Hello, my name is Stephan and welcome to this in-depth exploration of an assembly code example designed
to calculate the sum of numbers from zero to a given value.
This tutorial aims to guide you through the intricate workings of the code, breaking down each instruction
to unveil the underlying mechanisms of register manipulation, loop constructs and interfacing with
the external functions such as values printf here.
So by the end of this lecture, you will have gained a profound understanding of these components,
come together to create a functional program.
And now let's delve into the specifics of the code.
We will first declare the external function declaration.
The External Directive announces the existence of the printf function, which resides externally, implying
that its implementation will be linked later.
So here we will write extern printf.
And after that we will create a section data and this is the data section.
And here we will define a 64 bit quad word.
Here we will number number equal five.
Here we are defining the 64 bit quad word, which is eight bytes.
Uh, let's actually write it down here.
Eight bytes.
Quadword.
We are defining the eight bytes Quadword variable named number with an initial value of five and we
are here.
Fmt db.
We will add a string here.
The sum from zero to.
L d.
Is.
Is old.
And here we will pass ten.
Atlas.
Here, ten and zero.
Here we are defining a null terminated string format suitable for printf, and we have also developed
the section BSS.
This is the section section uninitialized data.
Here currently empty and this section is used to reverse space for variables and this that will be initialized
during program execution.
We will also define the section text.
Section Text.
We will create a global main here and you can define main as the global entry point of the program.
We will create the main.
Now this is a function prologue here.
Posh RVP.
Here and move RVP.
RWP rsp.
Rtsp.
And here in this line of code, we are preserving the value of base pointer rbpp by pushing it on to
the stack.
And here we are setting the base pointer rbpp to the current stack pointer RSP.
And now we will go to initialization path phase here.
So we will use move rc.
Number.
The number we defined here?
Five.
Here.
Here we are.
Load the.
We are telling the assembly to load the value stored in the memory location number into the loop contour
here, which in this case is rc x and we will move racks.
Zero.
We are initialize telling the assembly to initialize the accumulator racks to zero.
And now we will create this loop here.
B Loop.
So this is the loop body.
We will add like this.
We will add Rex.
Actually.
Why?
This here is like this.
So bloop ad rex.
Are racks and ask X.
Here we are telling the assembler to add the value of the looping counter rc x to the accumulator rax
rax.
And now we will loop.
Up here.
Now we will decrement the loop counter arcs and jump back to bloop bloop.
If RC X is not zero and here we can write our loop termination and here.
Move the.
Are the I fmtm here.
Load the address of the format string into RDA or RDA.
Or die here.
Yeah.
MOV RSI.
Number.
There are a lot of the value of number.
This is the sum to be displayed into RSI and move the X to racks.
So load the computer to sum of value from racks into RDX.
Move racks.
Zero.
Clear racks in preparation for the printf call.
Now we will also call the printf here.
Call printf.
We are invoking the printf function to display the formatted output.
And now we will write our function epilogue here.
MOV rsp.
R'p E.r.v are telling the Assembly to restore the stack pointer rsp to its original value.
Cleaning up the stack and we will pop the restore the original value of the base pointer are P and after
that we will return from the main function.
This.
The assembly code showcased here efficiently calculates the summation of numbers starting from zero
up to a specified value.
The key mechanism is the loop construct in which the RC X register serves as a loop counter with each
iteration.
RC x is automatically decremented and the the loop body adds its value to the accumulator racks or racks.
And once the loop concludes, the accumulated sum is presented through the printf function, and a thought
provoking experiment arises by setting number to a notably larger value such as 1 billion.
By doing so, you can observe the program's execution time using the Linux Time command, which we will
do that in the next lecture.
Leading to valuable insights into its performance characteristics.
And this enlightening tutorial empowers you to grasp the intricate interplay of registers, memory and
control flow in assembly language programming.
Fostering a deeper understanding of low level computer operation.