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- =========================
- 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 coalesced 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.