# 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. ## Lesson 4: Subroutines Lesson four introduces two new pairs of instructions: `push` & `pop`, and `call` and `ret`. The first two push values onto the stack and then pop them off. The latter two call a subroutine and then return from it; `call` pushes the address of the next instruction onto the stack, and `ret` pops it off and sets the IPR (Instruction Pointer Register) to the calling routine. In these examples, I think I've engaged in what is known as *callee cleanup*, which means that the subroutine has the responsibility for restoring the registers after using them. Then again, I may be hopelessly confused. Hopefully, future lessons will clear up the `cdecl()` and other assembly conventions. As is clear in [the commit](https://github.com/elfsternberg/asmtutorials/commit/89b58186fbc54508891c0077cc3e32b3fed8d7cb) and in the comments itself, I've hopelessly abused convention by storing the results in the EDX and RDX registers, rather than EAX as is the convention. On the one hand this is definitely *unstylish ASM*, on the other hand it's something one can do in hand-written ASM, saving exactly one cycle (register copies are *cheap*, people) on my computer that (checks `lshw`) executes approximately 2,870,000 instructions per **second**. More to come... I hope... ## Authors Yours truly! Elf M. Sternberg . ## 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.