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freshened literals.md

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Perry Kivolowitz 2023-04-05 11:28:05 -05:00
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62
section_2/float/literals.md
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@ -20,30 +20,32 @@ To load a `float`, you could translate the value to binary and do
as the following:
```asm
.text // 1
.global main // 2
.align 2 // 3
// 4
main: str x30, [sp, -16]! // 5
ldr s0, =0x3fc00000 // 6
fcvt d0, s0 // 7
ldr x0, =fmt // 8
bl printf // 9
ldr x30, [sp], 16 // 10
mov w0, wzr // 11
ret // 12
// 13
.data // 14
fmt: .asciz "%f\n" // 15
.end // 16
.text // 1
.global main // 2
.align 2 // 3
// 4
main: str x30, [sp, -16]! // 5
ldr s0, =0x3fc00000 // 6
fcvt d0, s0 // 7
ldr x0, =fmt // 8
bl printf // 9
ldr x30, [sp], 16 // 10
mov w0, wzr // 11
ret // 12
// 13
.data // 14
fmt: .asciz "%f\n" // 15
.end // 16
```
The above code is found [here](./t.s).
The above code is kind of found [here](./t.s) - the file is used
for miscellaneous testing.
`Line 6` puts the translated value of 1.5 into `s0` (since the value
is a `float` it goes in an `s` register). The assembler performs some
magic getting a 32 bit value seemingly fit into a 32 bit instruction.
See [below](./literals.md#fitting-32-bits-into-a-32-bit-bag).
`Line 6` puts the translated value of 1.5 into `s0` (since we are
thinking of the value as a `float` it goes in an `s` register). The
assembler performs some magic getting a 32 bit value seemingly fit into
a 32 bit instruction. See
[below](./literals.md#fitting-32-bits-into-a-32-bit-bag).
`Line 7` converts the single precision number into a double precision
number for printing.
@ -136,6 +138,9 @@ Cool huh?
## Fitting 32 bits into a 32 bit bag
**This section is currently LINUX-centric - in the future it will
address both native Apple and Linux equally.***
AARCH64 instructions are 32 bits in width. Yet, `line 6` from
[this](./t.s) program reads:
@ -195,15 +200,16 @@ Scan downward to find `0x7a0`:
0x7a0 <main+32> .inst 0x3fc00000 ; undefined
```
Hey look! Here's our literal float. The `.inst` is an ARM
specific GNU assembler directive what allows the programmer
to encode their own instruction. Note, the encoded instruction does not
have to make any sense - instead the compiler has emitted a make believe
instruction that happens to have the value of our literal.
Hey look! Here's our literal float. The `.inst` is an ARM specific GNU
assembler directive says: `¯\_(-)_/¯`.
Note, the encoded "instruction" does not have to make any sense -
instead the compiler has emitted a make believe instruction that happens
to have the value of our literal.
What we're seeing the actual `line 6` doing is reaching ahead a short
distance to load the value of another "instruction" when really it is
our constant.
distance to load the value of another location in memory where our
constant is really found.
Let us take this explanation further. Notice we see:

BIN
section_2/float/literals.pdf
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Binary file not shown.