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78 lines
2.4 KiB
Markdown
78 lines
2.4 KiB
Markdown
# `fmov`
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The `fmov` instruction is used to move floating point values in and out
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of floating point registers and to some degree, moving data between
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integer and floating point registers.
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## Loading Floating Point Numbers as Immediate Values
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Just as we saw with integer registers, some values can be used as
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immediate values and some cannot. It comes down to how many bits are
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necessary to encode the value. Too many bits... not enough room to fit
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in a 4 byte instruction plus the opcode.
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For example, this works:
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`mov x0, 65535`
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but this does not:
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`mov x0, 65537`
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The constraints placed on immediate values for `fmov` are much tighter
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because floating point numbers are far more complex than integers.
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Make sure you have read and understand [this chapter](./what.md) before
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proceeding.
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Let's take a look at some code:
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```text
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fmov d0, 1.0 // works
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fmov d0, 1.5 // works 2**-1
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fmov d0, 1.75 // works 2**-1 + 2**-2
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fmov d0, 1.875 // works 2**-1 + 2**-2 + 2**-3
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fmov d0, 1.9375 // works the preceding + 2**-4
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fmov d0, 1.96875 // Zoinks!
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```
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From this we can see that an immediate value for an `fmov` has
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4 bits available for the mantissa. In fact, the only values that work
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as immediate values will be those floating point values whose fractional
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values are combinations of:
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* 1/2
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* 1/4
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* 1/8 and
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* 1/16
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As far as exponents go, `fmov` can accommodate 3 bits. So, exponents of
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plus or minus 2**7 can be used.
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A sign bit makes the total number of bits available for immediate moves
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to be 8.
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## Loading / Storing Floating Point Numbers in General
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When in doubt, load fixed floating point numbers from memory. This is
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covered [in this chapter](./literals.md).
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## SIMD
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`fmov` can also deal with the more complicated special cases induced by
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SIMD instructions. `fmov` is able to move values between the various
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register widths such as single precision to double precision. **However,
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no conversion of value is performed - `fmov` just copies bits.**
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If you need to change the precision of a floating point value, the
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`fcvt` family of instructions must be used instead.
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## Movement To / From Integer Registers
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`fmov` can copy *bits* between the integer and floating point registers.
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We emphasize the *bits*. No conversions are done using `fmov`. There
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exist other instructions for that. See [this chapter](./rounding.md) for
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more information.
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