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added fmov chapter
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Perry Kivolowitz
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74
section_2/float/fmov.md
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74
section_2/float/fmov.md
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# `fmov`
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The `fmov` instruction is used to move floating point values in and out
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of floating point registers and to some degree, moving data between
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integer and floating point registers.
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## Loading Floating Point Numbers as Immediate Values
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Just as we saw with integer
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registers, some values can be used as immediate values and some cannot.
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For example, this works:
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`mov x0, 65536`
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but this does not:
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`mov x0, 65537`
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The reason is that all AARCH64 instructions must fit within a 32 bit
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instruction that must hold the instruction's op code, its flags and
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other bits and bobs plus any immediate value. In the above example we
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can see that the `mov` instruction provides up to 16 bits for an
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immediate value.
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The constraints placed on immediate values for `fmov` are much tighter
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because floating point numbers are far more complex than integers.
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Make sure you have read and understand [this chapter](./what.md) before
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proceeding.
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Let's take a look at some code:
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```text
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fmov d0, 1.0 // works
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fmov d0, 1.5 // works 2**-1
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fmov d0, 1.75 // works 2**-1 + 2**-2
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fmov d0, 1.875 // works 2**-1 + 2**-2 + 2**-3
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fmov d0, 1.9375 // works the preceding + 2**-4
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fmov d0, 1.96875 // Zoinks!
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```
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From this we can see that an immediate value for an `fmov` seems to have
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4 bits available for the mantissa. In fact, the only values that work
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as immediate values will be those floating point values whose fractional
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values are combinations of:
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* 1/2
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* 1/4
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* 1/8 and
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* 1/16
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As far as exponents go, `fmov` can accommodate 3 bits. So, exponents of
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plus or minus 2**7 can be used.
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## Loading / Storing Floating Point Numbers in General
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When in doubt, load fixed floating point numbers from memory. This is
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covered [in this chapter](./literals.md).
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## SIMD
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`fmov` can also deal with the more complicated special cases induced by
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SIMD instruction.
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## Movement To / From Integer Registers
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`fmov` can *bits* between the integer and floating point registers. We
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emphasize the *bits*. No conversions are done using `fmov`. There exist
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other instructions for that. See [this chapter](./rounding.md) for more
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information.
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@ -1,99 +1,13 @@
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.section __TEXT,__text,regular,pure_instructions
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.build_version macos, 13, 0 sdk_version 13, 1
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.globl _main ; -- Begin function main
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.text
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.p2align 2
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_main: ; @main
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.cfi_startproc
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; %bb.0:
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sub sp, sp, #80
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stp x22, x21, [sp, #32] ; 16-byte Folded Spill
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stp x20, x19, [sp, #48] ; 16-byte Folded Spill
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stp x29, x30, [sp, #64] ; 16-byte Folded Spill
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add x29, sp, #64
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.cfi_def_cfa w29, 16
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.cfi_offset w30, -8
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.cfi_offset w29, -16
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.cfi_offset w19, -24
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.cfi_offset w20, -32
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.cfi_offset w21, -40
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.cfi_offset w22, -48
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Lloh0:
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adrp x20, _d@PAGE
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Lloh1:
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add x20, x20, _d@PAGEOFF
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ldr d0, [x20]
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Lloh2:
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adrp x21, _f@PAGE
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Lloh3:
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add x21, x21, _f@PAGEOFF
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ldr s1, [x21]
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fcvt d1, s1
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str d1, [sp, #16]
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str d0, [sp, #8]
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str xzr, [sp]
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Lloh4:
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adrp x19, l_.str@PAGE
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Lloh5:
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add x19, x19, l_.str@PAGEOFF
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mov x0, x19
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bl _printf
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ldr d0, [x20, #8]
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ldr s1, [x21, #4]
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fcvt d1, s1
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str d1, [sp, #16]
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str d0, [sp, #8]
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mov w8, #1
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str x8, [sp]
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mov x0, x19
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bl _printf
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ldr d0, [x20, #16]
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ldr s1, [x21, #8]
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fcvt d1, s1
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str d1, [sp, #16]
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str d0, [sp, #8]
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mov w8, #2
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str x8, [sp]
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mov x0, x19
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bl _printf
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ldr d0, [x20, #24]
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ldr s1, [x21, #12]
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fcvt d1, s1
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str d1, [sp, #16]
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str d0, [sp, #8]
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mov w8, #3
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str x8, [sp]
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mov x0, x19
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bl _printf
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mov w0, #0
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ldp x29, x30, [sp, #64] ; 16-byte Folded Reload
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ldp x20, x19, [sp, #48] ; 16-byte Folded Reload
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ldp x22, x21, [sp, #32] ; 16-byte Folded Reload
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add sp, sp, #80
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.global test
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test: fmov d0, 1.0
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fmov d0, 1.5
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fmov d0, 1.75
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fmov d0, 1.875
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fmov d0, 1.9375
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fmov d0, 1.96875
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ret
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.loh AdrpAdd Lloh4, Lloh5
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.loh AdrpAdd Lloh2, Lloh3
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.loh AdrpAdd Lloh0, Lloh1
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.cfi_endproc
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; -- End function
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.section __DATA,__data
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.globl _d ; @d
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.p2align 3
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_d:
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.quad 0x3ff8e38da3c21188 ; double 1.555555
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.quad 0x40055554fbdad752 ; double 2.6666660000000002
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.quad 0x400e38e325d4a5df ; double 3.7777769999999999
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.quad 0x40138e38a7e73a36 ; double 4.8888879999999997
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.globl _f ; @f
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.p2align 2
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_f:
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.long 0x3f8e38e3 ; float 1.11111104
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.long 0x400e38e3 ; float 2.22222209
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.long 0x40555554 ; float 3.33333302
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.long 0x408e38e3 ; float 4.44444418
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.section __TEXT,__cstring,cstring_literals
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l_.str: ; @.str
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.asciz "index %ld double %f float %f\n"
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.subsections_via_symbols
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.end
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