CZ1101, Lab 5 (last), 13-18 October. Due Thursday, 23 October, 1997

You are given extra two days to do the problems. Submitting assembly programs, and report some of your test outputs. First problem comes from textbook, page 162.

1. (3.33) [30] Write a function string2int in MIPS assembly language to convert an ASCII string of digits to an integer. For example, if the input to the procedure is the string "1024", the return value should be the integer 1024. Your program should expect register $4 to hold the address of a null-terminated string containing some combination of the digit characters '0' through '9'. Your program should compute the integer value equivalent to this string of digits, then place the number in register $2. Your program need not handle negative numbers. If a non-digit character appears anywhere in the string, your program should stop with the value -1 in register $2. For example, if register $4 points to a sequence of three bytes 50_{ten}, 52_{ten}, 0_{ten} (the null-terminated string "24". Check the ASCII table for the encoding), then when the program stops, register $2 should contain the value 24_{ten}. Hint: use Horner's method of polynomial evaluation. Note that given the digits, 1, 3, 7, the number 137 can be computed as

   (((0 + 1) x 10 + 3 ) x 10 + 7).
   

   Test your procedure by writing a main program in MIPS assembly language, follow closely the MIPS register/stack usage convention.
   
   
2. [30] Write a procedure, P, in MIPS assembly language to compute the value of Legendre polynomials defined recursively as

      P(x, 0) = 1,
      P(x, 1) = x,
      P(x, n) = x (2 - 1/n) P(x, n-1) - (1 - 1/n) P(x, n-2).   n>1.
   

   The function P(x,n) takes two arguments. The first argument is a real number. We use double precision for x. The second argument is an integer. The return value should be double. Use the double precision floating point instructions add.d, sub.d, mul.d, div.d, li.d (load immediate double), l.d (load double), s.d (store double), and mov.d (move double). To convert an integer value in a CPU register, say, $6, to a floating point number of the same value, in FPU register $f8, (e.g., converting integer 3 to floating 3.0), you need the following instruction sequence

      mtc1 $6, $f10       # move the bit pattern to FPU reg $f10
      cvt.d.w $f8, $f10   # convert integer to double, result in $f8
   

   Follow closely the MIPS argument passing/stack usage convention. In particular, the argument x should be passed in $f12, and the second argument n should be passed in $6, and the return value should be in $f0. Test your assembly program with an interactive main program. Compute and report the following values P(0.0, 2), P(0.5, 3), and P(0.25, 5). How do you know that your answers are correct?