Description
Goals
These exercises are intended to give you more practice with function calls and manipulating
pointers in MIPS.
Setup
Copy the directory ~cs61c/labs/04 to an appropriate directory under your home directory.
Exercises
Exercise 1
This exercise uses the file listmanips.s.
We might have left Python behind with CS61A, but we definitely want to bring our friends map and
reduce along with us! In this exercise, you will complete an implementation of map in MIPS. In
general, map takes a function and a list as arguments and applies the function to each element in
the list. Our function will be simplified to mutate the list in-place, rather than creating and
returning a new list with the modified values.
Our map procedure will take two parameters; the first parameter will be the address of the head
node of a singly-linked list whose values are 32-bit integers. So, in C, the structure would be
defined as:
struct node {
int value;
struct node *next;
};
Our second parameter will be the address of a function that takes one int as an argument and
returns an int. We’ll use jalr (see below) to call this function on the list node values.
Our map function will recursively go down the list, applying the function to each value of the list
nodes, storing the value returned in that node. In C, this would be something like this:
void map(struct node *head, int (*f)(int))
{
if(!head) { return; }
CS61C Summer 2015 Lab 4 1/20/18, 3)16 PM
http://www-inst.eecs.berkeley.edu/~cs61c/su15/labs/04/ Page 2 of 3
head->value = f(head->value);
map(head->next,f);
}
If you haven’t seen the int (*f)(int) kind of declaration before, don’t worry too much about it.
Basically it means that f is a pointer to a function, which in C is used exactly like any other
function.
You’ll need to use an instruction you might not have learned before to implement this: jalr. jalr is
to jr as jal is to j. It jumps to the address in the given register and stores the address of the next
instruction (i.e., PC+4) in $ra. So, if I didn’t want to use jal, I could use jalr to call a function like
this:
# I want to call the function garply, but not use jal.
la $t0 garply # so I use la to load the address of garply into a register ($t0)
jalr $t0 # and then use jalr to jump and link to it.
There are 7 places (6 in map and 1 in main) in the provided code where it says “#### YOUR CODE
HERE ####”. Replace these with instructions that perform as indicated in the comments to finish the
implementation of map, and to provide a sample call to map with square as the function argument.
The sample list is already created for you in create_default_list. When you’ve filled in these
instructions, running the code should provide you with the following output:
List Before: 9 8 7 6 5 4 3 2 1 0
List After: 81 64 49 36 25 16 9 4 1 0
Checkof
Show your TA your test run.
Exercise 2
Add the prologue and epilogue to the code in nchoosek.s so that it computes “n choose k”, the
number of combinations of n distinct elements taken k at a time. (This is also the (n,k) entry in
Pascal’s triangle.)
Checkof
Show your TA your code and its test run.
CS61C Summer 2015 Lab 4 1/20/18, 3)16 PM
http://www-inst.eecs.berkeley.edu/~cs61c/su15/labs/04/ Page 3 of 3
Exercise 3
Write two versions of a function named first1pos (starting from first1pos.s) that, given a value
in $a0, returns in $v0 the position of the leftmost bit that is set to 1 in the word in $a0. If $a0
contains 0, store -1 in $v0. You are allowed to modify $a0 in the process of finding this position.
Positions range from 0 (the rightmost bit) to 31 (the sign bit).
The first version repeatedly shifts $a0 to the left, checking the sign bit at each shift. The second
version starts a mask at 0x80000000 and repeatedly shifts it right to check each bit in $a0.
Checkof
Show your TA both versions of the function and its test run.
