Why Should Computers Have a Single Central Processing Unit?

Current computer systems often integrate a single central processing unit (CPU) which acts as the command center, handling all computational tasks. However, the question arises: why can't we have a dedicated processing unit for each unit within the device to interpret its own commands?

Exploring the Role of Peripherals

Let's delve into the components hooked up to a typical personal computer (PC) to understand the rationale behind their design:

Keyboard: Features a programmable or preprogrammed microcontroller to scan keys and communicate requests via the USB bus. Mouse: Contains a microcontroller with DSP capabilities, essential for optical mice. Monitor: Equipped with a microcontroller to generate OSD menus, handle DDC (Display Data Channel) communication, and possibly manage HDMI functionalities. Graphics Card: Houses a programmable processor with its own memory and parallel processing capabilities. Hard Drives (HDD/SSD): Comprise their own microcontrollers to manage data storage interactions with the CPU. Printer: Operates with its own processor to handle various communication protocols and manage printing functionalities. WLAN Stick: Incorporates a specialized microcontroller for firmware updates. Card Reader: Equipped with a microcontroller to manage various memory card formats. DVD-RW Drive: Utilizes a microcontroller to control hardware communications with the host and CD playback.

These components demonstrate that while the CPU acts as the central command hub, many peripherals have their own processing capabilities, thus enhancing efficiency and reducing latency.

The Role of Microcontrollers in Hard Drives

HDD and SSD drives are connected to the CPU via the SATA interface, a serial interface. Communication between these drives and the CPU occurs through a protocol using specific storage commands. By sending a READ command (code: 20), the CPU requests specific sectors from the disk drive, which then processes the command and returns the data to the CPU. Each sector has a unique number, typically specified using LBA (Logical Block Addressing).

This interaction shows how the CPU and peripheral devices communicate using their own specific commands, similar to how an early computer like the Apple II or IBM PC directly controlled storage devices.

Modern CPU Architecture

Today, CPUs like AMD Ryzen and ARM processors used in mobile phones are similar in architecture to what we might propose. Previously, the CPU directly controlled storage devices; for example, in the case of a floppy disk or cassette tape. However, contemporary CPUs, including those with higher processing speeds, utilize the superscalar architecture, which offers significant parallel processing capabilities.

Superscalar architecture allows for multiple instructions to be processed in parallel within a single clock cycle. This is achieved through the conversion of machine instructions into micro-operations (uOps) and pipelining them through various execution units. For instance, in a modern core, a single clock cycle can handle up to 4 instructions, significantly boosting performance.

The Intel AVX (Advanced Vector Extensions) technology allows for the processing of up to 8 numbers in parallel, further enhancing the speed and efficiency of computational tasks.

In essence, while the concept of dedicated processing units for each device is not entirely new, the integration of such functionality into a single, highly optimized CPU, through innovations like superscalar architecture, offers a more efficient and scalable solution for modern computing needs.

Conclusion

The architecture of today's CPUs is designed to handle the increasing demands of modern computing while maintaining backward compatibility. While it may seem advantageous to have dedicated processing units for each peripheral, the current design strikes a balance between performance, efficiency, and functionality, ensuring that modern computing systems continue to meet the evolving needs of users.