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Add FFLAGS and FRM CSRs (#657)
The PR addresses Issue #559 and adds the remaining F CSRs. - `FFLAGS` - `FRM` Most of the information for the two CSRs is also included in `fcsr.yaml` since `fcsr` is `frm` + `fflags`.
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‎arch/csr/F/fflags.yaml

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# yaml-language-server: $schema=../../../schemas/csr_schema.json
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$schema: "csr_schema.json#"
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kind: csr
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name: fflags
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long_name: Floating-Point Accrued Exceptions
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address: 0x001
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description:
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- id: csr-fflags-purpose
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normative: true
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text: |
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The accrued exception flags indicate the exception conditions that have arisen on any floating-point arithmetic
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instruction since the field was last reset by software.
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- id: csr-fflags-fptrap
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normative: false
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text: The base RISC-V ISA does not support generating a trap on the setting of a floating-point exception flag.
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- id: csr-fflags-reasoning
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normative: false
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text: |
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As allowed by the standard, we do not support traps on floating-point exceptions in the F
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extension, but instead require explicit checks of the flags in software. We considered
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adding branches controlled directly by the contents of the floating-point accrued
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exception flags, but ultimately chose to omit these instructions to keep the ISA simple.
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priv_mode: U
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length: 32
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definedBy: F
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fields:
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NV:
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alias: fcsr.NV
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long_name: Invalid Operation
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location: 4
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description: |
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Set by hardware when a floating point operation is invalid and stays set until explicitly
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cleared by software.
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type: RW-H
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reset_value: UNDEFINED_LEGAL
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sw_write(csr_value): |
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CSR[fcsr].NV = csr_value.NV;
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return csr_value.NV;
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DZ:
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alias: fcsr.DZ
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long_name: Divide by Zero
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location: 3
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description: |
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Set by hardware when a floating point divide attempts to divide by zero and stays set until explicitly
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cleared by software.
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type: RW-H
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reset_value: UNDEFINED_LEGAL
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sw_write(csr_value): |
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CSR[fcsr].DZ = csr_value.DZ;
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return csr_value.DZ;
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OF:
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alias: fcsr.OF
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long_name: Overflow
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location: 2
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description: |
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Set by hardware when a floating point operation overflows and stays set until explicitly
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cleared by software.
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type: RW-H
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reset_value: UNDEFINED_LEGAL
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sw_write(csr_value): |
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CSR[fcsr].OF = csr_value.OF;
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return csr_value.OF;
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UF:
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alias: fcsr.UF
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long_name: Underflow
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location: 1
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description: |
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Set by hardware when a floating point operation underflows and stays set until explicitly
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cleared by software.
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type: RW-H
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reset_value: UNDEFINED_LEGAL
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sw_write(csr_value): |
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CSR[fcsr].UF = csr_value.UF;
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return csr_value.UF;
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NX:
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alias: fcsr.NX
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long_name: Inexact
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location: 0
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description: |
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Set by hardware when a floating point operation is inexact and stays set until explicitly
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cleared by software.
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type: RW-H
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reset_value: UNDEFINED_LEGAL
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sw_write(csr_value): |
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CSR[fcsr].NX = csr_value.NX;
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return csr_value.NX;
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sw_read(): |
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return
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(CSR[fcsr].NV `<< 4) |
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(CSR[fcsr].DZ `<< 3) |
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(CSR[fcsr].OF `<< 2) |
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(CSR[fcsr].UF `<< 1) |
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CSR[fcsr].NX;

‎arch/csr/F/frm.yaml

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# yaml-language-server: $schema=../../../schemas/csr_schema.json
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$schema: "csr_schema.json#"
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kind: csr
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name: frm
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long_name: Floating-Point Dynamic Rounding Mode
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address: 0x002
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description:
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- id: csr-frm-encodings
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normative: false
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text: |
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Rounding modes are encoded as follows:
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[[rm]]
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.Rounding mode encoding.
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[%autowidth,float="center",align="center",cols="^,^,<",options="header"]
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!===
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!Rounding Mode |Mnemonic |Meaning
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!000 !RNE !Round to Nearest, ties to Even
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!001 !RTZ !Round towards Zero
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!010 !RDN !Round Down (towards latexmath:[$-\infty$])
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!011 !RUP !Round Up (towards latexmath:[$+\infty$])
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!100 !RMM !Round to Nearest, ties to Max Magnitude
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!101 ! !_Reserved for future use._
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!110 ! !_Reserved for future use._
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!111 !DYN !In instruction's _rm_ field, selects dynamic rounding mode; In Rounding Mode register, _reserved_.
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!===
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- id: csr-frm-reserved
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normative: false
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text: |
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The behavior of floating-point instructions that depend on rounding mode when
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executed with a reserved rounding mode is _reserved_, including both static
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reserved rounding modes (101-110) and dynamic reserved rounding modes (101-111).
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- id: csr-frm-rmfield
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normative: false
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text: |
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Some instructions, including widening conversions, have the _rm_ field but are
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nevertheless mathematically unaffected by the rounding mode; software should set
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their _rm_ field to RNE (000) but implementations must treat the _rm_ field as
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usual (in particular, with regard to decoding legal vs. reserved encodings).
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priv_mode: U
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length: 32
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definedBy: F
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fields:
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ROUNDINGMODE:
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alias: fcsr.FRM
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location: 2-0
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description: |
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Rounding mode data.
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type: RW-H
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reset_value: UNDEFINED_LEGAL
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sw_write(csr_value): |
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CSR[fcsr].FRM = csr_value.ROUNDINGMODE;
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return csr_value.ROUNDINGMODE;
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sw_read(): |
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return
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CSR[fcsr].FRM;

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