The Stack

Notes

• When a function is called, it normally needs to manage the stack
• It creates something know as
  • an activation record
  • a stack frame
  • These two are equivalent.
• The activation record contains
  • Information needed to manage the stack (old base pointer)
  • Local variables
  • Values of preserved that will be used locally
  • parameters to functions to be called
  • the return address
• Let's write some code
  • A Linear congruential generator is a way to generate pseudo random numbers
    • X_n+1 = (a * X_n + c) mod m
    • We need integer division
      • (Jorgensen 7.5.4)
      • div divisor (the thing we are dividing by)
      • rdx:rax contains the dividend (the thing we are dividing)
      • after: rax contains the quotient, rdx contains the remainder
      • Or adjust the registers as you see fit.
    • Parameters
      • X₀ is the initial seed value, default 1
      • glibc uses
        • m = 2³¹
        • a = 1103515245
        • c = 12345
    • Implement RAND
      • A global SEED
      • Constants for M, A, and C
      • No parameters
      • Return the value in rax
  • Test this with RAND_TEST
    • Pass in n, the number of times to call the RNG
    • Print out these values.
  • Write RAND_IN_RANGE
    • Pass in the size of the range in rdi
    • Pass in the minimum value in rsi
    • Return rand % range + min in rax
    • It is probably not needed in this case, but to be safe
      • In the preamble
        • set up a stack frame
        • save rsi and rdi
      • In the epilogue
        • restore the stack if necessary
        • Restore the previous stack frame
  • Write SIMPLE_RANGE_TEST
    • Pass the number of times to test in rdi
    • Pass the low value of the range to test in rsi
    • Pass the high value of the range to test in rdx
    • In the preamble
      • Set up the stack frame
      • Work in R12, r13, r14 so save these on the stack.
      • treat n, the number of iterations as a local variable, so put this on the stack.
    • Do the computation: print n random numbers in the range low to high
    • Restore the stack
      • remove rdi
      • Restore r14, r13, r12
      • Restore the old stack frame
  • Write BALANCED_DIE_CHECK
    • Pass in the number of sides: rdi
    • Pass in the number of trials: rsi
    • Set up space for the count array on the stack
    • Roll the die n times, counting the number of times each side appears.
    • Print the resulting table.
    • Clean up in the epilogue