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Re: ARM 920t queries with eCos


"When system is booting MMU is off and bootROM is mapped at address 0.
Therefore bootMonitor (the boot loader) gets executed. After systems boots,
MMU is turned on and a remapping happens. During remapping, RAM is mapped to
address 0 and bootROM maps to somewhere else

Yes.


(I suppose this should be EBI reason. Actully it is already there, at boot time this is aliased to
bottom). The address I see in target.ld reflact address after remapping has
taken place (MMU on)."

Can you explain me, How the RAM gets to 0x0000.0000 during boot time? Is it by some Hardware means in the EBI (like in Atmel ARM chips)?


There are several advantages in having RAM mapped to VMA 0x0.
Since you mentioned about the bootMonitor; Each third party boot monitor (like Angel) might have its own method of dealing with the HW vectors (0x00 ~ 0x20). It would be very difficult for eCOS or any other generic piece of SW for that matter to work with each of these monitors. It might be a bad idea to completely eliminate the onboard monitor (but platforms of different archs do), so we keep the monitor until Redboot starts up & after which complete control over the HW vectors is assumed by redboot by ensuring 100% code path within Redboot/eCOS. If we have RAM at 0x0, then Reboot can fix all the HW vectors with its own code, there by eliminating the monitor completely. The onboard third party monitor gets dummied out at this point <after Redboot starts>.


If the RAM is at 0x0 even before Redboot starts, then VMA 0x0000.0000 will or can be mapped to physical 0x0 (RAM).

Cheers,
-Mike.

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