167 lines
7.4 KiB
Markdown
167 lines
7.4 KiB
Markdown
# Doing Things with Assembly Language and NASM
|
|
|
|
This is just a list of short assembly language programs that I used to
|
|
reboot my assembly language skills, this time in X86 and X86_64. ("This
|
|
time" because the last time I wrote assembly language I was writing for
|
|
the Motorola 68000 line.)
|
|
|
|
The tutorial I based this off of is at http://asmtutor.com/ The source
|
|
code for the original tutorial, as well as the website, is by GitHub
|
|
contributor Daniel Givney at
|
|
[Assembly Tutorials](https://github.com/DGivney/assemblytutorials).
|
|
|
|
I'll see if I can't scrape together some other, more esoteric examples
|
|
in the future.
|
|
|
|
## Getting Started
|
|
|
|
There's a Makefile. It has a nice help¹.
|
|
|
|
You will need to be running Linux on an Intel platform. These lessons
|
|
do not apply to ARM chips like those on the Raspberry Pi (although it
|
|
would be super cool if they did!).
|
|
|
|
You will need a copy of [nasm](https://www.nasm.us/), the Netwide
|
|
Assembler, the most popular assembler currently in widespread use.
|
|
There are other assemblers, such as GAS (Used by the GNU GCC project),
|
|
MASM (from Microsoft), and so forth, but NASM is popular,
|
|
well-understood, and well-supported. You will also need a linker; the
|
|
Makefile assumes you have the linker suppled with GNU Binutils. On
|
|
Ubuntu-based platforms this comes with the `build-essentials` package.
|
|
If you have a different distribution, consult your archive. If you can
|
|
compile a **C** program, you're fine.
|
|
|
|
## Lesson 2
|
|
|
|
There is no Lesson 1. Okay, there *is*, but I didn't do it. While I
|
|
was looking around for tutorials I found a couple that taught different
|
|
things, and one of the things they all agreed on was a proper exit
|
|
command. Since all Lesson 2 does is add that command, that's what I
|
|
did.
|
|
|
|
I also used a few NASM features not in the ASM Tutorial. The `%define`
|
|
Nasm preprocessor allows you to provide named constants, and I've used
|
|
them here.
|
|
|
|
The syntax `equ $-msg` basically means "The address from HERE, the first
|
|
byte of this named data segment, minus the address named," which puts
|
|
into `len` the length of the string. It only works because `len` is the
|
|
immediate next data segment.
|
|
|
|
### Differences between the 32 and 64 bit versions.
|
|
|
|
The biggest difference that I see is that the Syscalls have all be
|
|
redefined. "Write" and "exit" were 4 & 1 in 32-bit Linux, but 1 & 60 int
|
|
64-bit, respectively. The ASM Tutorial was 32-bit only, and used the
|
|
first four registers. When I ported it to the 64-bit version, the
|
|
syscall for `write()` uses different registers.
|
|
|
|
The 32 bit version uses `int 80h` to interrupt the kernel. The 64 bit
|
|
uses `syscall`. The
|
|
[Linux System Call Table](http://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/)
|
|
is handy here.
|
|
|
|
### Lessons
|
|
|
|
So far, the assembly language programs have two `sections`: one for
|
|
constant data, the other for the actual program. Before either section
|
|
there are macros and directives. Right now the only macros I'm using
|
|
define constants.
|
|
|
|
We aren't allocating any memory that's not in a `.data` segment. And
|
|
that's okay. Everything is happening inside registers. The CPU has 16
|
|
of them. Some of them have side-effects and optimizations, and others
|
|
are *required* for some operations. The AX register, for example, used
|
|
to be the destination for mathematical operations. The X86_64 CPU
|
|
architecture is built around stack-based operations, and the command
|
|
`push reg` will push a value (either a register or memory contents) onto
|
|
the stack pointed to by the SP and BP registers, *and then increment
|
|
those registers*. So, you know, there are quirks to memorize.
|
|
|
|
The Makefile contains compiling and linking instructions. They're
|
|
different for 32 and 64 bit programs, and learning those differences
|
|
would be useful if you intend to write a lot of assembly language.
|
|
|
|
## Lesson 3
|
|
|
|
Lesson 3 is a lot like lesson 2, only instead of knowing the length of
|
|
the string, we're going to calculate it, using the NULL value as our
|
|
end-of-string marker. This also introduces comparison and jump
|
|
commands!
|
|
|
|
The question embedded in my comment in the source file is legitimate.
|
|
At the time, I didn't know if `sub` sets things like the "is zero" flag
|
|
when two values are the same value, the way `cmp` does. The
|
|
[Intel X86 Manual](https://software.intel.com/sites/default/files/managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf)
|
|
(**Warning**: PDF, and very big!) doesn't say they do, and the contents
|
|
of those flags should probably not be regarded as robust or reliable
|
|
after a `sub` operation.
|
|
|
|
With the 64-bit version, rather than blindly copy the ax/bx/cx/dx
|
|
sequence of registers, I deliberately chose to use `RSI` (the Source
|
|
Index Register) for my data source. While the first eight registers are
|
|
considered "general purpose," RSI is (somewhat) optimized to read data
|
|
out of memory and its use is a signal to the CPU's predictive cache. I
|
|
don't know if that's any use to me yet, but it's something I'm aware of
|
|
and I might someday have a use for it.
|
|
|
|
### Memory addressing syntax
|
|
|
|
Lesson three also introduces the `cmp byte [rax], 0` syntax, which does
|
|
a few things. First, there are a *crazy* number of opcodes for the X86
|
|
architecture, and `cmp` is only one-half. An opcode is the numeric
|
|
representation of an instruction to the chip; it's bit sequence
|
|
literally instructs which nanoscopic wires in the chip to light up to
|
|
perform an operation. Not including the wild stuff, an Intel chip has
|
|
something like 1,900 opcodes. But you'll only need to know about 20 of
|
|
them.
|
|
|
|
The `[rax]` syntax tells nasm to generate the `cmp` opcode for which the
|
|
first operand is an address in memory; `cmp` will fetch the thing at
|
|
that address first before doing the comparison. (I'm not sure if this
|
|
occupies another register or what. The manual doesn't say!) The `byte`
|
|
command says that the comparison is on a byte-by-byte basis, so that's a
|
|
*different* opcode, but I suspect nasm makes it easy to remember which
|
|
is which with mnemonics. You don't need to know different ASM commands
|
|
for "compare two registers," "compare a memory location with a
|
|
register," and "compare a memory location with a constant," because
|
|
nasm's syntax makes it easy to understand those operations.
|
|
|
|
What I do know is that the one thing you *can't* do is compare two
|
|
memory locations directly. `cmp` works with two registers, or a
|
|
register and a memory location, or a register and a constant, but no
|
|
other combination.
|
|
|
|
More to come... I hope...
|
|
|
|
## Authors
|
|
|
|
Yours truly! Elf M. Sternberg <elf.sternberg@gmail.com>.
|
|
|
|
## License
|
|
|
|
Daniel Givney does not specify a license for his code, but it is his
|
|
copyright. I did type in, modify, and write these examples on my own (I
|
|
find that I only *learn* things in my brain if they go through my
|
|
fingers, so I rarely cut-and-paste anything), and unless Daniel has a
|
|
complaint, I'm tagging my code with the MIT License. See the
|
|
`LICENSE.txt` file for the full details.
|
|
|
|
## Acknowledgements
|
|
|
|
* Daniel Givney, of course.
|
|
* [The NASM Documentation](https://www.nasm.us/doc/) is very well-written!
|
|
* [Nayuki](https://github.com/nayuki) has added much to my understanding
|
|
* [David Evans](http://www.cs.virginia.edu/~evans/cs216/guides/x86.html)
|
|
helped with my understanding of syntax and register use.
|
|
* [Ray Toal](http://cs.lmu.edu/~ray/notes/nasmtutorial/)'s notes on NASM
|
|
are also useful.
|
|
|
|
---
|
|
Footnotes!
|
|
|
|
¹ I firmly believe that no command, typed blindy, should modify the
|
|
contents of your hard drive. `Make` takes target arguments, and you
|
|
should specify the targets you want built. So `make` by itself only
|
|
issues help.
|