cpct_setTaps_glfsr16

Returns a pseudo-random byte uniformly distributed using fast method (33*Seed mod 257)

C Definition

u8 cpct_setTaps_glfsr16 (u16 tapset) __z88dk_fastcall;

Input Parameters (2 bytes)

Assembly call

call cpct_setTaps_glfsr16_asm

Parameter Restrictions

• tapset could theoretically be any 16-value.  However, there are only 1024 values that will produce complete sequences of 65535 16-bit pseudo-random numbers without repetition.  For those sequences, a set of 1024 enumerated values <GLFSR16_TAPSET_YYYY> (with YYYY from 0000 to 1023) are defined.  If any of this values is used as tapset, a sequence of 65535 16-bit pseudo-random numbers without repetition is guaranteed.  With any other value, shorter sequences will be produced, with no guarantee on their final frequency.

Known limitations

• This function will not work from a ROM, as it uses self-modifying code.

Details

This function sets the concrete sequence that cpct_getRandom_glfsr16_u8 and cpct_getRandom_glfsr16_u8 will produce.  The sequence is picked up from 1024 different complete 65535-value sequences codified in TAPSETs (Check <GLFSR16_TAPS>).  Therefore, this is like reordering the 65535 possible values that random generators will produce.  There are 1024 different ordenations, this function picks one, and then pseudo-random numbers are retrieved in order when calling cpct_getRandom_glfsr16_u8 or cpct_getRandom_glfsr16_u8.

Destroyed Register values

A

Required memory

9 bytes

Time Measures

   Case     | microSecs (us) | CPU Cycles
-----------------------------------------
   Any      |      13        |    52
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