Assembly Language Programing 2

In this episode we’ll write a hello world program in assembly. We’ll talk about what CPU architecture looks like, registers, RAM, and more.

Transcript

The following transcript was automatically generated by an algorithm.

00:00 : hello there I'm gonna continue the
00:01 : series on assembly programming so last
00:04 : time we got set up with a Debian Linux
00:10 : operating system running within backer
00:13 : if you're on a Mac computer or a Windows
00:17 : computer but if you use a Linux computer
00:20 : already then you can install Nathan
00:24 : yourself nay Sam is the assembler that
00:28 : we're using for this series we got that
00:33 : set up and we got docker for me I'm
00:36 : using Mac OS 10 so I got a Debian Linux
00:40 : running within docker on my Mac and the
00:44 : way I can start that instance is by
00:46 : using a doctor coming on like docker run
00:49 : IT means run the show within it and I'm
00:53 : gonna map a directory my home directory
00:55 : in the slash boom and then finally I
00:59 : specified the name of the container
01:03 : which I created a I created a public
01:10 : docker image which you can also use for
01:14 : this purpose and then finally I can
01:16 : write the bash shell has to come in I
01:19 : want to execute and I write all that
01:22 : then again I get this this show which is
01:26 : running inside Debian Linux now and this
01:29 : is to map a drive so that if I see be
01:32 : into the slash home then this slash home
01:37 : mirrors what I have within my home
01:41 : directory and then my home directory I
01:44 : have a playground directory that thing
01:48 : that I have a lot of playground projects
01:50 : the one I'm gonna use I'm ever gonna
01:53 : make up new directory I'm gonna make
01:57 : directory that's called it
02:00 : tutor because STM tutor.com is the site
02:07 : I'm gonna be pulling some lessons from
02:12 : however before I start getting into the
02:15 : lesson I'm gonna take this line and pull
02:19 : it out into my bash profile so that I
02:23 : don't have to write this very very long
02:25 : line every time I want to do this I
02:27 : actually have already done this alias I
02:31 : created an alias and that name of the
02:33 : alias is Debian and I've set they're the
02:37 : result of the alias to this long command
02:40 : that I just wrote and then so you can at
02:44 : this syntax here into your that bash
02:49 : profile file save it and to save it you
02:53 : do ctrl o to write out and then you can
02:57 : re-execute this file by typing dot and
03:01 : then the path to your bash profile file
03:04 : and then you have this alias set up so
03:07 : now I can just type Debian Debian and
03:14 : then I'm into the shell well how fast it
03:17 : was so now they would sell and again if
03:22 : you exit it it just sort of cleaned up
03:26 : everything not the entire container the
03:31 : entire instance of that minute computer
03:33 : has been blown away and cheered up into
03:35 : smithereens so the only reason we are
03:40 : able to perceive our work while we're in
03:43 : the Debian instance is because of the
03:46 : map drive so I need to make sure
03:47 : everything of the work I'm doing I want
03:50 : to do it inside the map drive which is
03:53 : slash home and then I'm going to again
03:57 : go into my playground directory and
04:00 : Azzam tutor to do my work in here so
04:06 : that my work is preserved because it's
04:10 : mapped to my
04:11 : actual Mac computer whenever I write
04:14 : files into this directory one more thing
04:17 : I'm gonna do is to set the ps1 variable
04:24 : so that I can have a shorter show so my
04:28 : show is just one dollar sign and that's
04:30 : what I prefer because I don't look I
04:33 : don't prefer a long shelf okay so now
04:36 : I'm in this directory that I just
04:38 : created and it has nothing in it and now
04:41 : let's write some assembly code let's go
04:44 : for a lesson one so let's read this so
04:52 : assembly language is bare-bones
04:54 : the only interface the programmer has
04:56 : above the actual hardware is the kernel
04:59 : itself in order to build useful programs
05:02 : in assembly we need to use the Linux
05:05 : system calls provided by the kernel so
05:08 : in the environment of a Linux operating
05:10 : system when we're making system costs
05:17 : what a system call a system call is sort
05:19 : of like an API function API provided by
05:28 : by the operating system and when they
05:32 : say kernel when they say kernel really
05:35 : that's just the core of the operating
05:37 : system so you can think of kernel as the
05:40 : same thing as operating system for most
05:42 : cases so when you're making a system
05:45 : call you're calling into an API of the
05:49 : operating system and you can take a look
05:52 : at there's a table here that it links to
05:55 : here that shows you a table of all the
05:59 : system calls that Lennox has and in the
06:05 : first in the following example we're
06:08 : gonna be using sis right which is to
06:11 : write something into the terminal and
06:14 : we're gonna be used six adjust to accept
06:17 : the program because just to exert a
06:18 : program into the make a system call so
06:21 : we'll do that
06:25 : and let's write our program there are
06:30 : these registers which will may kind of
06:35 : freak you out at the beginning but these
06:38 : registers you can think of them as
06:40 : variables inside of a CPU you can think
06:44 : of a CPU as this program or this program
06:50 : so like last time I we said a CPU it
06:56 : kind of looks like this and you can
07:00 : think of it as well inside the CPU
07:03 : there's this arithmetic unit I think
07:08 : it's called the aru which does the
07:13 : calculations like add subtract multiply
07:15 : and divide and stuff other stuff like
07:19 : that
07:20 : but we in order to do that it has to get
07:24 : the inputs fed into it and then its
07:28 : output it has after it computes
07:31 : something it has to turn out an output
07:34 : which also has to go somewhere and those
07:37 : values are gonna come either from or to
07:44 : these things called registers maybe r1
07:49 : r2 but they don't have names like r1 r2
07:53 : instead they have names like edx e CX e
08:02 : b x and so forth and there are more of
08:06 : them so these are some of them so these
08:09 : guys will store values there you can
08:14 : think of them as variables that are in
08:17 : the cpu and if you want the sort of
08:21 : calculator the arithmetic unit aru to
08:25 : calculate something need to load some
08:28 : values into these registers first and
08:30 : then ask this aru to do the computation
08:33 : based on
08:34 : the values that are stored in certain
08:37 : registers and then once it's done the
08:39 : calculation it'll get the computer value
08:42 : and store it into another one of your
08:45 : registers so that's how things work is
08:48 : how things work with computation but
08:50 : that's also how things work with
08:52 : syscalls
08:53 : so with Cisco's you are telling you're
08:59 : telling Lennox the arguments that are
09:03 : you're passing to the Cisco you're
09:05 : putting in them in these registers and
09:08 : then once you load them up with the
09:12 : values that you want
09:13 : you're gonna tell Lennox hey can you do
09:16 : this system call for me and they'll look
09:18 : at you to register the values inside the
09:21 : register you put in to do the
09:24 : appropriate system call based on those
09:27 : values so that's what we're gonna do so
09:30 : let's write this little Halliwell
09:33 : program as is I'm gonna type it in and
09:36 : you can type it in as well of course you
09:40 : can don't have to look at me type it in
09:41 : you can go to the ACM tutor.com website
09:45 : to look at the code there but I'm gonna
09:49 : type it in in front of you I'm gonna use
09:51 : Nano and since they use the filename he
09:55 : uses hello world am i will follow I'm
10:01 : not gonna bother typing in his comments
10:04 : I'm just gonna like these are common the
10:06 : words that come after the semicolon
10:09 : those are comments and I'm not gonna
10:11 : bother typing those in so this is it's
10:17 : the data section is a place where you
10:19 : can store string literals and other
10:22 : literals they could also be numerical
10:24 : literals - this is saying we're gonna
10:27 : make a msg constant this you can name
10:32 : this guy whatever you want you can call
10:34 : it poop but let's call it message
10:42 : and you can use tabs or spaces doesn't
10:45 : matter to separate this one from DB DB
10:49 : means I forget what DB means but it just
10:53 : means some data here and then here's
10:55 : this string literal this Oh 8 H the H is
11:01 : actually special code that means
11:03 : hexadecimal if you don't put the H here
11:06 : oops if you don't put the H here then
11:09 : it's interpreted as decimal numbers and
11:12 : OAH the hexadecimal number of Oh a is
11:18 : actually 10 and if you are not good at
11:22 : converting hexadecimal numbers to
11:24 : decimal numbers in your head you can use
11:27 : this online hex calculator and ok you
11:30 : can type in Oh what is Oh amen
11:32 : it'll calculate for you and tell you
11:35 : that's 10 so this Oh a H is actually
11:38 : equivalent to writing 10 and that's fine
11:42 : I'm gonna write 10 and what the heck is
11:44 : 10 10 is actually the ASCII code for the
11:48 : newline character so if you go to ASCII
11:50 : table com you show you all of the ASCII
11:54 : codes and you tell you the decimal
11:56 : number 10 that's code for a new line
11:59 : that and in JavaScript that would be /n
12:03 : but in assembly language that's been
12:07 : number 10 okay so now let's do the next
12:14 : section this text section in low-level
12:19 : programming speak text means the actual
12:22 : program so that's what we'll do and the
12:28 : underscore start is the entry point of
12:31 : the program and you have to name it that
12:34 : for some reason for for the Assam for
12:38 : the NASM compiler okay and if you're
12:42 : familiar with c programming at all and C
12:45 : this would be called main
12:47 : but in Assam Disko underscore start so
12:51 : now we're gonna actually write the first
12:53 : line over
12:54 : toad there's five lines in this program
12:57 : these move statements okay what what are
13:04 : these lines
13:05 : okay so first line number one let's move
13:10 : the value of 13 into the register EDX
13:15 : remember we have a CPU it looks like
13:20 : that whatever CPU it looks like that and
13:25 : inside of it it's got these registers
13:27 : and one of them is called EDX well let
13:32 : me make more room here so I have a EDX
13:36 : register which is able to store
13:40 : information in it and this instruction
13:44 : is it can you move the value 13 into the
13:47 : EDX register so this that's what that
13:51 : would do okay and then number nine
13:56 : number two is well we have another
13:58 : register that's called ECX and somewhere
14:03 : within the memory chip remember what
14:06 : does the memory trip look like memory
14:08 : trip maybe looks something like this
14:11 : okay so this is the memory chip or Way
14:16 : RAM random access memory so we actually
14:23 : are nathan has taken care to store this
14:28 : hello world exclamation point and then a
14:31 : 10 which stands for new line somewhere
14:34 : in there so somewhere in this memory
14:35 : chip there's this hello world string in
14:39 : there and this is saying hey wherever
14:47 : you stored this guy on the memory trip
14:49 : can you take that address value the
14:53 : address value is some number I don't
14:55 : know
14:57 : 23 I don't know it's a number that
15:02 : addresses this spot inside the memory
15:05 : chip and he said can't just take that
15:07 : number and store it in this ECX register
15:11 : so if the computer runs this instruction
15:17 : it'll put that number in here and then
15:20 : the next one is doing the same with the
15:23 : EBX register so we have EBX and this is
15:28 : gonna put one in there and then there's
15:30 : another register called EAX and this is
15:34 : gonna put four in there and then we're
15:37 : gonna trigger this interrupt interrupt
15:42 : is saying interrupts the flow of this
15:45 : program and send this number 80 in
15:50 : hexadecimal
15:51 : what is the number 80 in hexadecimal
15:53 : let's see
15:55 : 80 in hexadecimal calculator it's 128 so
15:59 : this would have been equivalent to
16:01 : writing 128 and this program would be
16:04 : equivalent but sending an interrupt is
16:08 : gonna sort of temporarily pause this
16:11 : program and then ask the operating
16:14 : system to get involved and in this case
16:16 : it's Linux and Linux is gonna come in
16:20 : and look at these the contents of these
16:23 : four register and and then determine
16:26 : what it needs to do and in this case
16:28 : because we're dealing with Linux here
16:31 : [Music]
16:34 : it's going to base and this this
16:39 : reference will actually tell you what
16:41 : registers Linux is looking into and so
16:46 : this kind of matches up it says Oh
16:50 : Lennox is going to take the number from
16:55 : the EAX register to determine which of
16:59 : these system costs to actually execute
17:02 : in this case because EAX has
17:05 : the number for in it that cross bonds to
17:10 : this floor and therefore it's gonna make
17:13 : the SIS right system call
17:15 : so Lennox is preparing to make this
17:18 : sister right call and sis right is
17:23 : actually a function call with three
17:27 : parameters and this this contract
17:32 : basically tells you in order to get well
17:35 : it's gonna have an FD it's gonna have a
17:38 : buff and it's gonna have a count and
17:42 : it's basically saying I need to get this
17:45 : information from the other three
17:47 : registers I'm gonna get that hefty
17:49 : information from EBX what's in EDX of 1
17:53 : so FD is gonna be one buff I'm gonna get
18:00 : buff from EC X that's this guy so the
18:04 : buff is gonna be one two three four five
18:08 : and then I'm gonna get the value of
18:10 : count from EDX it's 13 so count is 13 so
18:18 : basically the operating system kernel is
18:22 : making a call to assist right with these
18:25 : guys as the argument values what do they
18:30 : mean well
18:31 : FD is the file descriptor and it is like
18:36 : if you open a file you get a number a
18:40 : unique ID for that the particular opened
18:44 : file in the operating system but the
18:48 : number one for file descriptors is
18:51 : special it's a special file handle that
18:54 : stands for standard out and standard out
18:59 : is whenever you will write a command
19:00 : line program you just print something
19:03 : like console that log or printf if
19:05 : you're in C it prints the stuff to this
19:07 : special file called standard out which
19:10 : has always has the file descriptor
19:13 : number of one so we're just printing
19:16 : something out essentially buff is the
19:20 : string data that the file the operating
19:23 : system is gonna read from to get the
19:26 : string and we have one two three four
19:28 : five which is the location here of the
19:31 : memory chip that's pointing to our hello
19:33 : world message and with a new line here
19:38 : so it's gonna print that and the count
19:41 : is how many character from that from
19:46 : that location to print this is gonna
19:48 : print we've set the count to 13 that was
19:52 : set here and that just happens to cross
19:55 : bun - how many letters there are total I
19:58 : believe because this is one two three
20:02 : four five six seven eight nine ten
20:04 : eleven twelve and then thirteen is the
20:07 : new line so it will also print the new
20:09 : line yeah and so that is what completes
20:13 : the system call let's see that actually
20:18 : happened so now I'm gonna save this file
20:22 : and I'm going to compile this compile
20:29 : this program using this command but
20:34 : nessam with a dash F that means the
20:39 : format of the output it wants it to be
20:42 : in the ALF
20:43 : format which is a format that Linux uses
20:47 : for its binary and then we're gonna use
20:50 : the linker called LD to take the the
20:55 : file
20:55 : so the first step generated this hello
20:58 : world dot o so so this actually created
21:08 : the hollow world oh and then the second
21:11 : step will take this hollow or that old
21:13 : and then generate this file which is now
21:18 : here so we first start from here to
21:21 : generate this and then take this and
21:24 : generate this and now
21:29 : this hello world we can do that / hello
21:32 : world to run this program and we get
21:35 : hollow oil and then we get a Sigmund
21:37 : tation fault it's okay it's expected the
21:40 : the creator of the tutorial oh I have to
21:44 : give him credit
21:45 : let me find his name unfortunately I'm
21:50 : not able to locate the name of the
21:53 : author for Assam tutor.com if you know
21:57 : who he is or if you are that person
21:59 : please let me know and I would like to
22:02 : give him or her credit for creating
22:05 : these tutorials but anyway so this
22:09 : segmentation fault
22:11 : is expected and lesson 2 is actually
22:13 : going to teach us how to fix it and the
22:17 : way to fix it is you need to actually
22:18 : make another system call to tell the how
22:23 : the operating system to allow you to
22:26 : gracefully exit and that system Cora's
22:29 : system says exit which is system call
22:31 : number one and and then it takes one
22:36 : argument instead of three as was the
22:40 : case with sis right so says exit takes
22:42 : just one argument and it wants to be
22:45 : placed in EBX and error code in UNIX is
22:50 : has the convention of zero meaning
22:53 : success but one meaning failure and
22:56 : there may be more than one code for
22:59 : failure I don't know open in our case we
23:01 : just care to say that we returned
23:05 : successfully so now I'm gonna try to not
23:09 : even look at the tutorial and and and do
23:13 : this and try to do this correctly
23:15 : because I just have to move some values
23:18 : into EAX and EBX and then call it the
23:21 : int the interrupt command and that
23:25 : should be okay so I want to place the
23:28 : value of one into EAX and then I wanna
23:34 : place a value of zero for the error code
23:38 : to EBX and then I have to trigger this
23:44 : interrupt to tell the operating system
23:46 : to get involved so I'm gonna save this
23:50 : I'm gonna up arrow to get back the two
23:55 : compilation commands I had from before
23:58 : that nation one and then this load come
24:01 : in and now I'm gonna run my hello world
24:03 : and I get a hello world without a
24:05 : segmentation fault no let me see yes
24:10 : that's exactly what he did and now we
24:12 : have a hollower program that doesn't
24:14 : error so I'm gonna conclude this video
24:17 : here hope you enjoyed it I hope you also
24:20 : follow along and get some hands-on
24:22 : experience with this and I hope you hope
24:28 : to see you next time

Assembly Language Programing 2 - Hello World

Transcript