2.2 KiB
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 before proceeding.
Let's take a look at some code:
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.
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 for more
information.