CS208 Introduction to Computer Systems
Friday, 6 March 2026

+ Questions

+ Paged virtual memory (textbook section 13.3)
- Why would we want every process to believe it has 2^64 bytes (16 exabytes) of memory?
- How does the computer deal with that?
- Components of VM
    - pages (typically 2^12 = 4096 bytes)
        address = page number || page offset
    - mapping via some kind of table
        - whoa, how big are those tables?
        - where do we put the tables? in memory? but wait...
    - MMU = memory management unit

+ Memory hierarchies
- pull a register into CPU for addition or subtraction or something
    1 clock cycle
    my Mac ~ 4.5GHz
    time = 1 / 4.5 x 10^9 ~ 2.2 x 10^-10 seconds
- read a byte from RAM
    typically 100 clock cycles ~ 2 x 10^-8 seconds
- read a byte from SSD
    typically .1 - 1 ms ~ 10^-4 - 10^-3 seconds
- roughly 1 (reg) : 100 (ram) : 1,000,000 (ssd)
    - yikes--let's never read from a file again

To avoid that SSD penalty
    - caches (scratch space where we keep copies of data
        we have used before)
    -