Query regarding physically discontiguous memory

[Pritam Bankar]

Hi,
I just started reading Memory Managment chapter from book Professional
Linux Kernel Architecture.  At very start of chapter I came across terms
FLATMEM, DISCONTIGMEM and SPARSEMEM. Out of these DISCONTIGMEM and
SPARSEMEM are available to NUMA systens only. Book also says UMA systems
can also choose DISCONTIGMEM option if there address space contains large
holes.  Some googling says DISCONTIGMEM is prefered when we have physical
discontiguous memory.
This is what I don't understand what does it mean by

containing large holes?

physical discontiguous memory?

Is it like when I put 2 RAM chips in a system then memory for them will be
discontiguous?

Would /proc/iomem give me details I wanted?

Per my understanding RAM memory will ALWAYS be contiguous whether its 512
MB or 4 GB.

Please give me graceful example of discontiguous physical memory
That will really help me out.
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