mirror of
https://github.com/pkivolowitz/asm_book.git
synced 2026-06-21 05:46:51 +08:00
74 lines
2.2 KiB
Markdown
74 lines
2.2 KiB
Markdown
# `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.
|
|
|
|
For example, this works:
|
|
|
|
`mov x0, 65536`
|
|
|
|
but this does not:
|
|
|
|
`mov x0, 65537`
|
|
|
|
The reason is that all AARCH64 instructions must fit within a 32 bit
|
|
instruction that must hold the instruction's op code, its flags and
|
|
other bits and bobs plus any immediate value. In the above example we
|
|
can see that the `mov` instruction provides up to 16 bits for an
|
|
immediate value.
|
|
|
|
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` seems to have
|
|
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.
|
|
|
|
## 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.
|
|
|
|
## Movement To / From Integer Registers
|
|
|
|
`fmov` can *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.
|