I was reading this article that explains the difference between a thread and a core and it says the following:
A CPU core is a hardware component and is called the ‘brain’ of a CPU. It is like a small CPU within the bigger CPU.
My question is, what does the CPU contain apart from the core? All this time I thought the CPU itself was "the core".
Can someone please help clarify this?
In the old days the CPU and the CPU core were the same thing. There was nothing else on the die.
Gradually more things were added as technology progressed. First cache was added (formly on the motherboard). Then memory controllers started to be brought onto the die (formerly in the chipset). Then multiple processors were tiled into the same piece of silicon (formerly you would have two physical CPUs if you needed two processors). Gradually caches were adapted such that individual cores had their own private cache and the system gained a last level cache that was shared between the cores. Next PCIe and GPUs were integrated into CPUs (although even today not all integrate a GPU). More and more chipset functions were brought over too, with some CPUs now having USB, image processing hardware and even Thunderbolt controllers on the CPU. Take for example Intel's Ice Lake mobile parts:
Thunderbolt, an image processing unit, PCIe, display controllers, GPU, last level cache. The die photographs drive this home:
At least in the mobile parts, most of the die is NOT the CPU cores.
(taken from: https://en.wikichip.org/wiki/intel/microarchitectures/ice_lake_(client) )
For a very long time, a CPU could only process one execution flow at a time. Read instruction, decode instruction, execute instruction, and repeat.
At some point it was found that it would be interesting to process multiple execution flows at the same time — in parallel.
This started by having multiple CPUs, a so-called multiprocessor setup.
After a while, manufacturers starting putting multiple processors in the same chip. These processors inside the chip are often called cores, while the chip itself it called a multi-core CPU. But this is just one convention, and from the operating system or software point of view, the multiple cores are often considered as multiple CPUs.
Each core will contain a number of units used in the processing of an execution flow, but there may be additional stuff in the chip that is not part of any core, especially cache memory (there may be a small cache within each core and a larger cache shared by multiple cores), the necessary stuff to manage arbitration between the multiple cores, some execution units may be shared, there will be RAM interfaces (or built-in RAM in some cases), I/O interfaces (USB, SATA, PCIe…) and in recent “CPU” chips which contain a lot more than execution units, video-related stuff (VPU encoder/decoder, 3D GPU, video input and/or output), sometimes flash, etc.
So a “CPU” can either refer to a single core inside a chip which contains multiple cores and a lot of other stuff, or the whole chip. The latter is often called a SoC nowadays (system on chip) as more and more components which used to be external get integrated into a single chip.
In the PC world nowadays you’ll have nearly everything in the chip except RAM and some I/O, while in the world of mobile phones, tablets, embedded devices and recent Mac models even RAM is often integrated.
Actually, it's a processor or microprocessor in this context that contains CPU cores, not a CPU.
Some texts use the established terminology, unfortunately some misuse it through misunderstanding or choose alternatives through preference, like many things in naming.
A Central Processing Unit (CPU) is a logic circuit capable of executing instructions and programs. Designers may refer to their CPU IP as a CPU core.
A microprocessor is a CPU plus external bus interface circuitry, cache controller etc, all on a single IC. To implement a computing device, a microprocessor requires external ICs such as memories and I/O devices. These days, microprocessor is often shortened to processor.
A microcontroller (MCU) is a CPU, bus interfacing, memories and I/O devices all on a single IC. To implement a computing device, it doesn't require external ICs.
(MCU is an abbreviation of Microcontroller Unit, a term dating back to 1976 and (coined for) Intel's MCS-48 family, which is where I first saw it. MPU for Microprocessor Unit came along a bit earlier from Motorola.)
CPU core is a single logical unit of a CPU. Meanwhile, the CPU itself could be just a single core, or more commonly, multiple cores. If you, for example, have a eight-core CPU, then you have one CPU and eight cores.
That's the straight-to-the-point answer, without all the unnecessary and distracting barely-relevant fluff included in the other answers.
