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more about fmov but still mystified about mov immediate

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Perry Kivolowitz 2023-04-05 11:17:24 -05:00
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section_2/float/fmov.md
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@ -6,23 +6,19 @@ integer and floating point registers.
## Loading Floating Point Numbers as Immediate Values ## Loading Floating Point Numbers as Immediate Values
Just as we saw with integer Just as we saw with integer registers, some values can be used as
registers, some values can be used as immediate values and some cannot. 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: For example, this works:
`mov x0, 65536` `mov x0, 65535`
but this does not: but this does not:
`mov x0, 65537` `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 The constraints placed on immediate values for `fmov` are much tighter
because floating point numbers are far more complex than integers. because floating point numbers are far more complex than integers.
@ -40,7 +36,7 @@ Let's take a look at some code:
fmov d0, 1.96875 // Zoinks! fmov d0, 1.96875 // Zoinks!
``` ```
From this we can see that an immediate value for an `fmov` seems to have 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 4 bits available for the mantissa. In fact, the only values that work
as immediate values will be those floating point values whose fractional as immediate values will be those floating point values whose fractional
values are combinations of: values are combinations of:
@ -56,6 +52,9 @@ values are combinations of:
As far as exponents go, `fmov` can accommodate 3 bits. So, exponents of As far as exponents go, `fmov` can accommodate 3 bits. So, exponents of
plus or minus 2**7 can be used. 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 ## Loading / Storing Floating Point Numbers in General
When in doubt, load fixed floating point numbers from memory. This is When in doubt, load fixed floating point numbers from memory. This is
@ -64,11 +63,16 @@ covered [in this chapter](./literals.md).
## SIMD ## SIMD
`fmov` can also deal with the more complicated special cases induced by `fmov` can also deal with the more complicated special cases induced by
SIMD instructions. 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 ## Movement To / From Integer Registers
`fmov` can *bits* between the integer and floating point registers. We `fmov` can copy *bits* between the integer and floating point registers.
emphasize the *bits*. No conversions are done using `fmov`. There exist We emphasize the *bits*. No conversions are done using `fmov`. There
other instructions for that. See [this chapter](./rounding.md) for more exist other instructions for that. See [this chapter](./rounding.md) for
information. more information.