RISC-V: Difference between revisions

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<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">{{Warning: |Many parts of RISC-V are not yet finallyfinal. Things mightcan and will change! Look at the official specification for the most up-to-date information</div>.}}
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RISC-V is not a single ISA, rather a meta-ISA. It defines basics and boundaries for a family of implementations.
The specification is published under a Creative CommonCommons License and actively developed on [[#External Links|githubGithub]].
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An implementation consists of one of the [[#Base ISA|Base ISAs]] and zero or more [[#Extensions|Extensions]].
 
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=== Hardware Threads ===
The RISC-V ISA specifies hardware threads, called ''harts''. A ''hart'' is defined byhas its own instructionstate and serves as RISC-V's term for a logical fetchCPU unitcore.
A processor may contain multiple ''harts'', and has at least one.
Each hart has an ID associated with it and at least one ''hart'' has ID 0.
 
=== Exceptions, Traps and Interrupts ===
In RISC-V the term ''Exception'' refers to an unusual condition at run-time associated with an instruction in the current hardware thread.<br/>
A ''Trap'' is a synchronous transfer of control to a trap handler and is caused by an exceptional condition within a RISC-V thread. The trap handlers usually execute in a more privileged environment.<br/>
An external event that occurs asynchronously to the current thread will cause an ''Interrupt''. When an interrupt occurs, somethe currently running instruction(s) finish or are cancelled and isthe selectedCPU toexecutes experiencethe ainterrupt traphandler.
 
=== Privileges ===
The spec defines a debug mode and 4 priviledgeprivilege modes, of which a valid combination has to be implemented. The modes are:
 
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* One level: Machine mode only for embedded systems
* Two levels: Machine and User mode, small systems
* Three levels: Machine, Supervisor and User mode, ''Complex''complex systems able to run Unix-like operating systems
* Four levels: Machine, Hypervisor, Virtual supervisor and User mode, complex systems supporting virtualization in hardware
 
==== Changing privilege ====
In RISC-V, the privilege mode of a ''hart'' can be changed by:
* A trap or interrupt in the thread caught by a higher privilege mode
* The execution of the <code>mret</code> or <code>sret</code> instructions
* Intervention by a hardware debugger
* A hart being reset (it always starts in Machine mode)
When a trap or interrupt occurs, the ''hart'' determines which privilege mode will catch this exception and jump to the address stored in <code>mtvec</code> or <code>stvec</code>, possibly with an offset, for exceptions caught by Machine and Supervisor mode respectively. This mode with an offset into an [https://en.wikipedia.org/wiki/Interrupt_vector_table interrupt vector table] is called vectored interrupts. RISC-V traps do not use an offset. This has the side effect of making the interrupt vector table a table of jump instructions as opposed to a table of addresses.
 
The <code>mret</code> and <code>sret</code> instructions update the privilege mode to that specified by <code>mstatus</code> or <code>sstatus</code> respectively before resuming in said mode at the address stored in <code>mepc</code> or <code>sepc</code> respectively. These instructions are intended for returning from trap/interrupt handlers and explicit privilege mode switches.
 
 
 
== Base ISA ==
 
The base ISA specifies RV32I and RV64I, 32 and 64-bit respectively,; most of what is said about RV32I also applies to RV64I. Additionally, there is also RV32E, a reduced version of RV32I for embedded systems, and RV128I, a 128-bit version, which is mostly a placeholder so far.
 
==== RV32I ====
RV32I offers 31 general-purpose registers (x1-x31) which hold integer values, while the x0 register is hardwired to zero,; all registers are 32 bits wide. It specifies a number of logical and arithmetic operations (and, or, xor, shift left and right, addition and subtraction), all of which are available with a source register or an immediate.
 
==== RV32E ====
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==== RV64I ====
RV64I is very similar to its 32-bit counterpart, but offers 64-bit wide registers and can read the CSRs in one operation instead of requiring the programmer to read the upper and lower half separately. AdditionalyAdditionally, there are some instructions to work with words (32 bit) instead of double-words.
 
==== WikipediaRV128I ====
Like RV64I but with 128bit register length. This base ISA is not ratified and no widely adopted CPUs that support RV128I exist.
 
 
== Extensions ==
 
An extensionsextension can be one of the officialyofficially defined ones or a vendor-specific one (there is opcode space explicitelyexplicitly reserved for this) and can define additional elements, including opcodes and registers.
 
The following Standard Extensions are noteworthy here:
==== Zicsr ====
This extensions defines some kind of secondary address space for '''Control and Status Register''', which areis used for controlling
thinsthings like interrupts, privilege level, hart-local timers and such.
These registers can be accessed via the ''CSR instructions''.
 
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==== RVF & RVD & RVQ (Floating Point Extensions) ====
Adds additional instructions to work with floating points and also the floating point registers.
The different extensions differ in the length|/precision of the floating point numbers. The extensions F, D and Q are for single, double and quad precision respectively.
 
==== RVV (Vector Instruction Extension) ====
The RISC-V V extension adds a vector register file and instructions for operating on vectors. Note that RISC-V's vectors have a very different model from e.g. x86 in that a vector register's width is an implementation detail of the ''hart'' instead of an ISA-mandated specific size. These vector instructions conceptually work more like a stream of operations rather than operating on a predetermined amount of values in batches. Be warned that there are differences between RVV versions 0.7 and 1.0 which may break some software.
 
==== C (Compressed Instructions Extension) ====
Allows 16-bit variants of common instructions with a reduced register set (16 instead of 32) and
is intentedintended to increase code density. It can be '''freely mixed''' with 32-bit instructions. Note that this also has the effect of relaxing the alignment of 32-bit instructions to a 16-bit boundary.
 
 
== Running RISC-V ==
=== Emulator, Simulators, etc ===
For advanced setups (multiple privilege modes, etc.), QEMU is probably the way to go. Other simulators [https://riscv.org/software-status/#simulators exist],
but the author has not yet tried them :-)
 
=== Real Hardware ===
Some fairly inexpensive SoCs are becoming available, notably VisionFive 2 from StartFive [https://doc-en.rvspace.org/Doc_Center/visionfive_2.html] and Star64 form Pine64 [https://wiki.pine64.org/wiki/STAR64].
There does not exist that much real RISC-V hardware. The most notable example would probably be the
Both of these are Raspberry Pi like devices based on JH7110 Quad-Core SiFive U74 64-Bit CPU with
[https://www.sifive.com/boards/hifive-unleashed Hifive Unleashed], which is able to run linux, has
MMU, GPU, GPIO, Ethernet, et al and run Linux.
multiple cores and is expandable.
Also note
[https://www.sifive.com/boards/hifive-unleashed HifiveHiFive Unleashed], which is also able to run linuxLinux, has
multiple cores, and is expandable.
 
The PolarFire SoC Icicle Kit [https://www.crowdsupply.com/microchip/polarfire-soc-icicle-kit] by Microchip also contains cores by SiFive and can run Linux, but is cheaper than the HiFive Unleashed.
 
Most other hardware implementations (which are for sale for normal consumers) are more in the
category of "microcontrollers" and lackslack parts like a Memory Management Unit (MMU), I/O for humans
(keyboard, graphics, sound), etc).
 
The Kendryte K210 (e.g. in the MAIX Bit-board) is a RV64IMACFD_Zicsr_Zifence processor with two harts
and quite cheap.
 
== See Also ==
 
=== Articles ===
* [[HiFive-1 Bare Bones]]
* [[RISC-V_Bare_Bones|RISC-V Bare Bones]]
=== Wikipedia ===
* [[RISC-V_Meaty_Skeleton_with_QEMU_virt_board|RISC-V Meaty Skeleton with QEMU virt board]]
* [[Wikipedia:RISC-V|RISC-V]]
 
=== External Links ===
* [[Wikipedia:RISC-V|RISC-V on Wikipedia]]
* [http://www.riscv.org/ riscv.org]
* [https://riscv.org/software-status/ RISC-V Software Listingriscv.org]
* [httphttps://riscv.org/specificationsexchange/software/ RISC-V SpecificationsSoftware Listing]
* [https://riscv.org/technical/specifications/ RISC-V Specifications]
* [https://github.com/riscv/riscv-asm-manual/blob/master/riscv-asm.md RISC-V Assembly Programmer's Manual]
 
[[Category:RISC-V]]
[[Category:Instruction Set Architecture]]