Re: Linux x uClinux for ColdFires: what is the best for what?

Hi David, Ricardo,

David Brown wrote:
> Ricardo Raupp wrote:
>>  
>> Hi people
>> Could someone show me a comparsion between Linux and uClinux for 
>> ColdFires?
>> I´m deciding between a V2 (5274L) x a V4 (54450) core.
>> The main diference I noticed ( or almost the) is V4 has MMU, wich 
>> enable Linux on it.
>> The chips cost is very close...I´m tending to a V4 chip..
>> Thanks
>>  
> 
> The difference between ucLinux and Linux is that ucLinux works without 
> an MMU (you can also use it on a cpu with an MMU if you want).  ucLinux 
> works mostly like Linux, but has a few limitations:
> 
> You don't get protection between processes memory spaces (obviously), so 
> a bad process can stomp on good processes.
> 
> You can't use "fork" - you must use "vfork" to replace "fork + exec" 
> combinations.  This may mean changing existing Linux code to port it to 
> ucLinux - for most applications, it's a minor issue, but it can be a 
> problem if you want to run traditional unix-style forking servers.
> 
> You can't use virtual memory.  Even if you are not using external page 
> files, virtual memory and the MMU can help avoid fragmentation of main 
> memory.
> 
> I'm not sure on this one, but I don't think you can nmap files in ucLinux.

You can use mmap(), but there is limitations.
For the details have a look at the file Documentation/nommu-mmap.txt
in any modern 2.6.x kernel version source tree.

Regards
Greg



> For small systems where you have complete control over which processes 
> will run on the system, ucLinux is often perfectly good, and can be 
> faster than plain Linux since MMU's can add a delay to memory access.
> 
> 
> As for the V2 versus the V4 - you get more processing power for your 
> money with a V4, but more microcontroller-style peripherals with most V2 
> devices, and a V2 board is going to be easier and cheaper to make.  But 
> that depends on the sort of boards you are used to making.
> 
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