Xtensa GPR and ABI

1 Overview

Two different ABI:

• Windowed register ABI, works with the Windowed Register Option (ESP32 use this ABI by default)
• CALL0 ABI, used with any Xtensa processor (worse performance)

Instruction CALL0 is for the CALL0 ABI

Instruction CALL4/CALL8/CALL12 is for the windowed register ABI

2 Windowed Register ABI

2.1 Argument passing

          Return Addr  Stack Ptr       arg0, arg1, arg2, arg3, arg4, arg5
          -----------  ---------       ----------------------------------
call4       a4           a5              a6,   a7,   a8,   a9,  a10,  a11
call8       a8           a9             a10,  a11,  a12,  a13,  a14,  a15
call12     a12          a13             a14,  a15   ---   ---   ---   --- 

2.2 Return value

          Return Value
          -----------
call4       a6
call8       a10
call12      a14

3 CALL0 ABI

3.1 Argument passing

          Return Addr  Stack Ptr       arg0, arg1, arg2, arg3, arg4, arg5
          -----------  ---------       ----------------------------------
call0       a0           a1              a2,   a3,   a4,   a5,   a6,   a7

3.2 Return value

          Return Value
          -----------
call0       a2

4 Reference

• http://wiki.linux-xtensa.org/index.php/ABI_Interface