What is assembly?

Before anything else: a CPU (central processing unit) is the chip at the heart of your computer, and it really does only one thing: it carries out a long list of tiny commands called instructions, one after another, billions per second. Each instruction is simple on its own (add two numbers, move a value, jump somewhere). Assembly is how we write those instructions in text a person can read.

Assembly is the human-readable form of the instructions a CPU actually runs. Each line is (almost) one machine instruction. There are no loops, objects, or strings handed to you. You move numbers between registers and memory, one tiny step at a time.

This simulator runs RISC-V, a clean, modern instruction set used in real chips and taught in universities. What you learn here maps directly to hardware.

RISC stands for Reduced Instruction Set Computer. This means the hardware is designed to execute a small set of simple instructions extremely fast, rather than having complex instructions that do many things at once. We build complex behavior by combining these simple, fast steps.

Registers: the CPU's hands

First, what is a bit? It's a single 0 or 1, the smallest piece of information a computer can store. Line up 32 of them and you get a 32-bit number, which can count from 0 up to about 4 billion. Every register and every memory slot in RISC-V holds exactly one 32-bit number.

A register is a tiny, ultra-fast slot holding one 32-bit number. RISC-V has 32 of them. Registers sit right inside the processor core, which makes them far faster to use than your computer's main RAM, by orders of magnitude. You'll mostly use a few: t0–t2 (temporary scratch), a0–a2 (arguments / results), zero (always 0), sp (stack pointer), ra (return address).

# Print the number 42, then exit.
# a7 picks the service; a0 carries the value.
li a7, 1        # service 1 = print integer
li a0, 42       # the value to print
ecall           # do it

li a7, 10       # service 10 = exit
ecall

Read it line by line. li means load immediate: it drops a constant into a register. li a7, 1 selects service 1 (print integer); li a0, 42 loads the value to print into a0. ecall hands control to the system, which prints whatever is in a0. Finally li a7, 10 selects the exit service and ecall stops the program. Press Run, then change 42 and run again.