asm_book/section_2/float/fmov.md

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 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 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.

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 for more information.