lui helps build large constants, and to see why it is needed, start with a limitation. Every RISC-V instruction is itself just 32 bits wide, and those bits must encode the operation and its registers as well as any constant. That leaves only about 12 bits for a constant inside a normal instruction — enough for small numbers, but nowhere near enough for a full 32-bit value. So large constants cannot be loaded in one instruction; they have to be assembled in pieces.

lui (load upper immediate) handles the top piece. It takes a 20-bit constant and places it in the *upper* 20 bits of the destination register, setting the low 12 bits to zero. The form is lui rd, value.

The usual recipe for a full constant is two instructions: lui lays down the high 20 bits, then an addi adds the low 12. That pairing is exactly what the li pseudo-instruction does for you automatically, which is why you will rarely write lui by hand for ordinary numbers — li is friendlier.

Where lui does appear directly is loading round addresses whose low bits are already zero, such as the fixed addresses of hardware devices, where a single lui supplies the whole value. Its close sibling is auipc, which builds an address relative to the current location instead of an absolute number.