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Dr. Jay
Account closed
Registered: Jan 2003
Posts: 32 |
Random Numbers
Just wanted to share. This has probably been discussed a million times, but in case it hasn't ... was doing a search for random numbers. Some of the old hardware tricks like $d800 don't work with specific versions of the machine and certainly not in all emulators. I found this snippet of code which, if I remember correctly, was from boulderdash:
random eor $dc04
eor $dc05
eor $dd04
adc $dd05
eor $dd06
eor $dd07
rts
Tried using it - I always test random numbers by using the sequence to plot pixels. If you have a bad generator, you can easily see patterns. I could tell this pattern above was clustering on byte-boundaries after watching a few minutes in the plotter. I felt like it was because it was synched too much to the timers and that something would have to throw it out. Then I started thinking about how stable rasters are a $#$@#$@$ and if the call to this routine doesn't always come on a stable raster then we'll have some cycles out of synch ... so I made this simple modification:
random lda #$00
eor $dc04
eor $dc05
eor $dd04
adc $dd05
eor $dd06
eor $dd07
sta random+1
rts
And it appears to work like a charm - much more random! Just uses a seed to feed into the timer sequence, rather than the timer sequence alone!
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CyberBrain
Administrator
Posts: 392 |
what a kewl and *elegant* way to make random numbers! |
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xIII
Registered: Nov 2008
Posts: 210 |
Sorry for bringing this very old thread back but is this still a good way to make random numbers ?
How should I change this routine to get random numbers from 0 to 9 ? |
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chatGPZ
Registered: Dec 2001
Posts: 11136 |
this has never been a good way in the first place :) its not even faster than a proper 16bit lfsr for that matter....
to get numbers in a specific range, the "correct" way (which preserves even distribution of all numbers) is to generate a number in range 0...1, and then multiply the result by your range.
if you can live with somewhat uneven distribution, first generate a number in 0...255 range, then use the result as index into a one page table that contains all numbers you want to get |
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Oswald
Registered: Apr 2002
Posts: 5023 |
http://codebase64.org/doku.php?id=base:6502_6510_maths#random_n.. |
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xIII
Registered: Nov 2008
Posts: 210 |
Thx Groepaz.
I did some more research @codebase64 and found this random generator and in combination with the table you suggested it works fine for me.
.var sr = $ff
lda sr+1
asl
asl
eor sr+1
asl
eor sr+1
asl
asl
eor sr+1
asl
rol sr
rol sr+1
tax
lda rnd_table,x
sta rnd_number
rnd_table: .for(var i=0;i<256;i++) .byte random()*9+48
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xIII
Registered: Nov 2008
Posts: 210 |
Thx Oswald :) |
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chatGPZ
Registered: Dec 2001
Posts: 11136 |
Quote:.byte random()*9+48
i'd use something like .byte (i%9)+48 instead ... if only because it will always assemble into the same table |
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xIII
Registered: Nov 2008
Posts: 210 |
Quote: Quote:.byte random()*9+48
i'd use something like .byte (i%9)+48 instead ... if only because it will always assemble into the same table
Kickassembler doesn't seem to like this ?
Maybe because I use C64 Kickass IDE v1.2 which is old? |
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chatGPZ
Registered: Dec 2001
Posts: 11136 |
no idea what kickass likes really :=) it must have some kind of modulo operator, i guess =P |
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xIII
Registered: Nov 2008
Posts: 210 |
Quote: no idea what kickass likes really :=) it must have some kind of modulo operator, i guess =P
mod(random()*9,9)+48 seems to work.
But it's no problem if the table assembles different everytime ;) |
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