cpct_getRandom_mxor_u8

Gets a high-quality 8-bit pseudo-random number using Marsaglia’s XOR-shift algorithm (Using a 32-bits state)

C Definition

u8 cpct_getRandom_mxor_u8 ();

Assembly call

call cpct_getRandom_mxor_u8_asm

Return value (Assembly calls, return L=random 8-bits)

Known limitations

• This function cannot be used from ROM, as is uses self-modifying code.

Details

This function uses cpct_nextRandom_mxor_u32 to produce a stream of pseudo-random 32-bits numbers, and use them as groups of 4 8-bits values.  Then, it returns each one of the last 4 8-bit values produced before calling cpct_nextRandom_mxor_u32 again.  It uses cpct_mxor32_seed as storage buffer for the last 4 pseudo-random values got from cpct_nextRandom_mxor_u32.

As cpct_nextRandom_mxor_u32 produces (2^32)-1 32-bits numbers without repetition, this function will produce (2^34)-1 8-bits values without repetition.

It is important to know that this function will have 2 different behaviors

• [available] 3 out of every 4 calls, it will only return the next random-byte from the buffer (cpct_mxor32_seed).
• [production] 1 out of every 4 calls, it will also call cpct_nextRandom_mxor_u32 to produce 4 new random bytes.  This behaviour is reflected in the time measures.

This function uses Marsaglia’s XOR-shift standard algorithm with a concrete shift tuple.  Check cpct_nextRandom_mxor_u32 to know details on how this is produced.

Destroyed Register values

AF, BC, DE, HL

Required memory

76 bytes divided in,

• 37 bytes (this function’s code)
• +35 bytes (from cpct_nextRandom_mxor_u32 function’s code)
• +4 bytes (from cpct_mxor32_seed)

Time Measures

   Case     | microSecs (us) | CPU Cycles
-----------------------------------------
 available  |      23        |     92
 production |      88        |    352
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 average-4  |     39,25      |    157
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