memory remap

What is meant by remapping of memory. I occasionaly read this term
during the initialisation sequence of any microcontroller, but never
understood. Can someone throw some light ?

"Tutu" <bytespedition@googlemail.com> wrote in message 
news:1169253737.342366.241770@38g2000cwa.googlegroups.com...
> What is meant by remapping of memory.
>
In the early days of micros (circa late 1970s) it referred to problem of 
CPUs (most commonly the 8080/Z80 variety) that used 0x0000 as the 
restart/power up address.  This required a ROM in low memory, but that was 
also the location of the interrupt/RST vectors, which often had to be 
modified by programs.  One solution was to boot up in the ROM when reading, 
but to a RAM when writing at the same address.  It was then possible to copy 
the ROM to itself, in effect making a RAM image of the boot ROM.  An I/O 
port was then used to switch bus reads to RAM.  The ROM was mapped out of 
the processor address space.  You can still see this feature on the current 
Zilog eZ80 family of processors.

You may also be referring to more modern CPUs that have an integral memory 
management unit (MMU).  RAM regions that may be fragmented in the physical 
address space (for instance the many ARM7 variants where peripherals and 
buses are memory mapped) can be remapped by the MMU into one large and 
contiguous program address space.  The MMU can also be used to map out 
peripherals for user programs, protecting them from crashes.
  Jack Peacock 

Tutu wrote:
> What is meant by remapping of memory. I occasionaly read this term
> during the initialisation sequence of any microcontroller, but never
> understood. Can someone throw some light ?

It's any process that has the same memory (including its contents) end 
up visible under a different address than it was before.

This can be done either by a memory management unit (MMU) built into the 
CPU, or by external logic circuitry if the memory is external, too.  A 
very simple example would be a port pin controlling an inverter spliced 
into one of the address lines of an external memory bus.  Toggle the 
pin, and two areas of memory trade places in address space.  You've thus 
changed the mapping from CPU addresses to physical memory cells, i.e. 
you've "re-mapped" memory.

On Jan 22, 4:20 am, Hans-Bernhard Br=F6ker <HBBroe...@t-online.de>
wrote:
> Tutu wrote:
> > What is meant by remapping of memory. I occasionaly read this term
> > during the initialisation sequence of any microcontroller, but never
> > understood. Can someone throw some light ?It's any process that has the=
 same memory (including its contents) end
> up visible under a different address than it was before.
>
> This can be done either by a memory management unit (MMU) built into the
> CPU, or by external logic circuitry if the memory is external, too.  A
> very simple example would be a port pin controlling an inverter spliced
> into one of the address lines of an external memory bus.  Toggle the
> pin, and two areas of memory trade places in address space.  You've thus
> changed the mapping from CPU addresses to physical memory cells, i.e.
> you've "re-mapped" memory.


I can give another scenario.Some DSP processors (TI) have their own
internal Boot ROM (Primary Bootloader) code that will get copied to the
address where you will be placing your interrupt vector tables of your
bootloader(secondary bootloader) & application. In that case you may
need ot re-map (shift) the address of Reset Vector (Virtual - Physical
Mapping) so that it doesnt get overwritten during the initial boot
process of the processor. (Otherwise, your IVT will be overwritten and
your Bootloader & application will not Come Up ).

Karthik Balaguru