into the cult.

It does require a slightly different mindset; a resource constrained environment needs small, lean code to get the most out of a slow processor or a tight memory limit. To understand the approach I present we'll first review the mechanisms for register access in such an environment.

Hardcore embedded developers can probably skip ahead; otherwise here's the view from 10, feet. Most embedded code needs to service hardware directly.

This seemingly magical act is not that hard at all. Some kinds of register need a little more fiddling to get at than others, but you certainly don't need an eye-of-newt or any voodoo dances. The exact mechanism depends on how your circuit board is wired up. The common types of register access are:. Bus separated It's harder to control devices connected over a non-memory mapped bus. I 2 C and I2S are common peripheral connection buses. In this scenario you must either talk to a dedicated I 2 C control chip whose registers are memory mappedtelling it what to send to the device, or you manipulate I 2 C control lines yourself using GPIO [ 1 ] ports on some other memory mapped device.

Each device has a data sheet that describes amongst other things the registers it contains, what they do, and how to use them. Registers are a fixed number of bits wide - this is usually determined by the type of device you are using.

Comparing C to machine language

This is an important fact to know: some devices will lock up if you write the wrong width data to them. With fixed width registers, many devices cram several bits of functionality into one register as a 'bitset'. The data sheet would describe this diagrammatically in a similar manner to Figure 1. So what does hardware access code look like? Using the simple example of a fictional UART line driver device presented in Figure 1, the traditional C-style schemes are:.

Direct memory pointer access. It's not unheard of to see register access code like Listing 1, but we all know that the perpetrators of this kind of monstrosity should be taken outside and slowly tortured.

It's neither readable nor maintainable.

A Technique for Register Access in C++

This low-level purgatory is where we use C's volatile keyword. This is just the behaviour we need for hardware register access. Every time we write code that accesses a register we want it to result in a real register access.

Don't forget the volatile qualification! Pointer usage is usually made bearable by defining a macro name for each register location.

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