Lecture 07-15

H[3] = address of H[0] + 3 * 2 where H is an array of half-words 3 * 2 is the offset

A: .space 400… is that equivalent to A: .word 100 ?todo You gotta do B: .word 42:100 cuz that initialises the array to 42

When doing la with a label, A and B are in RAM right? So are the addresses not the same as the 32 ones in the register file?todo oh right it’s lw or sw for load/store word

For the optimization, sll has two registers (i and i * 4) so the idea is to not keep the i and just have i * 4 and increment it by 4 (cuz that’s the size of a word) each iteration

Optimization two: instead of calling j and bge every loop, we can just jump once and branch each loop.

Strings!

Array of char (byte, not a word!) .word, .space, these are directives (anything that starts with .) So .asciiz is the directive for strings. The assembler now knows the characters become bytes! The null terminator is added implicitly.

It understands things like \n

Structs!

Structs store many data types!
Remember for lw or sw, we need to access thingys aligned (you cant load a word at location 1 for instance cuz words are aligned on addresses divisible by 4).
- For instance, a struct of char and int would be in total 5 bytes. But that’s not possible because if the struct starts at position 0, then char would be at 0 and int would be at 1. That’s not aligned!
- Instead, you treat the char as a 4 byte value. It is a bit wasteful but it’s necessary
- This is padding. This struct is actually 8 bytes (not 5)

#include <stdio.h>
 
struct a {
    int a;
    char b;
};
 
int main() {
    printf("%d", sizeof(struct a)); // This prints 8, not 5!!!!
}

Functions

An “interface” for a bunch of lines of code
This interface asks for inputs and specifies outputs. It has a name and content where values are returned, too.

Parameters

Option #1: just use the registers for return values (v0, v1) and args (a0 to a3)
- But if you need more args and return values you’re out luck.
- Also, what if you need to call another function?! Recursion would be pain
Option #2: We use the stack!
- Set some memory for this stack
- $sp is the register that points to the top of the stack (push-ing onto the stack)
- A function caller would push parameters to the top of the stack such that it grows!
- ”Then the function pops the parameters from the stack using $sp”todo wat. It was making sense before this
I think the stack goes “up” in that, if you want to add a new thing, the index goes down??
- So if the stack starts at, idk $sp = 1000. If you want to push to the stack, then $sp = 996 depending on the size of the stack element what is this called

Implementation time

Idea: the function is a label, but instead of jumping to it directly, we also cache where the function was called. The caller will push the params on stack in order (a, b, c). When we pop them we pop c, b, a (reverse order!)

These are your calling conventions (ways the caller and callee agree on how params and stuff work between them — order, mechanism, etc.) Return values again are stored by pushing it onto the stack! Or you can use $v0 and $v1 To go back to what we called, we use $ra (return address) instead of storing it in the stash
This is done with jal (jump and link) which jumps to the function and stores the next line’s address (PC + 4) into $ra
But what about functions in functions!? $ra would be overridden!
todo i didn’t catch this part

Functions r kinda slow

”Expected lines” = “predictability” = “optimisations” “Jumps” = “no predictability” = ‘no optimisations’

Functions in functions

Problems: $ra will be overridden. Also if $t temp values are used by both functions then uh oh, also overridden
The order in which you push is important. Remember the calling conventions.
We only need to push $t2 because $t0 and $t1 are not needed past the firstmax calltodo go over this again
t0-t9 are caller-saved (the caller is responsible for “perserving” the value). s0-s7 are callee-saved (the callee is responsible)todo I thinkt his means like, the caller shouldn’t override it if it needs it
plz go over the slides (82 specifically)

todo STAB52 ⇐ I wanna murder this C43 C09 MUZA80 (sans c10) CSC236

🪴 Quartz 4.0

Explorer