[U-Boot] problem with mpc837x start.S

[shawn Bai]

> Date: Wed, 3 Aug 2011 10:48:39 -0500
> From: scottwood at freescale.com
> To: programassem at hotmail.com
> CC: u-boot at lists.denx.de
> Subject: Re: [U-Boot] problem with mpc837x start.S
> 
> On Wed, 3 Aug 2011 00:48:32 +0000
> shawn Bai <programassem at hotmail.com> wrote:
> 
> > ----------------------------------------
> > > Date: Tue, 2 Aug 2011 10:21:09 -0500
> > > From: scottwood at freescale.com
> > > To: programassem at hotmail.com
> > > CC: u-boot at lists.denx.de
> > > Subject: Re: [U-Boot] problem with mpc837x start.S
> > >
> > > On Tue, 2 Aug 2011 04:11:25 +0000
> > > shawn Bai <programassem at hotmail.com> wrote:
> > >
> > > > > When flash is enlarged to 4GiB, it repeats throughout the address space.
> > > >
> > > > After flash is enlarged to 4GiB, why will it repeat through the address space? the whole 4GiB address space?How does it happen?
> > >
> > > It's just how the hardware works -- the flash chip only sees the address
> > > bits that are relevant to its size. The higher address lines can be
> > There must be something needed to learnt by myself to understand "how the hardware works".
> > And, when you mention "its size", you mean 8MBytes,or 4GiB?
> 
> If the flash is 8MiB (as determined by physical reality, not OR0), it only
> decodes the low 23 bits of the address. The upper bits must hit in BR0/OR0,
> but after that they are discarded. So for example, if OR0 maps a 4GiB
> window, 0x00123456, 0x00923456, 0xff123456, and 0xe0123456 all point to the
> same location in flash, because the flash only sees 0x123456.
> 
> -Scott



When enlarging Nor Flash to 4GiB, the AM in OR0 is 0x0000_0, where the number of zero is 17. 

According to what is said in datasheet, if the bit value of some bit in address mask is 0, 
then the corresponding bit in address will be masked.

So, the higher 17 bits in address will be masked, is it right ?

If so, the range accessed in flash is just 32KBytes from the BA in BR0. 
Is that right ? But Not the same with what you mean.

And from what you replied before in this question, may I say the 32KBytes will repeat through 4GiB address space ? not 8MBytes ?

And with the effect of address mask in OR, as you say above, the higher bits is masked.
Now, we go back to CONFIG_SYS_MONITOR_BASE + in_flash, whatever value of the sum is, higher of it will be masked, 

so the label 'in_flash' will be located in flash, that is, 
both "in_flash" and "CONFIG_SYS_MONITOR_BASE + in_flash" can locate at where in_flash is in flash.

Even branching to location CONFIG_SYS_MONITOR_BASE + in_flash, it's the instruction that locates at "in_flash" to be run.

then, what is the effect of CONFIG_SYS_MONITOR_BASE in this ABSOLUTE branch as uboot comment indicates ?
 
Thanks.

-Shawn

>