Rnd

On May 4, 2010, at 2:43 PM, Ian Bogost wrote:

Yes, Randomness is a good area to consider here. It brings up all sorts of issues of encapsulation in programming and expression. A random number generator is a powerful and nontrivial piece of engineering compared to the function one invokes in "10 print."

The smallest effective 8-bit RND I know of in 6502 assembly is 9 bytes long, and considerably longer than the entire "10 print" program when typed in:

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Rand8

lda random ; get seed (must be non-zero)

asl ; shift byte

bcc Rand8.no_eor ; branch if carry flag not set after left shift

eor #$CF ; otherwise EOR with literal $CF

Rand8.no_eor

sta random ; save number as next seed

Although it's interesting to note that in "10 print" the BASIC RND (which generates a number between 0 and 1) is actually less efficient than simply ORing the C64 TOD clock's tenth-of-seconds register with $01 — although that clock outputs in binary-coded decimal (BCD), so it's less truly random. The need to do the RND(1)*2 is sort of an artifact of this process.

Incidentally, the BASIC RND function on the C64 is at ROM address $E09A, so it's possible just to jump there to generate a random number into the 6502's accumulator:

jsr $E09A

I think it was possible on the C64 to get random values from the Sound Interface Device (SID) chip. Does anyone remember how?

reading from register 27 (decimal) returns the current value of oscillator 3. If it's set to noise, you get a random number generator. (http://www.waitingforfriday.com/index.php/Commodore_SID_6581_Datasheet#SID_Control_Registers)

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