Goals

Checklist

  • Use the submit command: /u/cs126/bin/submit 9 readme toy.c option.toy. Do not use different file names. Do not use 5a instead of 9, as this will submit your files into the Assignment 5 directory. (We don't have access to the source code for the submit program, so this is the easiest "hack.")

  • Review Lectures 4, 5, and 6.

  • readme
  • name, precept number
  • description of problems encountered
  • high level description of code
  • include the output of your TOY program, including the initial and final "core dump" (print out a running count of the number times that your option is in the money after each of the 255 simulations)
  • indicate any help (if any) you received

  • executing
  • compile toy.c with lcc and type a.out < option.toy (you may obtain errors with the cc compiler since // is used for commenting)

  • toy.c
  • first, make sure you understand the TOY simulator in C
  • once you understand the C code, the "upgrade" should be easy

  • option.toy
  • It's possible to write the whole TOY program in 16 instructions (not including the LFBSR code provided). Keep in mind that you only have 8 registers (variables) to work with, so these are a scarce resource. (In the old days, programmers only had 4!) So, you will need to carefully plan out your program.
  • You will want to remove the print statement that displays each random bit - this will ultimately produce 30 * 255 lines of output, which aren't very useful.
  • Start from the bits.toy program that prints out 30 pseudo-random bits. The TOY "for-loop" is created by setting R6 to 30 (actually 1E in hex) and using opcode 7 (jump and count) to decrement R6. Pay careful attention to the mechanics of a "for-loop" since you will have to write one later.
  • Now, add code to perform 1 simulation. Store the current stock price of $50 in register R4. For each of the 30 pseudo-random bits, increment the stock price by $1 if the bit is 1; decrement it by 1 if the bit is 0. There is no if-else construct in TOY; you can obtain the same effect using opcode 6 (jump if positive).
  • Use register R5 to count the number of experiments in which the option is in the money. If after 30 iterations, the stock price is $56 or more, increment R5 by 1. Here, it will be especially convenient to use the updated opcode 3 (compare and increment).
  • Once everything else is working, write an outer loop to run through this experiment 255 times. Use register R7 to countdown from 255 to 0. Be sure to do this in hex!
  • Using the LFBSR to generate pseudo-random bits, the option will be in the money 49 of 255 times. This provides a good approximation to the true mathematical answer of 194129627/1073741824 or roughly 18%. Part of the inaccuracy is due to performing only 255 trials; the rest could be fixed by a better pseudo-random number generator.

  • TOY debugging hints
  • Comment your TOY code. Update your comments if you modify your code.
  • The pc is initially set to 10.
  • Remember that all values, "line numbers", and arithmetic are in hex. This is by far the most common error.
  • 8 registers is not many, so you may need to reuse a register for multiple purposes. All registers are global variables, so be careful.
  • check the initial "core dump" to be sure that the TOY simulator read in your program as expected (this should fix common errors like having two or more instructions with the same line number, or forgetting the colon after the memory address)
  • Watch out for jump statements - if you insert a new line of code between existsing lines, the location that you want to jump to may change. A common mistake is to update only the "comments", not the actual instructions. Also, after updating the line numbers, check that there are no inadvertent "gaps" in line numbering.
  • Be careful to use consecutive "line numbers" - if you don't specify the initial contents of some memory address, it is set to 0000 which means halt. A useful work-a-round is to insert "dummy" instructions to act as placeholders. A good choice is D000 which does absolutely nothing.
  • Use opcode 4 to print out the value of a particular register.
  • Add extra dump() and dumpreg() commands in your TOY simulator to print out the contents of the pc, memory, and registers, as necessary. Remove these before submitting toy.c.