Assembly is the lowest level you can go besides writing machine code directly. There are multiple assembly variants, like x86, RISC-V, 6502 and such.

This will focus on the x86 variant.

Registers are like variables, but very fast and only a few.

Fun fact: a CPU’s bitness (16-bit, 32-bit, 64-bit, …) refer to the size of the registers (e.g a 32-bit CPU can hold a number up to 4.294.967.295)

Some special registers – by convention, not rule. These can be used as general purpose registers but some instructions may read/write to them automatically.

There are also ‘reserved’, auto-set registers that shouldn’t be accessed directly by the program:

Write values to registers.

To assign a value to a register, you can write mov rax, 5 to move 5 to the rax register. You can also move between registers with mov rax, rbx (copy contents of rbx to rax)

Add values to registers.

You can do add rax, 2 to add 2 to the rax register.

Subtract values from registers.

To subtract 3 from the rbx register, do sub rbx, 3.

Multiply a register with a value.

To multiply rax with 8, do mul rax, 8 (e.g if rax is 2 then it will be 16 after mul)

See also

Most of this is taken from https://github.com/hackclub/some-assembly-required a really great introduction to assembly.

NASM Assembly Language Tutorials - asmtutor.com → this seems like a good place to jump to after some-assembly-required.

https://defuse.ca/online-x86-assembler.htm#disassembly → this could be useful to practice writing x86/64 assembly and see its machine code representation (how it’s actually interpreted by the computer, as bytes)

As sidenote knowledge (debugging) go through Assembly - Nightmare to prep assembly knowledge.