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David Howellsb920de12008-02-08 04:19:31 -08001 =========================
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 coelesced 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.