How to define, allocate, and also initialise the values of an array of user defined length - mips

Question

I am quite new to MIPS32 and am working on an assignment that requires me to first ask the user for the length of the array they would like to define, and then ask them what the respective values are. I have written a rough C code which does the same, which is as follows
int main()
{
int N;
scanf("%d\n", &N); // will be the first line, will tell us the number of inputs we will get
int i=0, A[N]; // (1)
// Loop to enter the values to be sorted into an array we have made A[]. the values are entered as a1, a2.... and so on.
while(i != N)
{
scanf("%d\n", &A[i]);
i++;
}
}
I am mainly having trouble with how I write the code above, mainly line (1) in MIPS32. I know that defining the size of the array in the data section itself is not an option, but I am unsure about how to dynamically define an array of size N and then also store values into the array. Any help or advice on what I can do would be really helpful.

Answer 1

Arrays can be stored in global, stack or heap memory.
Global memory
Global memory is essentially fixed-sized at program build time — you put a label in your .data and reserve some constant amount of space, using .space or other data directive.
One approach here is to have a maximum (say 100), so reserve space for that many, and program a limit test to make sure the code doesn't try to use more than the pre-defined maximum.
As an exception, the last global data item can be used to to store an array of relatively unknown size.  This happens to work in QtSpim and MARS, because a fair amount of space behind the global data is there for use to use.  This approach is not very professional, since the code can't really know at what size this will no longer work, but is valid approach for sample toy programs and throw away assignments.  Put a label at the end of your global data and reserve no space or just one word of space.
Integer element arrays have alignment requirements, so when putting global data after string data often requires use of alignment (as a separate directive or by reserving a word, e.g. .word, which will inject alignment automatically).
Heap memory
Heap memory can be allocated using MARS/QtSpim syscall #9.  If the allocation fails, the size was too large, though if it succeeds you have all the space that was asked for.  The syscall #9 returns a pointer to the newly allocated memory in $v0, and you will generally want to store that value somewhere (register or global) for later use.
Stack memory
The stack grows in the downward direction: stack memory can be allocated by decrementing the stack pointer.  The stack pointer — after a decrement — refers to the newly allocated memory.  You can decrement the stack pointer by a fixed amount or by a variable amount.  In your case, you would use a variable amount.  It is generally required that the stack pointer maintain alignment, so in computing the amount to decrement, we would round up.  If you need multiple entities, you can decrement the stack pointer multiple times, or, sum the sizes together and decrement once, which would be the more common approach.
Before (or as) a function returns to its caller, the stack pointer must be returned to the value it had upon function entry.  This releases any allocated stack memory and returns to the caller the same stack environment that it had when it made the function call.  It should stand to reason that it would be a logic error to return released memory to a caller, so this approach cannot be used within a function that needs to return an array to its caller.
Any function that uses syscall #10 to terminate the program does not have to honor this requirement, since the program terminates immediately upon that syscall.  This approach is often used to exit the main — MARS requires it, since it doesn't "call" the main, whereas QtSpim, by default, inserts a small startup stub that does "call" main.