In our terminilogy "address space" means a set of:
Microcontrollers usually have more address spaces. Some is used inside only, some can be used for external memories.
Address space does not store any value. It just specifies range of
addresses by start address (which is not necessarily zero) and
size which the CPU can provide when it tries to access a memory
"Memory chip" is a circuit which can hold values in
cells. Cells are indexed from 0 up to size-1. Each cell stores some
(usually 8) bits.
Addresses coming from an address space via address bus must be routed
to memory and translated to cell indexes. This is done by the
"address decoder". It listens addresses on the bus and control
lines and enables exactly one memory chip. This way cells of the
memory chip appear in the address space.
It is possible that some addresses are not decoded. Writing to such an address results data to be lost. Reading of a non-decoded address results random value. Pullup or pulldown resistors can be applied to data bus lines to provide a specific value for read operations.
As it shown above, it is also possible that some cells of a chip is not mapped to any address.
Because each cell of a memory chip uses same read/write control, in real world it is not possible to share individual cells of a chip between address spaces. In µCsim simulator we can define as many address decoders as we want so it is possible to map any cell to any address.