RISC-V is an open standard instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. Unlike most other ISA designs, RISC-V is provided under open source licenses that do not require fees to use. This gives RISC-V the potential to become a unified industry standard across academia and commercial implementations.
General RISC-V Questions
Here are some common interview questions about RISC-V in general:
What is RISC-V?
RISC-V is an open, free instruction set architecture (ISA) based on established reduced instruction set computer (RISC) principles. It was originally designed for research and education at UC Berkeley but is now managed by the RISC-V Foundation.
What are the benefits of RISC-V?
Key benefits include:
- Open and free – anyone can use RISC-V ISA for any purpose
- Modular design – only base ISA is required, extensions can be added
- Simplified instruction set for smaller and efficient hardware
- Extensible to support embedded, IoT, and other domains
- Momentum and industry support from major companies
What are the key features of the RISC-V ISA?
Some key features are:
- RISC load-store architecture
- Small standard base ISA with optional extensions
- Variable instruction length, predominantly 32-bit instructions
- Register-register based operands
- Simplified memory access and addressing modes
- Modular design to only include required functionality
How is RISC-V different from ARM or x86?
RISC-V differs in several ways:
- Open and free ISA – not proprietary like ARM or x86
- Modular ISA design vs fixed ISA
- Simpler RISC architecture focused on efficiency
- Positioned as a unified industry standard ISA
- Supports modern architecture principles like RV64GC
What are some example uses cases of RISC-V?
RISC-V is used in:
- Research – easily study computer architectures
- Academic projects – no licensing constraints
- Startups – reduce costs and time-to-market
- Purpose-built chips – customize ISA for application
- Embedded devices like IoT where cost and power matter
Technical RISC-V Questions
Here are some more technical questions that may come up in RISC-V interviews:
Explain the modular design of the RISC-V ISA
RISC-V has a small standard base ISA with optional extensions modules:
- Base ISA – foundation required for all implementations
- Standard extensions – additional common functionality
- Custom extensions – can be defined as needed by application
This modular approach allows only required functionality to be implemented, minimizing cost and complexity.
What are some standard RISC-V extensions?
Common standard extensions include:
- M – Integer Multiplication and Division
- A – Atomic Instructions
- F – Single Precision Floating Point
- D – Double Precision Floating Point
- G – Additional base General Instructions
There are 30+ standard extensions defined for RISC-V ISA.
Explain RV32I and RV64GC RISC-V ISA variants
RV32I and RV64GC refer to specific RISC-V ISA configurations:
- RV32I – Base integer ISA only, 32-bit architecture
- RV64GC – Base + General + Standard Extension, 64-bit architecture
The notation RVXY defines:
- X – Register width (32 or 64)
- Y – Base ISA variant (I, E, G)
- C – Standard Extension modules added
So RV64GC is a full 64-bit ISA with common extensions.
What are the different register sets in RISC-V?
RISC-V defines several register sets:
- x – General purpose integer registers
- f – Single precision floating point registers
- s – Supervisor mode registers
- t – Temporary registers
- a – Atomic operation registers
The number of registers in each set depends on the specific RISC-V configuration.
Explain the RISC-V memory access and addressing modes
RISC-V uses a load-store architecture with several addressing modes:
- Base + Offset – Register + signed 12-bit immediate offset
- PC-Relative – PC + signed 20-bit immediate offset
- Register + Register – Two register operands
Memory access is simplified with separate load and store instructions only. Complex addressing modes are avoided.
What are some key RISC-V instructions?
Some common RISC-V instructions are:
- Load and Store – lw, sw, ld, sd
- Integer Arithmetic – add, sub, mul, div
- Logical – and, or, xor, shifts
- Control Flow – jal, jalr, branches
- Atomic – lr, sc
- Floating Point – flw, fsw, fadd, fmul
Instructions follow a regular naming scheme like add, sub, and, or, etc.
How are interrupts handled in RISC-V?
RISC-V has basic interrupt handling with:
- External interrupts delegated to supervisor mode
- Each interrupt has a number from 1 to MAX_INTERRUPTS
- mcause CSR indicates interrupt number and type
- mepc CSR contains return address to resume
- mstatus CSR used to set interrupt enables
Interrupt handling can be customized using RISC-V’s modular design.
RISC-V Project and Implementation Questions
Here are some example RISC-V project and implementation questions:
Have you implemented or worked on a RISC-V based project? Describe your role.
This allows you to explain your hands-on RISC-V experience. Focus on key contributions and implementation specifics if possible.
Describe a specific challenge you faced in a RISC-V project and how you solved it.
Shows ability to problem solve real RISC-V issues. Focus on technical details and the final solution.
How would you design and implement a RISC-V CPU core? What are the key steps?
Highlights overall understanding of full RISC-V CPU design process including:
- ISA configuration selection
- RTL implementation in Verilog/Chisel
- Functional simulation
- Synthesis to gates
- Backend implementation and timing analysis
Emphasize major milestones and deliverables.
What are some ways RISC-V can be optimized in hardware implementations?
Shows knowledge of RISC-V hardware optimization techniques like:
- Pipelining execution stages
- Cache design – caches, TLBs
- Branch prediction
- Out of order execution
- SIMD instructions
Focus on techniques relevant to the specific project or role.
How would you implement memory management for a RISC-V SoC?
Ability to explain memory subsystem design for RISC-V systems:
- TLB for virtual memory support
- Cache hierarchy – L1, L2 caches
- Main memory and memory controllers
- Memory-mapped IO
- Coherence for multi-core designs
Emphasize how RISC-V memory model impacts implementation.
Conclusion
RISC-V interview questions commonly aim to assess your general architecture knowledge, hands-on experience, technical proficiency, problem solving skills, and system-level understanding. Prepare by studying RISC-V principles and documentation, reviewing your own projects, and rehearsing responses to likely questions.
