Continuing with our series on embedded systems interrupt handling, let us explore the world of interrupt handling in the RISC-V architecture. In this comprehensive article, we'll explore the core concepts, the various components involved, and the flow of interrupt processing in this powerful open-source instruction set architecture.

Interrupts are a fundamental mechanism in computer systems that allow external events or internal conditions to be communicated to the processor, enabling it to respond promptly and efficiently. Just like any other architecture, interrupt handling in the RISC-V, is a crucial aspect of system design and optimization, and understanding its intricacies can greatly enhance your ability to develop robust and reliable embedded systems.

Let's dive in and unravel the mysteries of interrupt handling in RISC-V!

Overview of the RISC-V Architecture

RISC-V is an open-source and royalty-free ISA that has gained significant attention in the embedded systems and microcontroller domains. One of the key features of RISC-V is its modular design, which allows for a wide range of customization and scalability. The RISC-V architecture consists of a base integer instruction set, which is complemented by various optional extensions that can be selectively included based on the specific requirements of the application.

The RISC-V ISA is designed to be simple, yet powerful, with a focus on efficiency and performance. It features a reduced instruction set, which helps to minimize the complexity of the processor design and improve overall system performance. Additionally, RISC-V supports a wide range of hardware configurations, from small, low-power microcontrollers to high-performance, multi-core processors.

While the RISC-V can be 32- or 64-bit or 128-bit, we will consider the 32-bit variant for this topic. Further RISC-V supports a subset mode called ‘E’ that is optimized for embedded applications with a compressed instructions set support.
In the context of interrupt handling, the RISC-V architecture provides a comprehensive set of features and mechanisms to ensure seamless and efficient interrupt processing.

Privilege Levels in RISC-V Architecture

The RISC-V architecture defines three distinct privilege levels, which are essential for managing the execution of different software components and their access to system resources. These privilege levels are:

• Machine Mode (M-Mode): The highest privilege level, responsible for handling low-level system tasks, such as interrupt handling, memory management, and access to critical hardware resources.
• Supervisor Mode (S-Mode): The intermediate privilege level, typically used for operating system kernels and hypervisors, which manage the execution of user-level applications.
• User Mode (U-Mode): The lowest privilege level, where user-level applications execute, with limited access to system resources.

Understanding these privilege levels is crucial for designing and implementing interrupt handling mechanisms in RISC-V systems, as different interrupt sources and handling routines may operate at different privilege levels.

RISC-V General-Purpose Registers

The RISC-V architecture defines a set of 32 registers (x0-x31) used for storing data and addresses. These registers could be extended to 64/128 registers or limited to 16 registers based on the extension.

RISC-V General Purpose Registers

RISC-V Control and Status Registers (CSRs)

RISC-V also defined a set of special-purpose registers used for managing the processor's state, called the Control and Status Regisiters. These registers are defined at a specific privilege level and are accessible only to those levels. While there could be upto 4096 CSRs, RISC-V specification defines about 200 + CSRs.

Some of the key CSRs relevant to interrupt handling in Machine mode include:

• mstatus: Stores the current processor mode and various status flags, including the global interrupt enable/disable state.
• mtvec: Holds the base address of the interrupt vector table, which is used for handling interrupts.
• mepc: Stores the address of the instruction that was interrupted, which is used for resuming execution after the interrupt is handled.
• mcause: Indicates the cause of the current interrupt or exception.
• mie and mip: Provide the interrupt enable and pending status, respectively, for various interrupt sources.

Understanding the role and usage of these CSRs is crucial for implementing robust and efficient interrupt handling mechanisms in RISC-V-based systems.

Direct vs Vectored Interrupt Handling

RISC-V refers to an external asynchronous event that may cause a RISC-V core to execute an unexpected transfer of control as interrupt. Similarly, an unusual condition occurring at the run time (due to invalid instructions etc.) is referred to as exceptions. The term trap is used to collectively refer to interrupt or exception.

RISC-V supports two main trap handling mechanisms: direct and vectored. Based on the last 2 bits of the mtvec register (or stvec for ‘S’ mode), it could be either perform Direct handling (bit values – 00) or Vectored handling (bit values – 01).

Direct Interrupt Handling:

• In this mode, the processor jumps to a fixed address (specified by the mtvec CSR) when an interrupt occurs.
• The interrupt service routine (ISR) is responsible for determining the source of the interrupt and handling it accordingly.
• Direct interrupt handling is simpler to implement, but it may not be as efficient for systems with a large number of interrupt sources.

Vectored Interrupt Handling:

• In this mode, the processor jumps to an address that is determined by the interrupt source and the contents of the mtvec CSR.
• The mtvec CSR holds the base address of the interrupt vector table, and the processor calculates the appropriate jump address based on the interrupt source.
• Vectored interrupt handling is more efficient for systems with a large number of interrupt sources, as it allows for faster identification and handling of the interrupt.

The choice between direct and vectored interrupt handling depends on the specific requirements of the system, such as the number of interrupt sources, the complexity of the interrupt handling logic, and the performance requirements of the application.

HARTs in RISC-V

RISC-V supports the concept of Hardware Threads (HARTs), which are independent execution contexts within a single RISC-V core. HARTs can be used to improve the overall system performance and responsiveness, particularly in multi-threaded or multi-tasking applications.

In the context of interrupt handling, HARTs play a crucial role in managing and distributing interrupt sources among the available execution contexts. Each HART has its own set registers which allows for independent interrupt handling and state management.

The RISC-V architecture also provides mechanisms for inter-HART communication and synchronization, which can be leveraged to coordinate interrupt handling across multiple HARTs within a single core.

Conclusion

In this comprehensive article, we have explored the key concepts of the RISC-V architecture. We started by introducing the RISC-V ISA, followed by a deep dive into the privilege levels, register set, and control and status registers (CSRs) that are crucial for managing interrupts.

We then examined the differences between direct and vectored interrupt handling, as well as the role of hardware threads (HARTs) in RISC-V. We will continue with the interrupt halding in the upcoming article.