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Rewrite description of FCSR. #289

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Rewrite description of FCSR. #289

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12 changes: 9 additions & 3 deletions riscv-cc.adoc
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Expand Up @@ -50,9 +50,15 @@ calls if they are no larger than the width of a floating-point register in the
targeted ABI. Therefore, these registers can always be considered temporaries
if targeting the base integer calling convention.

The Floating-Point Control and Status Register (fcsr) must have thread storage
duration in accordance with C11 section 7.6 "Floating-point environment
<fenv.h>".
Floating-point control and status register is a special global register,
different bit fields has its own properties:

- Accrued exception flags field (bit 0 to bit 4 / `fflags`) are not preserved
across calls and could be any value upon entry.
- Rounding mode field (bit 5 to bit 7 / `frm`) might be modified by calls to
function which is documented as might change the floating point environment.
- All other bits are reserved for other standard extensions, procedures should
not modify the contents of those bits.

=== Vector Register Convention

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