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annotate src/libsys/rand.c @ 86:425ab6d987f3

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src/libsys: pieced together from Citrine
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 20 Jul 2018 20:36:19 +0000
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425ab6d987f3 src/libsys: pieced together from Citrine
Mychaela Falconia <falcon@freecalypso.org>
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1 /*
425ab6d987f3 src/libsys: pieced together from Citrine
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2 * This version of rand() has been lifted from Ancient UNIX, and modified
425ab6d987f3 src/libsys: pieced together from Citrine
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3 * to match the version used in TI's TCS211 GSM firmware, as revealed by
425ab6d987f3 src/libsys: pieced together from Citrine
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4 * disassembly of rand.obj in the rts16le_flash.lib binary library used
425ab6d987f3 src/libsys: pieced together from Citrine
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5 * by that semi-src package. TI's version (most likely from their compiler
425ab6d987f3 src/libsys: pieced together from Citrine
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6 * tools group, rather than the GSM fw group, but who knows) uses the
425ab6d987f3 src/libsys: pieced together from Citrine
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7 * same trivial implementation of rand() as the original Ancient UNIX libc,
425ab6d987f3 src/libsys: pieced together from Citrine
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8 * but with one change: TI's return value is right-shifted by 16 bits
425ab6d987f3 src/libsys: pieced together from Citrine
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9 * compared to what the Ancient UNIX rand() would have returned.
425ab6d987f3 src/libsys: pieced together from Citrine
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10 * The caller thus gets back only 15 pseudo-random bits rather than 31,
425ab6d987f3 src/libsys: pieced together from Citrine
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11 * but then the lower bits of the original rand() return value are
425ab6d987f3 src/libsys: pieced together from Citrine
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12 * known to be pretty bad.
425ab6d987f3 src/libsys: pieced together from Citrine
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13 *
425ab6d987f3 src/libsys: pieced together from Citrine
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14 * rand() is used by some FFS code in reclaim.c. If we don't provide our
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15 * own version of rand() and let the linker pull the version from newlib,
425ab6d987f3 src/libsys: pieced together from Citrine
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16 * the link fails because the latter uses malloc. This ancient implementation
425ab6d987f3 src/libsys: pieced together from Citrine
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17 * of rand() is quite poor, but my plan is to look into possibly adopting
425ab6d987f3 src/libsys: pieced together from Citrine
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18 * some better PRNG after we get the basic TI GSM firmware reintegrated.
425ab6d987f3 src/libsys: pieced together from Citrine
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19 */
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20
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21 static long randx = 1;
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22
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23 srand(x)
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24 unsigned x;
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25 {
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26 randx = x;
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27 }
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28
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29 rand()
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30 {
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31 return ((randx = randx * 1103515245 + 12345) & 0x7fffffff) >> 16;
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32 }