1. CS/COE 0447 (term 2181) Jarrett Billingsley
More Functions
CS/COE 0447 (term 2181)
Jarrett Billingsley

2. Class announcements If you're confused about Lab 2… Everyone is!
Hopefully today will help with that.
When doing the nested loops, work from the outside in.
When doing if-elses, do the first if first.
You can use Discord on your computers, too…
Since only 20 of you have responded so far:
Who has exams in other classes on 9/28?
10/3?
10/5?
9/19/2017
CS/COE term 2181

3. Which instruction do I use to put a value into…
We've seen a lot of data transfer instructions.
All of these only change the value of the register on the left.
Inst. li la
move lw
lh[u]
lb[u]
Use it for…
Putting a constant into a register
Putting an address into a register
Copying from one reg to another
Getting a 32-bit value from memory
Getting a 16-bit value from memory
Getting an 8-bit value from memory
The value will
stay in the reg
you're copying
from!!
9/19/2017
CS/COE term 2181

4. Saved and Unsaved registers
9/19/2017
CS/COE term 2181

5. Some toes are steppable, some aren't
Maybe you wrote a loop, and bad things happened.
li t0, 0
loop_top:
move a0, t0
li a1, 5
li a2, COLOR_RED
jal Display_SetLED
addi t0, t0, 1
blt t0, 4, loop_top
loop_end:
What is this supposed to do?
Let's see what really happens.
Now let's change t0 to s0.
9/19/2017
CS/COE term 2181

6. Another piece of the calling convention puzzle
When you call a function, it's allowed to change some registers.
Other registers must be left exactly as they were.
Functions are allowed to use these registers, as long as they put them back the way they were before they return.
You are allowed to use these registers freely, but when you call another function, you don't know what they'll hold afterwards!
Saved Registers
s0-s7 sp ra
Unsaved Registers
v0-v1
a0-a3
t0-t9
9/19/2017
CS/COE term 2181

7. Why it broke li t0, 0 loop_top: move a0, t0 li a1, 5 li a2, COLOR_RED
If we look at this code again…
t0 is our loop counter and
everything's fiiiine.
li t0, 0
loop_top:
move a0, t0
li a1, 5
li a2, COLOR_RED
jal Display_SetLED
addi t0, t0, 1
blt t0, 4, loop_top
loop_end:
uh oh.
We have no idea what's in t0 now!
9/19/2017
CS/COE term 2181

8. Whenever you call a function…
After calling a function, you have no idea what's in these registers.
...
jal Display_SetLED
Unsaved Registers
v0-v1
a0-a3
t0-t9
9/19/2017
CS/COE term 2181

9. Using s registers properly
You can't just use an s register. You have to follow the protocol.
If you want to use an s register in your function, save it (push it) at the beginning, and restore it (pop it) at the end.
my_func:
push ra
push s0
pop s0
pop ra
jr ra
Moving the papers off the desk.
The pops happen in reverse order!
Putting the papers back.
9/19/2017
CS/COE term 2181

10. Passing arguments and returning values
9/19/2017
CS/COE term 2181

11. The a and v registers Registers a0-a3 are the argument registers.
Registers v0-v1 are the return value registers.
This is just a convention. There's nothing special about them!
The caller…
Fills the argument registers
Uses jal to call the function
Looks at the return value registers
The callee…
Looks at the argument registers
Does its work
Puts something in the return value register(s)
Returns
9/19/2017
CS/COE term 2181

12. int add_nums(int x, int y) { return x + y; }
Input, output
We want to write a function like this:
int add_nums(int x, int y)
{ return x + y; }
Inside of our add_nums asm function…
What register will represent x?
What register will represent y?
What register will hold the sum that we return?
add_nums:
add __, __, __ v0 a0 a1
jr ra
9/19/2017
CS/COE term 2181

13. Calling that function li a0, 3 v0 = add_nums(3, 8) li a1, 8 print(v0)
Now let's call it from main. We want to do:
v0 = add_nums(3, 8)
print(v0)
How do we set 3 and 8 as the arguments?
How do we call add_nums?
How do we put the result in v0?
How do we print an integer?
Why do syscalls put the number of the syscall in v0?
What do you get when you cross an elephant and a rhino?
li a0, 3
li a1, 8
jal add_nums
move a0, v0
li v0, 1
syscall
9/19/2017
CS/COE term 2181

14. Which registers do I use?
90% of your code will use s and t registers.
Which register would you use to…
Hold a loop counter variable?
Hold a * 4 in the expression (a * 4) + b ?
Hold an address of a variable you'll load, and then not use the address again?
Hold the argument x in the function call check_block(x, y) ?
Hold x + 3 in return x + 3; ?
9/19/2017
CS/COE term 2181

15. Function recap
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CS/COE term 2181

16. Let's design a function which…
Draws a diagonal line of pixels on the display.
If we call it like draw_line(10, 10, 5), it will start at x = 10, y = 10, and draw 5 squares in a down-right direction.
What registers are the arguments given in?
Do we have to save ra?
What register will we use for the loop?
Do we have to save it?
How do we write the loop?
What do we call in the loop?
9/19/2017
CS/COE term 2181