The SPARC processor usually contains as many as 160 general purpose registers. At any point, only 32 of them are immediately visible to software – 8 are a set of global registers (one of which, g0, is hard-wired to zero, so only 7 of them are usable as registers) and the other 24 are from the stack of registers. These 24 registers form what is called a register window, and at function call/return, this window is moved up and down the register stack. Each window has 8 local registers and shares 8 registers with each of the adjacent windows. The shared registers are used for passing function parameters and returning values, and the local registers are used for retaining local values across function calls.