# Lab Exercise 7

Provided code: lab7.zip.

No compilation commands are being provided this week. It is part of the exercise to figure out how to make the code run. We expect it to be possible to assembly/link your code with the tools we have introduced in the course.

## Random Number Generator

For this question, you'll create a pseudo-random number generator (specifically, a linear congruential generator) function in lab7.S.

Our LCG's behaviour will be like this:

seed = 0
def randint():
#global seed
seed = seed * 6364136223846793005 + 1442695040888963407
#seed = seed % (2**64)
return seed


The commented-out lines would be necessary to express this as actual Python code (which needs global variables explicitly declared, and isn't limited to 64-bit integer arithmetic), but these are not necessary in an assembly translation of the logic.

Our random number generator will always start with the same seed (0). That means it will always generate the same sequence of "random" numbers, but that's fine for what we're doing.

The random seed will need to be a 64-bit integer in static memory, reserved and initialized in a .data section.

The constants in the calculation are larger than 32-bit constants that are allowed in most instructions, so the only way to use them is with movabs to first get the value into a register:

movabs $6364136223846793005, %r8  With those hints, write a function randint in lab6.S that generates pseudo-random unsigned integers as described. ## Random Array Write an assembly function fill_array that takes two arguments: a 64-bit signed integer array pointer and the number of elements. It should fill the array with random values (generated using your randint). We don't want our array values to be too big, so we'll keep the array elements in the range -128–127 (but still as 64-bit integers). Hint: call randint and$0xff, %rax
sub \$128, %rax


## Dot Product

Write an assembly function dot that takes two (64-bit signed integer) array pointers and a length (assumed to be the same for both arrays). It should return the dot product of the two arrays, i.e.

arr1[0]*arr2[0] + arr1[1]*arr2[1] + ⋯ + arr1[n]*arr2[n]


## Array of Structs

Instead of working with pairs of arrays, maybe we should work with arrays of pairs. The provided lab7.h defines this struct:

typedef struct {
int64_t a;
int64_t b;
} pair64_t;


In C, we know that array elements are guaranteed to be adjacent in memory, but struct members are also guaranteed to be arranged in memory in the order they're declared. We generally expect them to be adjacent, so this struct should take 16 bytes: the first 8 bytes are a and the next 8 are b. (It's actually a little more complicated than that because of padding and alignment, but we can ignore those for now.)

Repeat the dot product calculation, but taking a single array of pair64_t (and a length) as arguments.

## Speed

The included timing.c will do some timing of your implementations, as well as C implementations in lab7c.c. Have a look.

## Questions

Answer these questions in a text file answers.txt.

1. What commands did you use to compile/assemble and run your code for this lab?
2. How did the running times compare for the various dot product implementations?

## Submit

Submit your work to Lab 7 in CourSys.

Updated Mon April 15 2024, 10:42 by ggbaker.