linux/Documentation/mn10300/ABI.txt
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   1                           =========================
   2                           MN10300 FUNCTION CALL ABI
   3                           =========================
   4
   5=======
   6GENERAL
   7=======
   8
   9The MN10300/AM33 kernel runs in little-endian mode; big-endian mode is not
  10supported.
  11
  12The stack grows downwards, and should always be 32-bit aligned. There are
  13separate stack pointer registers for userspace and the kernel.
  14
  15
  16================
  17ARGUMENT PASSING
  18================
  19
  20The first two arguments (assuming up to 32-bits per argument) to a function are
  21passed in the D0 and D1 registers respectively; all other arguments are passed
  22on the stack.
  23
  24If 64-bit arguments are being passed, then they are never split between
  25registers and the stack. If the first argument is a 64-bit value, it will be
  26passed in D0:D1. If the first argument is not a 64-bit value, but the second
  27is, the second will be passed entirely on the stack and D1 will be unused.
  28
  29Arguments smaller than 32-bits are not coalesced within a register or a stack
  30word. For example, two byte-sized arguments will always be passed in separate
  31registers or word-sized stack slots.
  32
  33
  34=================
  35CALLING FUNCTIONS
  36=================
  37
  38The caller must allocate twelve bytes on the stack for the callee's use before
  39it inserts a CALL instruction. The CALL instruction will write into the TOS
  40word, but won't actually modify the stack pointer; similarly, the RET
  41instruction reads from the TOS word of the stack, but doesn't move the stack
  42pointer beyond it.
  43
  44
  45        Stack:
  46        |               |
  47        |               |
  48        |---------------| SP+20
  49        | 4th Arg       |
  50        |---------------| SP+16
  51        | 3rd Arg       |
  52        |---------------| SP+12
  53        | D1 Save Slot  |
  54        |---------------| SP+8
  55        | D0 Save Slot  |
  56        |---------------| SP+4
  57        | Return Addr   |
  58        |---------------| SP
  59        |               |
  60        |               |
  61
  62
  63The caller must leave space on the stack (hence an allocation of twelve bytes)
  64in which the callee may store the first two arguments.
  65
  66
  67============
  68RETURN VALUE
  69============
  70
  71The return value is passed in D0 for an integer (or D0:D1 for a 64-bit value),
  72or A0 for a pointer.
  73
  74If the return value is a value larger than 64-bits, or is a structure or an
  75array, then a hidden first argument will be passed to the callee by the caller:
  76this will point to a piece of memory large enough to hold the result of the
  77function. In this case, the callee will return the value in that piece of
  78memory, and no value will be returned in D0 or A0.
  79
  80
  81===================
  82REGISTER CLOBBERING
  83===================
  84
  85The values in certain registers may be clobbered by the callee, and other
  86values must be saved:
  87
  88        Clobber:        D0-D1, A0-A1, E0-E3
  89        Save:           D2-D3, A2-A3, E4-E7, SP
  90
  91All other non-supervisor-only registers are clobberable (such as MDR, MCRL,
  92MCRH).
  93
  94
  95=================
  96SPECIAL REGISTERS
  97=================
  98
  99Certain ordinary registers may carry special usage for the compiler:
 100
 101        A3:     Frame pointer
 102        E2:     TLS pointer
 103
 104
 105==========
 106KERNEL ABI
 107==========
 108
 109The kernel may use a slightly different ABI internally.
 110
 111 (*) E2
 112
 113     If CONFIG_MN10300_CURRENT_IN_E2 is defined, then the current task pointer
 114     will be kept in the E2 register, and that register will be marked
 115     unavailable for the compiler to use as a scratch register.
 116
 117     Normally the kernel uses something like:
 118
 119        MOV     SP,An
 120        AND     0xFFFFE000,An
 121        MOV     (An),Rm         // Rm holds current
 122        MOV     (yyy,Rm)        // Access current->yyy
 123
 124     To find the address of current; but since this option permits current to
 125     be carried globally in an register, it can use:
 126
 127        MOV     (yyy,E2)        // Access current->yyy
 128
 129     instead.
 130
 131
 132===============
 133SYSTEM CALL ABI
 134===============
 135
 136System calls are called with the following convention:
 137
 138        REGISTER        ENTRY                   EXIT
 139        =============== ======================= =======================
 140        D0              Syscall number          Return value
 141        A0              1st syscall argument    Saved
 142        D1              2nd syscall argument    Saved
 143        A3              3rd syscall argument    Saved
 144        A2              4th syscall argument    Saved
 145        D3              5th syscall argument    Saved
 146        D2              6th syscall argument    Saved
 147
 148All other registers are saved.  The layout is a consequence of the way the MOVM
 149instruction stores registers onto the stack.
 150
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