Re: [ECOS] Dual Port RAM on MPC8xx

[Gary Thomas <gary@mlbassoc.com>]

On Wed, 2005-04-20 at 11:59 +0100, Will Wagner wrote:
> Thanks very much for that. Not sure how I missed it.
> 
> In the alloc function instead of
> 
>      if ((bd < QUICC_BD_BASE) || (bd > QUICC_BD_END)) {
>          // Most likely not set up - make a guess :-(
>          bd = *nextBd = QUICC_BD_BASE+0x400;
>      }
> 
> Why not just check the reset has been called? Seems odd to need that guess?
> 

This was to handle the case where you ran an application with an old
RedBoot and resetting the CPM would break existing I/O setups.

> 
> Gary Thomas wrote:
> > On Wed, 2005-04-20 at 11:38 +0100, Will Wagner wrote:
> > 
> >>I'm trying to understand how the allocation of DPRAM is done in the 
> >>function _mpc8xx_allocBd in cpm.c.
> >>
> >>Can someone explain why nextBD is initialised as:
> >>
> >>static short *nextBd = (short *)(CYGHWR_HAL_VSR_TABLE + 0x1F0);
> >>
> >>Presumably this is so that redboot and an application can cooperate when 
> >>using the DPRAM? Why is this address chosen and when is it's contents 
> >>ever initialised?
> >>
> >>Presumably it may not have been initialised as in _mpc8xx_allocBd there 
> >>is this code:
> >>
> >>     if ((bd < QUICC_BD_BASE) || (bd > QUICC_BD_END)) {
> >>         // Most likely not set up - make a guess :-(
> >>         bd = *nextBd = QUICC_BD_BASE+0x400;
> >>     }
> >>
> >>I guess this is starting 0x400 into the DPRAM so that an apllication 
> >>doesn't reuse any DPRAM used by redboot? Is this correct? I can find 
> >>where redboot ever initialises this memory so I can't see how it works.
> >>
> >>Any help in understanding this is much appreciated.
> > 
> > 
> > Look about 10 lines below the "static short" line you quoted :-)
> > This value gets set when the CPM has been reset.
> > 
> > The point is to keep track [minimally] of what DPRAM is in use
> > and, yes, one wants applications to know and respect the allocations
> > that RedBoot has already made.
> > 
> > The choice was just an unused place in memory.  This region of
> > memory already holds similar data that is shared between RedBoot
> > and applications.
> > 
> 
> 
-- 
------------------------------------------------------------
Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
------------------------------------------------------------


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