53 lines
3.4 KiB
Markdown
53 lines
3.4 KiB
Markdown
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# NEORV32 `Zfinx` Floating-Point Extension
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The NEORV32 floating-point unit (FPU) implements the `Zfinx` RISC-V extension. The extensions can be enabled via the `CPU_EXTENSION_RISCV_Zfinx` top configuration generic.
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The RISC-V `Zfinx` single-precision floating-point extensions uses the integer register file `x` instead of the dedicated floating-point `f` register file (which is
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defined by the RISC-V `F` single-precision floating-point extension). Hence, the standard data transfer instructions from the `F` extension are **not** available in `Zfinx`:
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* floating-point load/store operations (`FLW`, `FSW`) and their compressed versions
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* integer register file `x` <-> floating point register file `f` move operations (`FMV.W.X`, `FMV.X.W`)
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:information_source: More information regarding the RISC-V `Zfinx` single-precision floating-point extension can be found in the official GitHub repo:
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[`github.com/riscv/riscv-zfinx`](https://github.com/riscv/riscv-zfinx).
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:warning: The RISC-V `Zfinx` extension is not officially ratified yet, but it is assumed to remain unchanged. Hence, it is not supported by the upstream RISC-V GCC port.
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Make sure you **do not** use the `f` ISA attribute when compiling applications that use floating-point arithmetic (`MARCH=rv32i*f*` is **NOT ALLOWED!**).
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### :warning: FPU Limitations
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* The FPU **does not support subnormal numbers** yet. Subnormal FPU inputs and subnormal FPU results are always *flushed to zero*. The *classify* instruction `FCLASS` will never set the "subnormal" mask bits.
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* Rounding mode `ob100` "round to nearest, ties to max magnitude" is not supported yet (this and all invalid rounding mode configurations behave as "round towards zero" (truncation)).
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## Intrinsic Library
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The NEORV32 `Zfinx` floating-point extension can still be used using the provided **intrinsic library**. This library uses "custom" inline assmbly instructions
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wrapped within normal C-language functions. Each original instruction of the extension can be utilized using an according intrinsic function.
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For example, the floating-point addition instruction `FADD.S` can be invoked using the according intrinsic function:
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```c
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float riscv_intrinsic_fadds(float rs1, float rs2)
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```
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The pure-software emulation instruction, which uses the standard built-in functions to execute all floating-point operations, is available via wrapper function. The
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emulation function for the `FADD.S` instruction is:
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```c
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float riscv_emulate_fadds(float rs1, float rs2)
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```
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The emulation functions as well as the available intrinsics for the `Zfinx` extension are located in `neorv32_zfinx_extension_intrinsics.h`.
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The provided test program `main.c` verifies all currently implemented `Zfinx` instructions by checking the functionality against the pure software-based emulation model
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(GCC soft-float library).
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## Resources
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* Great page with online calculators for floating-point arithmetic: [http://www.ecs.umass.edu/ece/koren/arith/simulator/](http://www.ecs.umass.edu/ece/koren/arith/simulator/)
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* A handy tool for visualizing floating-point numbers in their binary representation: [https://www.h-schmidt.net/FloatConverter/IEEE754.html](https://www.h-schmidt.net/FloatConverter/IEEE754.html)
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* This helped me to understand what results the different FPU operation generate when having "special" inputs like NaN: [https://techdocs.altium.com/display/FPGA/IEEE+754+Standard+-+Overview](https://techdocs.altium.com/display/FPGA/IEEE+754+Standard+-+Overview)
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