I just remembered a thing about “Random” which I encountered a short time ago, which might be worth to mention.
When playing around with Shongololo, one of the tasks was to draw random Shrooms on the screen. For that I wanted to generate random coordinates for a 16×16 grid. One further constraint was – no Shrooms were allowed to occupy the same location.
Using the BIOS function “Random” led to a black screen – and nothing happened.
What happened?
The Random function is not random enough. I wanted to draw more than 32 Shrooms! The Random was only able to generate 32 different pairs of random coordinates! Trying for another set of coordinates led to an endless loop.
Following code was used to generate the coordinates:
Resulting in following Shroom image:
Not very random looking :-(.
Than I remembered having read about different approaches – and remembered something I wrote in a “C” article of mine…
Ultra Fast Pseudorandom number generator for 8-bit
That translated to assembler resulted in following
Following is the code for possible usage, 4 variables used, one “init” function and a macro. But you might also use a subroutine – which is really straight forward.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 | ;*************************************************************************** ; see https://www.electro-tech-online.com/threads/ultra-fast-pseudorandom-number-generator-for-8-bit.124249/ BSS ORG $c880 ; start of our ram space random_a ds 1 ; vars for own "random" - is much better than the internal BIOS one! random_b ds 1 random_c ds 1 random_x ds 1 ;*************************************************************************** RANDOM_A_alt macro inc random_x lda random_a eora random_b eora random_x sta random_a adda random_b sta random_b lsra eora random_a adda random_c sta random_c endm ;*************************************************************************** code ORG 0 initRandom ; initialize random function ; the timer parts should be set from the title screen after the player ; presses a button to play ; ; will be pretty random than ; ; or just set it here with fixed values ; if you always want the same random seed. ; (vide != vectrex for the timer values) ; ; each value of the 4 "random" variable change causes ; quite different random outcomes ; so if you just want to test out different "shrooms" on screen ; keep changing e.g. "x" lda #2 sta random_x lda #$d4 sta random_a lda <VIA_t2_lo sta random_b lda <VIA_t1_cnt_lo sta random_c RANDOM_A_alt ; call random once rts |
is there no way to randomly set the seed value used by the random function?
Nope – than you would not need a random function :-).
But using the low part of a ever changing timer – after a “user-button” press – as a seed seems fairly random to me…