Symbian

Here we have a heritage taken from their experience with Psion PDA
operating systems. Starting in 1984, Psion produced low power budget PDAs
that could run on two AA batteries. To differentiate the hardware product, the
operating system and application software were optimized for multitasking—
the ability to do several things at once and have several applications open at
once, and to be able to move sideways from one task to another (and return to
the original task in its original state). This activity provides a good basis for
implementing the management and multiplexing of the rich media mix coming
from the MPEG-4 encoder in a 3G cellular handset, but multitasking comes at a
cost—additional memory and processor footprint.