Programmer’s View

RISC-V specifications allow many variations. This chapter provides more details about RISC-V variants available for the programmer. A global view of the CVA6 family is provided, as well as details for each verified configuration.

RISC-V Extensions

CVA6 family

The following extensions are available for the CVA6 family. Some of them are optional and are enabled through parameters in the SystemVerilog design.

RV32 denotes RISC-V 32-bit extensions. RV64 denotes RISC-V 64-bit extensions.

Extension	Optional	RV32 (in CV32A6)	RV64 (in CV64A6)	Note
I- Base Integer Instruction Set	No	✔	✔	Note 1
A - Atomic Instructions	Yes	✔	✔	Note 1
Zb* - Bit-Manipulation	Yes	✔	✔	Note 1
C - Compressed Instructions	Yes	✔	✔	Note 1
Zcb - Code Size Reduction	Yes	✔	✔	Note 1
Zcmp - Code Size Reduction	Yes	✔	✔	Note 1
D - Double precision floating-point	Yes		✔	Note 1
F - Single precision floating-point	Yes	✔	✔	Note 1
M - Integer Multiply/Divide	No	✔	✔	Note 1
Zicount - Performance Counters	Yes	✔	✔	Note 2
Zicsr - Control and Status Register Instructions	No	✔	✔	Note 2
Zifencei - Instruction-Fetch Fence	No	✔	✔	Note 2
Zicond - Integer Conditional Operations(Ratification pending)	Yes	✔	✔	Note 2

Notes:

• Note 1: These extensions have a slightly different definition between RV32 and RV64. They are therefore denoted with digits (e.g. RV32M).

• Note 2: These extensions do not differ between RV32 and RV64. They are therefore denoted without digits below (e.g. RVZifencei).

The following tables detail the availability of extensions for the various CVA6 configurations:

CV32A60AX extensions

These extensions are available in CV32A60AX:

Extension	Available in CV32A60AX
RV32I - Base Integer Instruction Set	✔
RV32A - Atomic Instructions	✔
RV32Zb* - Bit-Manipulation (Zba, Zbb, Zbc, Zbs)	✔
RV32C - Compressed Instructions	✔
RV32Zcb - Code Size Reduction	✔
RVZcmp - Code Size Reduction
RV32D - Double precision floating-point
RV32F - Single precision floating-point
RV32M - Integer Multiply/Divide	✔
RVZicount - Performance Counters	✔
RVZicsr - Control and Status Register Instructions	✔
RVZifencei - Instruction-Fetch Fence	✔
RVZicond - Integer Conditional Operations(Ratification pending)	✔

CV32A60X extensions

These extensions are available in CV32A60X:

Extension	Available in CV32A60AX
RV32I - Base Integer Instruction Set	✔
RV32A - Atomic Instructions
RV32Zb* - Bit-Manipulation (Zba, Zbb, Zbc, Zbs)	✔
RV32C - Compressed Instructions	✔
RV32Zcb - Code Size Reduction	✔
RVZcmp - Code Size Reduction	✔
RV32D - Double precision floating-point
RV32F - Single precision floating-point
RV32M - Integer Multiply/Divide	✔
RVZicount - Performance Counters
RVZicsr - Control and Status Register Instructions	✔
RVZifencei - Instruction-Fetch Fence	✔
RVZicond - Integer Conditional Operations(Ratification pending)

CV64A6_MMU extensions

These extensions are available in CV64A6_MMU:

Extension	Available in CV64A6_MMU
RV32I - Base Integer Instruction Set	✔
RV32A - Atomic Instructions
RV32Zb* - Bit-Manipulation (Zba, Zbb, Zbc, Zbs)	✔
RV32C - Compressed Instructions	✔
RV32Zcb - Code Size Reduction	✔
RVZcmp - Code Size Reduction	✔
RV32D - Double precision floating-point
RV32F - Single precision floating-point
RV32M - Integer Multiply/Divide	✔
RVZicount - Performance Counters
RVZicsr - Control and Status Register Instructions	✔
RVZifencei - Instruction-Fetch Fence	✔
RVZicond - Integer Conditional Operations(Ratification pending)

RISC-V Privileges

CVA6 family

CVA6 supports these privilege modes:

Mode
M - Machine
S - Supervior
U - User

Note: The addition of the H Extension is in the process. After that, HS, VS, and VU modes will also be available.

The following tables detail the availability of privileges modes for the various CVA6 configurations:

CV32A60AX privilege modes

These privilege modes are available in CV32A60AX:

Privileges	Available in CV32A60AX
M - Machine	✔
S - Supervior	✔
U - User	✔

CV32A60X privilege modes

These privilege modes are available in CV32A60X:

Privileges	Available in CV32A60X
M - Machine	✔
S - Supervior
U - User

CV64A6_MMU privilege modes

These privilege modes are available in CV64A6_MMU:

Privileges	Available in CV64A6_MMU
M - Machine	✔
S - Supervior	✔
U - User	✔

RISC-V Virtual Memory

CVA6 family

CV32A6 supports the RISC-V Sv32 virtual memory when the MMUEn parameter is set to 1 (and Xlen is set to 32).

CV64A6 supports the RISC-V Sv39 virtual memory when the MMUEn parameter is set to 1 (and Xlen is set to 64).

Within CV64A6, the hypervisor extension is available and supports Sv39x4 virtual memory when the CVA6ConfigHExtEn parameter is set to 1 (and Xlen is set to 64).

By default, CV32A6 and CV64A6 are in RISC-V Bare mode. Sv32 or Sv39 are enabled by writing the required configuration to satp register mode bits.

In CV32A6 the mode bit of satp register is bit 31. Sv32 is enabled by writing 1 to satp[31].

In CV64A6 the mode bits of satp register are bits [63:60]. Sv39 is enabled by writing 8 to satp[63:60].

When the MMUEn parameter is set to 0, CV32A6 and CV64A6 are always in RISC-V Bare mode; satp mode bit(s) remain at 0 and writes to this register are ignored.

By default, the hypervisor extension is disabled. It can be enabled by setting bit 7 in the misa CSR, which corresponds to the letter H.

When CVA6ConfigHExtEn parameter is set to 0, the hypervisor extension is always disabled; bit 7 in the misa CSR remains at 0 and writes to this register are ignored.

Even if the hypervisor extension is enabled, by default, address translation for Supervisor, Hypervisor and Virtual Supervisor are disabled. They can be enabled by writing the required configuration to satp, hgatp and vsatp registers respectively.

Sv39 is enabled for Supervisor or Virtual Supervisor by writing 8 to satp[63:60] or vsatp[63:60] respectively.

Sv39x4 is enabled for Hypervisor by writing 8 to hgatp[63:60].

Notes for the integrator:

• The virtual memory is implemented by a memory management unit (MMU) that accelerates the translation from virtual memory addresses (as handled by the core) to physical memory addresses. The MMU integrates translation lookaside buffers (TLB) and a hardware page table walker (PTW). The number of instruction and data TLB entries are configured with InstrTlbEntries and DataTlbEntries.

• The MMU offers a microarchitectural optimization featuring two levels of TLB: level 1 TLB (sized by InstrTlbEntries and DataTlbEntries) and a shared level 2 TLB. The shared level 2 TLB is enabled when the UseSharedTlb parameter is set to 1. The size of the shared TLB can be selected with the parameter SharedTlbDepth. The optimization has no consequences on the programmer’s view.

CV32A60AX virtual memory

CV32A60AX integrates an MMU and supports both the Bare and Sv32 addressing modes.

CV32A60X virtual memory

CV32A60X integrates no MMU and only supports the Bare addressing mode.

CV64A6_MMU virtual memory

CV64A6_MMU integrates an MMU and supports both the Bare and Sv39 addressing modes.

Memory Alignment

CVA6 does not support non-aligned memory accesses.

This is applicable to all configurations.

Harts

CVA6 features a single hart, i.e. a single hardware thread.

Therefore the words hart and core have the same meaning in this guide.

This is applicable to all configurations.