[uClinux-dev] Re: Porting Linux programs to uClinux and other

[Mike Frysinger]

On Sunday 04 November 2007, Michael Schnell wrote:
> While the recommended way is to do Kernel drivers for any hardware
> access (as it's required in standard Linux), with µCLinux it's possible
> to directly access any address (and thus even peripheral hardware) from
> userland. Unless interrupts are necessary this would work in appropriate
> cases.

this is not a correct blanket statement by any means.  the job of an MMU is 
two fold:
(1) virtual<->physical address mapping
(2) memory protection (for different processes and read/write/execute)

while some processors lack an MMU, then do not lack an MPU (memory protection 
unit).  for example, the Blackfin processor has an MPU which means the 
Blackfin Linux port can (and does) prevent user space from accessing 
peripherals.  we can (and have an experimental/in-development patch) also 
protect processes from other processes and kernel space from user space, but 
the overhead here seems to be noticeable since the hardware table can only 
contain 16 entries.
-mike
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