Register windows in SPARC

• 128 registers available in SPARC.
• Each window is divided in 3 areas:
  • 8 input registers (i0-i7)
  • 8 local registers (l0-l7)
  • 8 output registers (o0-07)

1
2
3
4
5
6
7

input // Function F1
local
output	input // Save instruction (F2 called in F1)
		local
		output	input // Save instruction (F3 called in F2)
				local
				output

As you can see, each time a function is called in another, the registers window “slides”: by the way, output registers of $F_n$ are equal to input registers of $F_{n+1}$.

Consequence #1 : 24 registers are consumed for F_1, 40 for F_2… Reg_total = 24 + (n-1) x 16, 1 <= n <= 6

Consequence #2 : we cannot have more than 6 function calls at a given time (i.e. F_1 calling F_2 calling F_3 […] calling F_6). In this case, 120 registers are used.

Consequence #3 : if a seventh function call happens, a window overflow trap will happen (exception 0x05, see Table 7-1 in the SPARC v8 ISA). This seventh function would need 16 more registers while only 8 registers only are available…