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Reserve e_flags bits 24 through 31 for custom use #200

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Aug 5, 2021
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Reserve e_flags bits 24 through 31 for custom use #200

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19 changes: 14 additions & 5 deletions riscv-elf.md
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Expand Up @@ -469,9 +469,9 @@ rules about 2×XLEN aligned arguments being passed in "aligned" register pa
* e_flags: Describes the format of this ELF file. These flags are used by the
linker to disallow linking ELF files with incompatible ABIs together.

Bit 0 | Bit 1 - 2 | Bit 3 | Bit 4 | Bit 5 - 31
-------|------------|-------|-------|------------
RVC | Float ABI | RVE | TSO | *Reserved*
Bit 0 | Bits 1 - 2 | Bit 3 | Bit 4 | Bits 5 - 23 | Bits 24 - 31
-------|------------|-------|-------|-------------|---------------------------
RVC | Float ABI | RVE | TSO | *Reserved* | *Non-standard extensions*
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I dropped the "Reserved for" as otherwise the table gets rather wide


* EF_RISCV_RVC (0x0001): This bit is set when the binary targets the C ABI,
which allows instructions to be aligned to 16-bit boundaries (the base RV32
Expand All @@ -497,9 +497,18 @@ rules about 2×XLEN aligned arguments being passed in "aligned" register pa
* EF_RISCV_TSO (0x0010): This bit is set when the binary requires the RVTSO
memory consistency model.

Until such a time that the *Reserved* bits (0xffffffe0) are allocated by
Until such a time that the *Reserved* bits (0x00ffffe0) are allocated by
future versions of this specification, they shall not be set by standard
software.
software. Non-standard extensions are free to use bits 24-31 for any purpose.
This many conflict with other non-standard extensions.

> NOTE:
> There is no provision for compatibility between conflicting uses of the
> e_flags bits reserved for non-standard extensions, and many standard RISC-V
> tools will ignore them. Do not use them unless you control both the
> toolchain and the operating system, and the ABI differences are so
> significant they cannot be done with a .RISCV.attributes tag nor an ELF
> note, such as using a different syscall ABI.

## <a name=sections></a>Sections

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