|
118
|
1 ---
|
|
|
2 layout: post
|
|
|
3 author: Paper
|
|
|
4 title: 'The Open Music System, part 1 - initialization'
|
|
|
5 nowplaying: 'The Radio Dept. - Brobygatan'
|
|
|
6 ---
|
|
|
7 <span>
|
|
|
8 Applications wanting to interact with OMS must first call into
|
|
|
9 <code>Gestalt</code> with the first parameter being the magic
|
|
|
10 bytes <code>" OMS"</code> (<code>0x204F3D53</code>) and the
|
|
|
11 second parameter being a pointer to a 32-bit variable that
|
|
|
12 receives the magic pointer.
|
|
|
13 </span>
|
|
|
14 {% comment %}
|
|
|
15 is there was a way to do this with newlines? this is ugly
|
|
|
16 {% endcomment %}
|
|
|
17 <figure><pre class="code-block"><code>UniversalProcPtr OMS_GetGestaltPtr(void)
|
|
|
18 {
|
|
|
19 OSErr err;
|
|
|
20 uint32_t x;
|
|
|
21
|
|
|
22 err = Gestalt(0x204F3D53 /* " OMS" */, &x);
|
|
|
23 if (err != noErr)
|
|
|
24 return NULL;
|
|
|
25
|
|
|
26 return (UniversalProcPtr)x;
|
|
|
27 }</code></pre></figure>
|
|
|
28 <span>
|
|
|
29 From here, applications must retrieve an array of functions
|
|
|
30 through calling that magic pointer. Note that users cannot
|
|
|
31 simply call this pointer directly, unless they are specifically
|
|
|
32 only targeting 68k. You must use the <code>CallUniversalProc</code>
|
|
|
33 function to safely call into 68k code.
|
|
|
34 </span>
|
|
|
35 <figure><pre class="code-block"><code>UniversalProcPtr oms_table[0x5e];
|
|
|
36
|
|
|
37 #define OMS_GESTALT_PROC_TYPE \
|
|
|
38 (kThinkCStackBased \
|
|
|
39 | RESULT_SIZE(kFourByteCode) \
|
|
|
40 | STACK_ROUTINE_PARAMETER(1, kTwoByteCode) \
|
|
|
41 | STACK_ROUTINE_PARAMETER(2, kFourByteCode))
|
|
|
42
|
|
|
43 int32_t OMS_Init(void)
|
|
|
44 {
|
|
|
45 uint32_t functable;
|
|
|
46 UniversalProcPtr omsptr;
|
|
|
47
|
|
|
48 omsptr = OMS_GetGestaltPtr();
|
|
|
49 if (!omsptr)
|
|
|
50 return -1;
|
|
|
51
|
|
|
52 functable = CallUniversalProc(omsptr, OMS_GESTALT_PROC_TYPE, 4, 0);
|
|
|
53 if (functable != (uint32_t)-1) {
|
|
|
54 memcpy(oms_table, (const void *)functable, sizeof(oms_table));
|
|
|
55 } else {
|
|
|
56 /* very old OMS */
|
|
|
57 uint32_t i;
|
|
|
58
|
|
|
59 functable = CallUniversalProc(omsptr, OMS_GESTALT_PROC_TYPE, 1, 0);
|
|
|
60 if (!functable)
|
|
|
61 return -1;
|
|
|
62
|
|
|
63 for (i = 0; i < ARRAY_SIZE(oms_table); i++, functable += 4)
|
|
|
64 oms_table[i] = (UniversalProcPtr)functable;
|
|
|
65 }
|
|
|
66
|
|
|
67 return 0;
|
|
|
68 }</code></pre></figure>
|
|
|
69 <span>
|
|
|
70 At this point, you can now call into any of the pointers in
|
|
|
71 <code>oms_table</code>, provided that you know what the parameters
|
|
|
72 are. The easiest way I've found to find the parameters is to open an
|
|
|
73 OMS-capable program in a reverse-engineering tool, search for uses of
|
|
|
74 <code>CallUniversalProc</code> that look roughly like the initialization
|
|
|
75 code above, and mark the offset they are copied to as the OMS table with
|
|
|
76 the correct size and type. You can then find other uses of
|
|
|
77 <code>CallUniversalProc</code> that use the pointers in the OMS table,
|
|
|
78 and mark down the procedure type (the second parameter to
|
|
|
79 <code>CallUniversalProc</code>). To decode the procedure type, I've
|
|
|
80 written a simple utility to do the heavy lifting
|
|
|
81 <a class="prettylink" href="https://hg.tflc.us/codedump/file/tip/decode-mixed-mode.c">here</a>.
|
|
|
82 Do take note that it does not support register-based calling yet.
|
|
|
83 Feel free to send me any patches if you add support for it ;)
|
|
|
84 </span>
|
