You've seen the pattern: lui with %hi(name), then a memory op with %lo(name). This lesson is the fine print — worth ten minutes now, because these pairs are on nearly every screen of assembly you'll read from here on.
A 32-bit address should split cleanly: top 16 bits in the lui, bottom 16 in the load's offset. Almost. The catch is that a load's offset field is signed. If the low half of an address is 0x8000 or above, the hardware treats it as negative, and upper + lower would land 65536 bytes too low.
The fix is baked into the definition of %hi: **when the low half is 0x8000 or more, %hi is the upper half plus one.** The oversized %hi and the negative %lo then cancel exactly. So %hi and %lo aren't literally "top half" and "bottom half" — they're two numbers engineered to sum to the address through a signed 16-bit offset. You never compute them; it's the linker's job. But now the names won't mislead you.
Until the game is linked, gTimer has no address — just a name. A compiled object file leaves the lui's immediate and the load's offset as zeros, plus a note telling the linker "patch these two slots with %hi/%lo of whatever address gTimer gets." That note is the relocation, and the diff view renders it inline, writing %hi(gTimer) where the raw zero sits.
This is good news for matching:
%lo(gRocketFuel), you've just learned a variable's name — free documentation, straight from the original developers' source.A few shapes to file away (you'll practice each in coming lessons):
lui + a load ⇒ read a global; the load's mnemonic tells the type.lui + a store ⇒ write a global.lui + addiu with %lo ⇒ no memory access at all — that's building the address itself as a value.lui half often drifts several lines from its %lo partner, and one lui can serve two accesses. Match pairs by the symbol name, not by adjacency.That's the machinery. From here, %hi/%lo pairs should read as a single mental token: "global access, this name." The next lessons put it to work in every direction.