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diff --git a/Documentation/mn10300/ABI.txt b/Documentation/mn10300/ABI.txt new file mode 100644 index 00000000000..1fef1f06dfd --- /dev/null +++ b/Documentation/mn10300/ABI.txt @@ -0,0 +1,149 @@ + ========================= + MN10300 FUNCTION CALL ABI + ========================= + +======= +GENERAL +======= + +The MN10300/AM33 kernel runs in little-endian mode; big-endian mode is not +supported. + +The stack grows downwards, and should always be 32-bit aligned. There are +separate stack pointer registers for userspace and the kernel. + + +================ +ARGUMENT PASSING +================ + +The first two arguments (assuming up to 32-bits per argument) to a function are +passed in the D0 and D1 registers respectively; all other arguments are passed +on the stack. + +If 64-bit arguments are being passed, then they are never split between +registers and the stack. If the first argument is a 64-bit value, it will be +passed in D0:D1. If the first argument is not a 64-bit value, but the second +is, the second will be passed entirely on the stack and D1 will be unused. + +Arguments smaller than 32-bits are not coelesced within a register or a stack +word. For example, two byte-sized arguments will always be passed in separate +registers or word-sized stack slots. + + +================= +CALLING FUNCTIONS +================= + +The caller must allocate twelve bytes on the stack for the callee's use before +it inserts a CALL instruction. The CALL instruction will write into the TOS +word, but won't actually modify the stack pointer; similarly, the RET +instruction reads from the TOS word of the stack, but doesn't move the stack +pointer beyond it. + + + Stack: + | | + | | + |---------------| SP+20 + | 4th Arg | + |---------------| SP+16 + | 3rd Arg | + |---------------| SP+12 + | D1 Save Slot | + |---------------| SP+8 + | D0 Save Slot | + |---------------| SP+4 + | Return Addr | + |---------------| SP + | | + | | + + +The caller must leave space on the stack (hence an allocation of twelve bytes) +in which the callee may store the first two arguments. + + +============ +RETURN VALUE +============ + +The return value is passed in D0 for an integer (or D0:D1 for a 64-bit value), +or A0 for a pointer. + +If the return value is a value larger than 64-bits, or is a structure or an +array, then a hidden first argument will be passed to the callee by the caller: +this will point to a piece of memory large enough to hold the result of the +function. In this case, the callee will return the value in that piece of +memory, and no value will be returned in D0 or A0. + + +=================== +REGISTER CLOBBERING +=================== + +The values in certain registers may be clobbered by the callee, and other +values must be saved: + + Clobber: D0-D1, A0-A1, E0-E3 + Save: D2-D3, A2-A3, E4-E7, SP + +All other non-supervisor-only registers are clobberable (such as MDR, MCRL, +MCRH). + + +================= +SPECIAL REGISTERS +================= + +Certain ordinary registers may carry special usage for the compiler: + + A3: Frame pointer + E2: TLS pointer + + +========== +KERNEL ABI +========== + +The kernel may use a slightly different ABI internally. + + (*) E2 + + If CONFIG_MN10300_CURRENT_IN_E2 is defined, then the current task pointer + will be kept in the E2 register, and that register will be marked + unavailable for the compiler to use as a scratch register. + + Normally the kernel uses something like: + + MOV SP,An + AND 0xFFFFE000,An + MOV (An),Rm // Rm holds current + MOV (yyy,Rm) // Access current->yyy + + To find the address of current; but since this option permits current to + be carried globally in an register, it can use: + + MOV (yyy,E2) // Access current->yyy + + instead. + + +=============== +SYSTEM CALL ABI +=============== + +System calls are called with the following convention: + + REGISTER ENTRY EXIT + =============== ======================= ======================= + D0 Syscall number Return value + A0 1st syscall argument Saved + D1 2nd syscall argument Saved + A3 3rd syscall argument Saved + A2 4th syscall argument Saved + D3 5th syscall argument Saved + D2 6th syscall argument Saved + +All other registers are saved. The layout is a consequence of the way the MOVM +instruction stores registers onto the stack. |