source: buchla-68k/lib700/rand24.s@ 84c0125

Last change on this file since 84c0125 was 84c0125, checked in by Thomas Lopatic <thomas@…>, 7 years ago

Fix * in comments.
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1| ------------------------------------------------------------------------------
2| rand24.s -- generate a 24 bit random number
3| Version 3 -- 1988-04-29 -- D.N. Lynx Crowe
4| ------------------------------------------------------------------------------
5| Synopsis:
6| long
7| rand24()
8
9| Based on:
10| Knuth, Donald E.
11| The Art of Computer Programming,
12| Volume 2: Semi-Numerical Algorithms
13
14| Computes:
15| S = [S * C] + K
16
17| Where:
18| K = 1
19| C = 3141592621
20| S = the seed (if zero, it gets set from the 200 Hz clock)
21
22| Returns:
23| S >> 8 (a 24 bit pseudo-random number)
24
25| Note: this function has an LSB with an exactly 50% distribution, so using
26| individual bits is probably not a good idea. Using more bits makes things
27| appear more random.
28| ------------------------------------------------------------------------------
29 .text
30
31 .xdef _rand24
32
33 .xdef _rseed
34
35| equates for things in the BIOS
36
37RCLOCK = 0x49E | LONG - 200 Hz clock
38
39| equates for stack offsets
40
41ARG1 = 8 | LONG / WORD - arg1 / MS bits
42ARG1L = 10 | WORD - arg1 LS bits
43ARG2 = 12 | LONG / WORD - arg2 / MS bits
44ARG2L = 14 | WORD - arg2 LS bits
45
46PART = -4 | LONG - partial product
47
48PI = 0xBB40E62D | LONG - PI as a hex value
49
50 .page
51
52| mult32 -- 32 bit signed multiply
53| ------ ----------------------
54mult32: link a6,#-4 | link stack frames
55 clr.w d2 | clear sign flags
56 tst.l ARG1(a6) | check sign of 1st argument
57 bge mult32a | ...
58
59 neg.l ARG1(a6) | make 1st argument positive
60 addq.w #1,d2 | log its sign as negative
61
62mult32a: tst.l ARG2(a6) | check sign of 2nd argument
63 bge mult32b | ...
64
65 neg.l ARG2(a6) | make 2nd argument positive
66 addq.w #1,d2 | log its sign as negative
67
68mult32b: move.w ARG1L(a6),d0 | generate 1st partial product
69 mulu ARG2L(a6),d0 | ...
70 move.l d0,PART(a6) | ...
71 move.w ARG1(a6),d0 | generate 2nd partial product
72 mulu ARG2L(a6),d0 | ...
73 move.w ARG2(a6),d1 | generate 3rd partial product
74 mulu ARG1L(a6),d1 | ...
75 add.w d1,d0 | add partial products
76 add.w PART(a6),d0 | ...
77 move.w d0,PART(a6) | ...
78 move.l PART(a6),d0 | ...
79 btst #0,d2 | adjust sign of result
80 beq mult32c | ...
81
82 neg.l d0 | ...
83
84mult32c: unlk a6 | unlink stack frames
85 rts | return
86
87 .page
88
89| _rand24 -- Generate a random number
90| ------- ------------------------
91_rand24: link a6,#0 | Link stack frames
92 tst.l _rseed | See if the seed is zero
93 bne rand01 | Jump if not
94
95 move.l RCLOCK,d0 | Pick up the 200 Hz clock
96 moveq.l #16,d1 | Shift it left
97 asl.l d1,d0 | ...
98 or.l RCLOCK,d0 | OR in current 200 Hz clock
99 move.l d0,_rseed | Use that as the seed
100
101rand01: move.l #PI,-(a7) | Put PI on the stack
102 move.l _rseed,-(a7) | ... and _rseed, too
103 bsr mult32 | Multiply them
104 addq.l #8,a7 | Cleanup stack
105 addq.l #1,d0 | Add 1 to the result
106 move.l d0,_rseed | Save as new seed
107 asr.l #8,d0 | Make it a 24 bit number
108 and.l #0x00FFFFFF,d0 | ...
109 unlk a6 | Unlink stack frames
110 rts | Return to caller
111
112| ------------------------------------------------------------------------------
113 .bss
114| ------------------------------------------------------------------------------
115
116_rseed: .ds.l 1 | random number seed
117
118 .end
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