data hazard

A hazard where an instruction needs a value that an earlier instruction hasn't finished producing yet (it must read after that write).

A read-after-write (RAW) data hazard occurs when an instruction reads a register (or memory location) that an earlier, still-in-flight instruction writes; it reflects a true dependence through the value and can be shortened by forwarding but never removed.

Examples

In add x1, x2, x3 then sub x4, x1, x5, the sub needs x1 before the add has written it back.