# `fmov`

The `fmov` instruction is used to move floating point values in and out
of floating point registers and to some degree, moving data between
integer and floating point registers.

## Loading Floating Point Numbers as Immediate Values

Just as we saw with integer registers, some values can be used as
immediate values and some cannot. It comes down to how many bits are
necessary to encode the value. Too many bits... not enough room to fit
in a 4 byte instruction plus the opcode.

For example, this works:

`mov    x0, 65535`

but this does not:

`mov    x0, 65537`

The constraints placed on immediate values for `fmov` are much tighter
because floating point numbers are far more complex than integers.

Make sure you have read and understand [this chapter](./what.md) before
proceeding.

Let's take a look at some code:

```text
        fmov        d0, 1.0         // works
        fmov        d0, 1.5         // works    2**-1
        fmov        d0, 1.75        // works    2**-1 + 2**-2
        fmov        d0, 1.875       // works    2**-1 + 2**-2 + 2**-3
        fmov        d0, 1.9375      // works    the preceding + 2**-4
        fmov        d0, 1.96875     // Zoinks!
```

From this we can see that an immediate value for an `fmov` has
4 bits available for the mantissa. In fact, the only values that work
as immediate values will be those floating point values whose fractional
values are combinations of:

* 1/2

* 1/4

* 1/8 and

* 1/16

As far as exponents go, `fmov` can accommodate 3 bits. So, exponents of
plus or minus 2**7 can be used.

A sign bit makes the total number of bits available for immediate moves
to be 8.

## Loading / Storing Floating Point Numbers in General

When in doubt, load fixed floating point numbers from memory. This is
covered [in this chapter](./literals.md).

## SIMD

`fmov` can also deal with the more complicated special cases induced by
SIMD instructions. `fmov` is able to move values between the various
register widths such as single precision to double precision. **However,
no conversion of value is performed - `fmov` just copies bits.**

If you need to change the precision of a floating point value, the
`fcvt` family of instructions must be used instead.

## Movement To / From Integer Registers

`fmov` can copy *bits* between the integer and floating point registers.
We emphasize the *bits*. No conversions are done using `fmov`. There
exist other instructions for that. See [this chapter](./rounding.md) for
more information.