A Random number library and tester.

• I use random numbers in a surprising number of exercises.
  • So I want a random number generator.
  • But there is not one in assembly.
  • So we need to make our own.
• A very old and simple method is the Linear congruential generator
  • Store a seed
  • Each time rand is called
  • seed = (a*seed + c) mod m
  • m = 2³¹, a = 1103515245, c = 12345 (glibc)
  • This is not good for anything but playing.
• One problem is that this needs a random seed.
  • We have used srand(time(null))
    • This is a system call (201)
    • Here is a better reference .
    • System calls are discussed in chapter 13 of Jorgensen.
      • we have covered most of this.
      • We will do files soon and get the rest.
      • But there is some good example code here if you want it.
    • By the way, time takes a single argument, the address to store the return
    • This is should be stored in rdi.
  • A second option is a the rdtsc instruction
    • a reference.
    • The intel chip has an internal counter.
    • This is apparently impacted by all cores.
    • The call returns a unsigned 64 bit "count" of actions.
  • A third way is to use the os' internal entropy buffer.
    • getrandom, a system call (318)
    • rdi contains the address to write results.
    • rsi contains the number of bytes to get.
• You can look at all three in action in myrandom.asm.
• I have implemented these, as well as the rng routine in lcg.asm.
• I have modified MY_IO library
  • PRINT_STRING:
    • rdi string address
    • rsi string length
  • PRINT_NEWLINE
  • PRINT_INT now no longer prints the newline
• And I have a reasonable example in randTest.asm