Lab Exercise 4

Provided code: lab4.zip.

For this exercise, you will write several assembly functions in lab4.S. The provided lab4.h gives details of the type signatures of the functions, and tests.c contains some reasonably-complete test cases.

Is A Number Prime?

Write a function is_prime that takes a single (unsigned integer) argument (which you can assume is >1), and returns 1 if it's a prime number, and 0 otherwise. We aren't going to worry about a clever algorithm here: we will simply check every number from 2 to n-1 to see if it divides n:

def is_prime(n):
i = 2
while i < n:
if n % i == 0:
return 0
i += 1
return 1


The div instruction divides unsigned integers and calculates their remainder, but it's unusual. It always divides the value %rdx:%rax (i.e. a 128 bit value with the high bits stored in %rdx and the lower bits in %rax) by its single operand. It always leaves the quotient in %rax and remainder in %rdx.

You can get %rdi divided by %r8 in %rax and remainder in %rdx like this. The first two instructions set %rdx:%rax to the value from %rdi (by putting bunch of leading zeros in %rdx).

    mov \$0, %rdx
mov %rdi, %rax
div %r8


Unsigned Overflow

Write an assembly function largest_power_unsigned that takes an unsigned integer as its argument and returns the largest power of that number that's representable as a 64-bit unsigned integer.

In order to find that value, you need to repeatedly multiply by $$n$$ until you see an unsigned overflow and return the value before you saw the overflow.

The value you return $$p$$ should be a power of $$n$$, so $$n^i=p$$ for some $$i$$, such that $$p<2^{64}$$ and $$np\ge 2^{64}$$. (But you can't detect it with math: you need to calculate and watch the carry flag.)

Signed Overflow

Write a function overflowing_subtract(a, b) that take two signed integer arguments. It should usually return a-b, unless that calculation is a signed overflow then it should return zero.

Recursion

Write an assembly function dumb(a, b) that takes two unsigned integer arguments and implements this recursive calculation (that's called "dumb" because it does a dumb calculation that makes no sense, but that's the task):

def dumb(a, b):
if a == 0:
return b
elif b == 0:
return a
else:
return a + b + b + dumb(a - 1, b - 1)


You must use the stack to store the a and b values across the recursive call.

Submit

Submit your work to Lab 4 in CourSys.

Updated Thu June 01 2023, 11:23 by ggbaker.