CS208 Introduction to Computer Systems
Wednesday, 22 October 2025

+ Assembly questions
- "I don't get what lea is for"
- "What's going on with a(b,c,d)?"
- "This whole thing is making my head hurt"
- ...?...

- Why?
    - Making a device that adds
        computer architecture
        electrical engineering
        digital logic
    - Making a device that does programmable computations
        computer architecture: datapath
        the instructions for these programs have to be
            arcane, fiddly combos of 1's and 0's
        these instructions, which physically drive the datapath
            to execute its program, are called "machine language"
        machine language is hard to work with
    - So, let's make synonyms of machine language instructions
        to make it easier for people to think about programs
        e.g., mov, add, jmp, etc. -- i.e., assembly language
        also need to write "assembler" -- i.e., a program that
            translates from assembly language to machine language
    - Later, since assembly language is also hard to think about,
        people develop higher-level languages. (1950s FORTRAN,
        COBOL, LISP, Algol,...). When you do this, you have to
        also write a compiler or an interpreter.
- Why us and assembly?
    - How does it work? -- important for experts to know
    - Security -- buffer overflow, in particular
    - More understanding of layers of abstraction
    - It's fun

- Flags: the EFLAGS register
    EFLAGS is 64 bits, and each bit represents some boolean memory
    
        addl $4, %eax

    Suppose this instruction results in a negative number. Then, it
    sets a bit in EFLAGS to 1 to indicate negativity

- LEA, LEAL, LEAQ, etc. "load effective address" (ugh.)
    better "do a little arithmetic"

    (1) compute a number
    (2) put it somewhere


+ The system stack

+ Using gdb in assembly language

+ The next homework