Field Programmable Gate Arrays (FPGAs) and Central Processing Units (CPUs) are two fundamental components used in many digital systems. But there are some key differences between their architectures and applications. This article will examine if an FPGA can be considered a type of CPU or if they are distinct technologies.
CPU Overview
A Central Processing Unit, or processor, is a digital circuit that executes instructions contained in software programs. CPUs implement fundamental computing operations like:
- Fetching instructions and data from memory
- Decoding instructions
- Executing arithmetic, logic and data processing operations
- Controlling program flow
- Communicating with peripheral devices
Modern CPUs utilize architectures like x86, ARM, RISC-V etc. They are fabricated as complex integrated circuits containing billions of transistors. CPUs are optimized for executing sequential instruction streams at high speed.
Key characteristics of CPUs:
- Fixed architecture optimized for general-purpose code execution
- Sequential Fetch-Decode-Execute cycle
- Caches, pipelines, branch prediction for performance
- Standard interfaces like memory, PCIe, USB
- Run software operating systems and applications
- Fabricated by silicon vendors like Intel, AMD, etc.
FPGA Overview
A Field Programmable Gate Array is an integrated circuit containing configurable logic blocks and programmable interconnects. Users can customize the FPGA to implement digital logic functions by programming it after manufacturing.
Capabilities of FPGAs include:
- Custom logic using Look-Up Tables (LUTs) and flip-flops
- Flexible I/O interfaces
- Low-latency parallel execution
- High bandwidth internal routing
- DSP slices and block RAM
- Reconfigurability and field updates
Key characteristics:
- Programmable logic cells and routing
- Parallel execution architecture
- No fixed instruction set
- Designs defined using HDL or schematic entry
- Used for hardware acceleration, prototyping, DSP
- Provided by vendors like Xilinx, Intel (Altera)
So in summary, the CPU is optimized for software programmability while the FPGA is optimized for hardware flexibility and customizability.
Comparing CPUs and FPGAs
Here is a comparison between some key attributes of CPUs and FPGAs:
| Parameter | CPU | FPGA |
|---|---|---|
| Architecture | Fixed | Configurable logic + routing |
| Execution | Sequential | Parallel |
| Programming | Software instructions | HDL or schematics |
| Performance | Generalized computing | Application-specific acceleration |
| Flexibility | Minimal | Highly flexible |
| Power efficiency | Lower | Higher |
| Cost | Low | High |
| Design effort | Low | High |
While CPUs and FPGAs both implement digital logic functions, they have fundamental differences in capabilities and use cases. CPUs are best suited for running software programs while FPGAs excel at application-specific hardware acceleration.
Can FPGAs Function like CPUs?
Even though they have different architectures, FPGAs can be configured to implement functionality quite similar to CPUs. This is achieved by programming the FPGA fabric using HDL code to realize components like:
- Instruction register and decoder
- Arithmetic logic unit (ALU)
- General purpose registers
- Control unit implementing a state machine
- Caches for instructions and data
- Bus interfaces like AXI, OCP, etc.
Using these CPU-like blocks, FPGAs can execute software binary code loaded into their internal memory and access external memories and peripherals.
While possible, such soft CPU architectures on FPGAs have lower performance compared to hard CPUs built as ASICs. They consume more power and area when implemented in programmable logic. Hence FPGAs are not a replacement for CPUs in running software.
Can CPUs be Used like FPGAs?
Modern CPUs also have some capabilities to improve flexibility and customizability typically associated with FPGAs:
Reconfigurable Instruction Sets
RISC-V, ARM and other latest CPU architectures have configurable instruction sets that can be extended with custom logic and accelerators.
Partial Reconfiguration
Features like Intel Management Engine allow parts of a CPU to be reconfigured without affecting the entire device.
FPGA Co-Processors
Intel and AMD CPUs integrate FPGA fabric as co-processors to enable application specific acceleration.
General Matrix Multiply Units
Programmable matrix multiply units in CPUs like Intel DL Boost provide an FPGA-like configurability.
However, these capabilities are quite limited compared to full-fledged FPGAs. The CPU flexible logic is tightly coupled to fixed cores and cannot be repurposed independently. CPUs are still firmly in the software programmability domain versus FPGAs designed for custom hardware flexibility.
Reasons FPGAs Are Not CPUs
While FPGAs can implement soft CPU cores, they differ from true CPUs in several fundamental aspects:
Sequential vs Parallel Execution
The Von Neumann architecture used in CPUs is optimized for sequential fetch-decode-execute. FPGAs employ a parallel architecture executing custom hardware blocks concurrently.
Fixed vs Configurable
CPUs have standardized fixed blocks like ALUs, caches, etc. In contrast, FPGAs are fully customizable at the logic and routing level.
Hardware vs Software Programming
CPUs execute software instructions. FPGAs are programmed using hardware description languages (HDLs) defining digital logic.
General Purpose vs Application Specific
CPUs are generalized computing engines. FPGAs are designed and optimized for a specific application.
Discrete Chips vs On-Chip Variants
Standard CPUs are manufactured as standalone chips. FPGAs are on-chip accelerators integrated with CPUs.
Due to these architectural differences, even though both implement digital logic, FPGAs cannot be classified as general-purpose CPUs. They complement CPUs with application-specific acceleration.
Reasons CPUs Are Not FPGAs
While CPUs are gaining more configurable logic, they are fundamentally not equivalent to FPGAs due to the following:
Instruction Driven vs Digital Logic
CPUs are constrained to an instruction set architecture. FPGAs allow designers to implement arbitrary digital logic unconstrained by instructions.
Limited Flexibility vs Highly Programmable
CPU configurable logic is minimal compared to the highly programmable FPGA fabric with millions of logic cells and robust routing.
Fixed Pipelines vs Arbitrary Datapaths
The CPU datapath is fixed. FPGAs can implement any custom datapath.
Full-Stack Software vs HDL Based Development
CPUs utilize rich full-stack software ecosystems. FPGA design relies on hardware description languages.
Generalized Cores vs Specialized IP
CPUs cores are generalized processors. FPGAs integrate application-specific accelerators and interfaces.
Due to these inherent differences, CPUs cannot match the custom hardware flexibility and parallelism offered by FPGAs.
Similarities Between FPGAs and CPUs
While they have distinct architectures and applications, FPGAs and CPUs have some high-level similarities:
- Both implement digital logic functions ultimately consisting of logic gates and flip flops at their foundation.
- They utilize similar semiconductor fabrication techniques like CMOS on silicon wafers.
- Standard CAD tools are used for designing and verifying both FPGA and CPU chips.
- They employ common interfaces like PCIe, DDRx memory, USB, etc. to communicate with peripherals.
- Higher level system integration happens over standard buses like AXI, OCP, Avalon, etc.
- They are combined together frequently as FPGA accelerated CPUs or SoC designs.
So while distinct internally, from an external perspective FPGAs and CPUs enable complementary benefits for digital systems designers.
Pros and Cons of FPGAs Compared to CPUs
FPGAs Advantages Over CPUs
- Customizable and reconfigurable logic
- Massive parallelism
- Optimized for application-specific needs
- Low latency, high bandwidth
- Algorithms directly in hardware
- Energy and performance efficiency
- Flexibility for future enhancements
FPGAs Disadvantages Relative to CPUs
- Higher design complexity
- Specialized toolset and skillset
- Redesign needed for new algorithms
- Limited by available gates
- Difficult optimization and debugging
- Higher cost
- Not software programmable
CPU Advantages Over FPGAs
- Generalized architecture
- Mature software ecosystem
- Easy integration and abstraction
- Quick to develop and modify
- Scalable through multicore
- Broad availability of standard CPUs
CPU Disadvantages Relative to FPGAs
- Less performance and power optimized
- Higher latency
- Limited parallelism
- Constrained by instruction set
- Not tailored for applications
- Less flexible for future enhancement
In summary, both technologies have pros and cons depending on the use case requirements, so they complement each other in building high performance digital systems.
Conclusion
While FPGAs and CPUs both provide digital logic capabilities, they have fundamentally different architectures optimized for their specific applications. CPUs are designed to efficiently execute software instruction streams. In contrast, FPGAs offer flexible digital logic that can be customized to accelerate specific workloads. FPGAs do not have a fixed instruction set or run software operating systems and are programmed using hardware description languages rather than a software toolchain. While FPGAs can implement soft CPU cores, they lack the full software programmability of a microprocessor. Similarly, CPUs are gaining more configurable logic but cannot match the hardware customizability of FPGAs. Both technologies have complementary strengths ideal for heterogeneous computing systems.
Top 5 FAQs about FPGAs and CPUs
Here are answers to some frequently asked questions about the relationship between FPGAs and CPUs:
Q1: Can an FPGA fully replace a CPU in a system?
A: Not usually, because FPGAs lack native software programmability. FPGAs excel at accelerating fixed hardware functions but CPUs are still needed to run software programs controlling the overall system.
Q2: Do FPGAs implement the same functions as a CPU internally?
A: No, FPGAs consist of configurable logic/routing while CPUs have fixed functional blocks like ALUs, caches, pipelines. The internal architecture is fundamentally different.
Q3: Can CPUs implement the same functionality as FPGAs?
A: To a limited degree, but CPUs have very constrained configurable logic compared to the massive programmable fabric in FPGAs. CPUs cannot match FPGA customizability.
Q4: Why can’t FPGAs fully replace CPUs if they can also execute software?
A: Soft processor cores in FPGAs have much lower performance than hard CPU cores. FPGAs lack native software programmability and OS support required in most systems.
Q5: Will FPGAs ultimately replace CPUs as technology improves?
A: Unlikely, since CPUs excel at generalized software execution which is easier to develop for. FPGAs and CPUs will continue to complement each other in heterogeneous compute systems.
In summary, while FPGAs and CPUs overlap in some capabilities, they are optimized for different applications and provide complementary strengths that make them both indispensable for modern digital designs.
