Re: VMA vs LMA?

dspfun wrote:
Thanks for your response!

So, the program is always linked with the VMA addresses, and never
linked with LMA addresses? Or how are references to LMA/VMA sections
resolved?


They are not resolved. Consider the different possible cases:

LMA/VMA are the same, because you are running entirely from RAM. There's no problem, and nothing more to think about.

LMA/VMA are different because you are using a processor with an MMU on a big system (such as a PC). Let the native tools handle the linking and let the OS handle the loading, and forget the details.

LMA/VMA are different because you are using a small system with an MMU but no OS (or only a limited OS). Forget it until you understand a bit more about what you are doing.

LMA/VMA are different because you have a program in flash, initialised data in flash, and variables in ram. This is extremely common in embedded systems - I'll assume this is the situation you are talking about here from now on.

For the code section, the LMA and the VMA are the same - the program runs directly from the flash it resides in. Similarly, the LMA and VMA for bss data (i.e., uninitialised data that is cleared at startup) is the same, although the LMA doesn't really matter as nothing is ever loaded. The only complication is the initialised data section, which can be thought of as two parts - a RAM block containing the variables (this is at the VMA), and a flash block containing their initial values (at the LMA). Startup code copies the initial values into the ram.

The linking process "resolves" accesses to symbols (code or data), using only VMA addresses - it is only interested in how the memory looks once the program has started. The only exception is that the startup code also refers to the LMA of the data section, so that it can do the initial copy.

When writing a program where one wants different LMA and VMA sections
the programmer himself/hersel has to make sure he/she writes the code
that copies the LMA section/data to its appropriate VMA address?


No, it's part of the crt0 code - the code generated by your compiler, library and linker in collusion, which runs before the start of main(). Sometimes it's useful to write this sort of stuff yourself, but that is considered relatively advanced and you don't want to go there until you understand the process better.


No, these are different situations. When an OS loads code from a file,
the OS does the copying. Usually the VMA and LMA are the same, so that
the OS loads the initialised data section directly into place. With a
ROM'ed program, where the program is run directly from the ROM rather
than being first copied into RAM (in which case you have the same
situation as an OS and a file), the code at the beginning of the program
copies the data across and also clears the bss - this is part of the
crt0 startup code.

Why does crt0 (C runtime 0) have code that copies data from LMA to
VMA? Is it there so the C programmer doesn't have to write his own
"LMA data to VMA data copy code"? If so, how would one make use of the
crt0 "LMA data to VMA data copy code"?


If you follow your compiler's documentation and examples, you'll see this initial copying happening "magically" - standard linker scripts will ensure the code is linked in. You did not say which processor you were using, but many gcc setups come with sets of linker scripts including "ram" and "rom" versions - the "rom" versions will do this initial copying.

Write yourself a tiny program with initialised data, generate the object files, and examine the result by reading the disassembly or by using a debugger to single-step at the assembly level through the startup code. That will make it clear what is happening.
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