Elf M. Sternberg
89b58186fb
There was a bug in the 64-bit subroutine lesson that popped the stack once too many times: there were two pushes but three pops. That this didn't generate a stack underflow condition confuses and worries me. I would have expected a segfault when I decremented the stack counter once too often. This makes me wonder if I understand this program well enough. The effect on the registers was, fortunately, harmless as the next step completely repurposed the registers to initialize the exit() routine. Once I'd figured out how to use the RSI and RAX registers correctly, I streamlined the 32-bit version so that the products of strlen(), the start and length of the message, were already in the registers when I issued my syscall to WRITE(). I'm sure this is, in some ways, *bad* assembly, but I'm not (yet) an ASM stylist. |
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| .gitignore | ||
| LICENSE.txt | ||
| Makefile | ||
| README.md | ||
| counted-hello32.s | ||
| counted-hello64.s | ||
| hello32.s | ||
| hello64.s | ||
| number.s | ||
| something.s | ||
| subroutine-hello32.s | ||
| subroutine-hello64.s | ||
| threads-x86_64.s | ||
README.md
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.
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, 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
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
(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 is very well-written!
- Nayuki has added much to my understanding
- David Evans helped with my understanding of syntax and register use.
- Ray Toal'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.