From 0d628b4706588a98479b68e1fdad9e93377ee7c6 Mon Sep 17 00:00:00 2001 From: Dolu1990 Date: Tue, 16 Mar 2021 14:44:31 +0100 Subject: [PATCH] fpu add doc --- README.md | 60 + assets/fpuDesign.svg | 2491 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 2551 insertions(+) create mode 100644 assets/fpuDesign.svg diff --git a/README.md b/README.md index eec2dd0..914f412 100644 --- a/README.md +++ b/README.md @@ -19,6 +19,7 @@ - [Adding a new CSR via the plugin system](#adding-a-new-csr-via-the-plugin-system) - [CPU clock and resets](#cpu-clock-and-resets) - [VexRiscv Architecture](#vexriscv-architecture) + * [FPU](#fpu) * [Plugins](#plugins) @@ -668,6 +669,53 @@ via the VexRiscv implementation: If you generate the CPU without any plugin, it will only contain the definition of the 5 pipeline stages and their basic arbitration, but nothing else, and everything else, including the program counter is added into the CPU via plugins. +### FPU + +Features : + +- Support IEEE 754 float and optionaly double +- Implement Subnormal (few cycles lost in case of subnormal load/store) +- Implement exceptions flags +- The FPU can be shared between multiple CPU +- Can be integrated inside or outside the CPU via the FpuPlugin +- Fully pipelined, can produce one result per cycle for most operations (add,sub, mul, fma, load, store), as long there is no inter-dependancies +- Implement multiplication using multiple sub multiplication operations in parallel ("FPGA friendly") +- Division done with radix 4 (2 bits per cycle) +- Square root done with radix 2 (1 bit per cycle) +- Currently only compatible with the DBusCachedPlugin for load and store +- 64 bits Load and store can be done in one cycle via the DBusCachedPlugin (even if VexRiscv is RV32) + +Accuracy, roundings (RNE, RTZ, RDN, RUP, RMM) and compliance: + +- Fully implemented excepted in the cases specified bellow +- In FMA, the result of the multiplication is truncated before the addition (keep mantissa width bits) +- A very special corner case of underflow flag do not follow IEEE 754 (rounding from subnormal to normal number) +- Very specific, but SGNJ instruction will not mutate the value from/to F32/F64 (no NaN-boxing mutation) + + There is a diagram of the FPU design and its CPU integration : + + ![fpuDesign](assets/fpuDesign.svg?raw=true "") + + The FPU can be parametrized with FpuParameter data structure : + + | Parameters | type | description | + | ------ | ----------- | ------ | + | withDouble | Boolean | Enable 64 bits floating point (32 bits always enabled) | + | asyncRegFile | Boolean | Implement the register file using combinatorial reads (instead of syncronous reads) | + | mulWidthA | Boolean | Specify the width of the left operand of multiplication blocks | + | mulWidthB | Boolean | Same than above but the the right operand | + +Synthesis results of the FPU itself, without the CPU integration, on the fast speed grade : + +``` +Fpu 32 bits -> + Artix 7 relaxed -> 135 Mhz 1786 LUT 1778 FF + Artix 7 FMax -> 205 Mhz 2101 LUT 1778 FF +Fpu 64/32 bits -> + Artix 7 relaxed -> 101 Mhz 3336 LUT 3033 FF + Artix 7 FMax -> 165 Mhz 3728 LUT 3175 FF +``` + ### Plugins This chapter describes the currently implemented plugins. @@ -692,6 +740,7 @@ This chapter describes the currently implemented plugins. - [MemoryTranslatorPlugin](#memorytranslatorplugin) - [DebugPlugin](#debugplugin) - [YamlPlugin](#yamlplugin) +- [FpuPlugin](#fpuplugin) #### IBusSimplePlugin @@ -1091,3 +1140,14 @@ The OpenOCD port is here: This plugin offers a service to other plugins to generate a useful Yaml file describing the CPU configuration. It contains, for instance, the sequence of instructions required to flush the data cache (information used by openocd). + +#### FpuPlugin + +Allow the integration of a internal or a external FPU into VexRiscv (See the FPU chapter) + +| Parameters | type | description | +| ------ | ----------- | ------ | +| externalFpu | Boolean | When false the FPU is instanciated in Vex, else the plugin has a `port` interface to which you can connect an external FPU | +| p | FpuParameter | Parameter with which the connected FPU will be created | + + diff --git a/assets/fpuDesign.svg b/assets/fpuDesign.svg new file mode 100644 index 0000000..947ac98 --- /dev/null +++ b/assets/fpuDesign.svg @@ -0,0 +1,2491 @@ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 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