CS208 Introduction to Computer Systems
Wednesday, 16 October 2024

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TO FIND THIS DOCUMENT:
- search online for "Jeff Ondich"
- go to my homepage
- click on the link for this course (CS208 Bridger's section)
- click on Class Notes
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+ Today
- if/else and loops: rip, jumps, cmp, test
- calling functions: the system stack, rsp, push, pop, call, ret
    (and what the heck is rbp?)
- your questions about the puzzles

+ Recap
- Architecture: Registers + Memory
- Sizes and names of registers
    rax (64 bits), eax (rightmost 32 bits of rax), ax (rightmost 16), al (rightmost 8)
- Conventional register uses
    - rax/eax -- return value
    - rdi/edi -- first parameter of a function
    - rsi/esi -- second parameter of a function
    - [see Tanya's diagram]
- Simple instructions
    - mov, add, sub
    - suffixes q (64 bits), l (32), w (16), b (8)
- The weird lea instruction
    - lea (%rax, %rbx), %rcx      -- rcx = rax + rbx
    - lea (%rax, %rbx, 4), %rcx   -- rcx = rax + 4*rbx
    - lea 5(%rax, %rbx), %rcx     -- rcx = rax + rbx + 5
    - lea 5(%rax, %rbx, 4), %rcx  -- rcx = rax + 4*rbx + 5

+ Playing with Compiler Explorer
- Settings (-Og flag; *not* Intel syntax; C, not C++)

- Write a function int f(int a, int b) that does:
    return a + b
  What does the corresponding assembly look like?

- Try some more
    return a * b
    return 5 * a + 9 * b - 50
    try other formulas

- Some of the instructions you might see
    - mov
    - add
    - leal
    - sub
    - ret
    - ...

+ History: Intel instruction set

    1971: 4004

    1970s-1980s: 8008, 8080, 8086
        8-bit registers
        registers named a, b, c, etc.

    1975: AMD cloned the 8080

    1982: 80186, 80286
        16-bit registers
        registers named ax, bx, cx, etc. (x = "extended")

    1985: 80386
        32-bit registers
        registers named eax, ebx, ecx, etc. (e = "extended", x = "extended")
        this language is called "x86"

    1991: AMD vs. Intel monopoly lawsuit

    1999: x86-64 (also known as x64)
        64-bit registers: rax, rbx, etc. (r = register)


+ Instruction-name weirdness

    movq $3, %rax  -- the q and the rax both say "64 bits"
    movl $3, %eax  -- the l and the eax both say "32 bits"
    movw $3, %ax   -- the w and the ax both say "16 bits"
    movb $3, %al   -- the b and the al both say "8 bits"