asm_book/section_1/kickstart.md

# Section 1 / Kickstart

In this section, we'll kickstart our exploration of AARCH64 assembly
language.

## Registers

At the dawn of time, central processing units (CPUs) could operate upon
memory directly as both were relatively the same speed. CPUs got smaller
and faster, leaving the speed of memory in the dust but also memory got
further and further away.

A CPU might be on one board while RAM was on another board and had to be
accessed over a shared bus. CPUs got still smaller and faster and might
be on one chip and RAM on another set of chips.

The idea of registers was introduced a very long time ago as being
super fast storage that is implemented directly in the CPU. Because they
are within the CPU, distance isn't really an issue. Similarly, because
they are in the CPU, they operate at the speed of the CPU itself.

Registers don't have addresses because they are not in memory. Instead
they have names and naming conventions. They have only a minimal concept
of typing: apart from integer, floating point (single or double
precision) and pointer, every other notion of "type" is syntactic sugar
provided by your language and its compiler.

ARM processors are RISC processors (Reduced Instruction Set Computers).
The rough idea behind RISC is to make instructions simpler so as to make
room for more registers. AARCH64 (what we are studying) has a lot of
registers. Thirty two integer registers and thirty two floating point
registers.

Also, the floating point registers can also serve as vector or SIMD
registers (later).

Certain registers are reserved for certain purposes (later).

rn means register "of some type" number n.

The kind of register is specified by a letter. Which register within a
given type is specified by a number. There are some exceptions to this.
Here is an introductory summary:

| Letter | Type |
| ------ | ---- |
| x | 64 bit integer or pointer |
| w | 32 bit *or smaller* integer |
| d | 64 bit floats (doubles) |
| s | 32 bit floats |

Some register types have been left out.

### Integers

| This declares an integer | This IS an integer |
| ------------------------ | ------------------ |
| char  | wn |
| short | wn |
| int   | wn |
| long  | xn |

Registers do not have to be declared. They simply ARE.

There are 32 integer registers but some, like x30, are used for specific
purposes. x31 has a special role (explained later).

When you want a 64 bit integer operation you use an x register. For all
other integer operations, you use a w register and further specify the
size by using different instructions, as you'll see later.

The x and w registers occupy the same space. w0 is the lower half of
x0 for example. Writing into x0 will overwrite w0 and vice versa.

### Pointers

| This declares a pointer | This IS a pointer |
| ------------------------ | ------------------ |
| *type* *  | xn |

All pointers are stored in x registers. X registers are 64 bits long but
many operating systems do not support 64 bit address spaces because
keeping track of that big of an address space itself would use a lot of
space. Instead OS's typically have 48 to 52 bit address spaces. So,
while a pointer sits in a 64 bit register, it is possible that not all
of the bits are actually used in making a pointer.

### Floats

There are 32 additional registers for floating point values ranging from
half floats to single precision to  double precision and beyond. What is
beyond double? Vector registers can support multiple values in a single
"very large" register.

If you've never heard of *half floats*, don't worry. After this course
you will likely never hear of them again.

## Instructions

Remember what RISC means? *Reduced Instruction Set*? Well, that was
then. This is now. AARCH64 has an enormous instruction set - hundreds of
instructions, each potentially with many variations. You will be
responsible for mastering every one.

Kidding - you won't be responsible for too too many. Relatively few.
Even so, you will be able to write sophisticated programs.

**EVERY** AARCH64 instruction is 4 bytes wide. Everything the CPU needs
to know about what the instruction is and what variation it might be
plus what data it will use will be found in those 4 bytes.

You might immediately wonder, how could you load a big big number into
a register if the whole instruction is exactly and always 4 bytes long?
Be patient, campers.

But we can draw a distinction between the RISC nature of ARM and the
CISC (Complex Instruction Set Computer) nature of the Intel processors.
The x86 and x64 ISA has variable length instructions ranging from 1 to
15 bytes in length! Complex indeed!

Every instruction is specified by a mnemonic consisting of some letters
which the *assembler* converts into numeric *op-codes*.

Most (but not all) AARCH64 instructions have three *operands*. These
are read in the following way:

```asm
    op     ra, rb, rc
```

means:

```asm
    ra = rb op rc
```

For a concrete example:

```asm
    sub    x0, x0, x1
```

means

```asm
    x0 = x0 - x1
```

An example of a two operand instruction is:

```asm
    mov    x0, x1
```

This means *copy* the 64 bit contents of x1 into the 64 bit register x0.

Or:

```asm
    x0 = x1
```

## Mixing Register Types

With few exceptions, different register types cannot be part of the same
instruction. Adding a 64 bit register to a 32 bit register cannot be
done. For example:

Given:

```asm
    mov    w0, 10       // puts 32 bit 10 in w0
```

You cannot do this:

```asm
    add    x1, w0, x1   // attempts to add w0 and x1 - BAD
```

But you can do this:

```asm
    add    x1, x0, x1   // This is fine
```

Putting a smaller-than-64-bit value into an integer register zeros out
the higher order bits (ignoring the sign bit - explained later). So, you
can safely view the x register when a value was placed into its
corresponding w register.

## Two Instructions for Dealing with Memory

With minimal exception, the AARCH64 ISA permits operations to be
performed only on data in registers. Two obvious instruction families
are those for transferring data from RAM to register(s) and those for
transferring data from register(s) to RAM.

Both loading and storing instructions must specify:

* The registers involved

* The address in RAM involved (always held in an x register)

Loading has the general form of:

```asm
    ldr    rn, [xm]
```

This is like:

```c
    type * ptr = some address;
    type var;

    var = *ptr;
```

Storing has the general form of:

```asm
   str   rn, [xm]
```

This is like:

```c
   type * ptr = some address;
   type var;

   *ptr = var;
```

**The analogies are not exact but close.**

Pairs of registers can also be stored and loaded with the `stp` and
`ldp` op codes.

Post increment and decrement of the pointer is also supported.

Pre increment and decrement of the pointer is also supported.

## Now You Are Ready to Proceed

Have fun!