A First Assembly Program

Objectives

We would like to :

Code, assemble, and execute a first program in x86 assembly
Make sure all students present understand how to use the tools/systems required to perform this task.

Notes

This is all me.
This is to understand the process
- And a little assembly language.
- We will review the technical details of the code later in much more detail
What you should already know/learn quickly
- How to log into prog1 or devweb1
  - If you don't know this, ask for help
- How to edit/transfer files on these machines
  - Probably visual studio code
  - I will use vi
  - I will use sftp/winscp/wget
- How to use a command line on these machines
  - Again, vsc will do this
  - I will use putty
- Basic *nix usage
  - Use my actions in class for an example
    - rm, cp, mv
    - mkdir, cd
    - ls, pwd
    - more/less cat
    - io redirection, pipes
  - We will discuss a few others at least
    - nasm, ld, g++
    - gdb
    - make
  - You will probably want to use make
    - It is the right tool for the job.
    - I don't care
  - Ask about things you do not know.
Log into prog1 or devweb1 (I prefer prog1)
I would make an assembly directiry and a prog1 directory inside of that.
- mkdir assembly
- cd assembly
- pwd
- mkdir program1
- cd program1
- pwd
- the pwd is, of course, optional.
Next download the starting files
- These are optional
- And we might change/remove some of these in the future.
- But for now, PLEASE JUST USE THEM.
- I will use wget
  - This is ideal for a remote shell accessing files on the web (web not generic internet)
  - Given a URL, this will fetch the file pointed to.
  - Apparently this is linux/unix
  - But curl is available on windows.
  - Or just right click and save in the browser.
- wget https://mirkwood.cs.edinboro.edu/~bennett/class/cmsc3100/spring2026/notes/intro/code/Makefile
  - Note, right click in a putty terminal pastes.
  - I have no idea how to paste in VSC, but I bet it is right click paste
  - Don't copy these from the notes, type wget and copy and paste the first url from the source code page.
  - Then use the up arrow and modify the command line for the followign:
- wget https://mirkwood.cs.edinboro.edu/~bennett/class/cmsc3100/spring2026/notes/intro/code/ioCalls.asm
- wget https://mirkwood.cs.edinboro.edu/~bennett/class/cmsc3100/spring2026/notes/intro/code/shell.asm
- wget https://mirkwood.cs.edinboro.edu/~bennett/class/cmsc3100/spring2026/notes/intro/code/CONSTANTS.h
- an ls is probably useful to see if you got the files.
We want to make a copy of shell.asm
- shell.asm is just a starting point to reduce typing.
- We want to write Hello World, so let's copy this to hello.asm
- cp shell.asm hello.asm
- You could also do mv shell.asm hello.asm, what is the difference?
Edit hello.asm
- Add the following lines to the data section:
- ```
 section .data
    GREETING: db 'Hello World!',0
    GREETING_FORMAT: db `%s\n`,0
```
- Note, the second line needs the back quotes ` (upper left hand corner of most keyboards), ' and " will not work.
- Add the following lines to main
- ```
main:
    ;mov rdi, format1
    ;mov rsi, -1
    ;call CallPrintf

    mov rdi, GREETING_FORMAT
    mov rsi, GREETING
    call CallPrintf
```
- Again, we will discuss this in much more detail later.
But for your homework
- db means a collection of bytes.
- `` is used for strings with escaped characters (\n)
- "" and '' works for code without escaped characters.
- Strings must end with the null terminator (0)
We need to modify the Makefile to assemble and link this new program.
- In your editor, change every reference to helloWorld to be hello
Assembly and link your program
- make
Run your program
- ./hello