Technology is constantly changing, and once innovative designs are now considered obsolete. Believe, or not, there was a time when a computer’s microprocessor only contained a single CPU. To maintain efficiency, the CPU delegated some of its workloads to secondary circuits; mainly two, which were [collectively] referred to as “the chipset”.
The northbridge, which had a direct connection to the CPU, was responsible for the high-speed interfaces. Devices, such as the video card and the RAM [which need very fast connections], were once managed by the northbridge.
Earlier designs placed the northbridge on the computer’s motherboard; but, as of 2011, its function are now integrated into the CPU, and it is referred to by some as the system’s agent (Intel’s sandy bridge and AMD’s fusion). There are some exceptions [mostly in high-end CPUs] where the northbridge still resides on the motherboard.
In the lesson on the memory controller, it states that the reads and the writes to the RAM are managed by the memory controller, which once was a part of the northbridge.
The southbridge [on the other hand] did not have a direct connection to the CPU; but, instead, it was connected to the northbridge. Devices that do not require high-speed connections, such as USBs, HDDs, and keyboards, were all once delegated to the southbridge.
Over the years, the purpose southbridge has become redundant. And its responsibilities are now managed by the PCH, which is connected to the CPU by the direct media interface.
So, the north and southbridge once helped to ease the burden of the CPU. But with advancements in technology, mainly due to fabricating on smaller dies, the responsibilities of “the chipset” are now moved onto the microprocessor itself. This integration which has resulted in lower latency also brings about some limitations, such as, locking down the CPU to only one specific type of memory.