CS208, Monday 7 Mar 2022

One-level page table structure

• Pretend addresses are 64 bits
• Pretend pages are 64KB (2^16 bytes)
• Pretend there's 16GB (2^34 bytes) of physical memory
• Goal: translate virtual page numbers to physical page numbers
• Q: How many virtual pages are there?
• Q: How many bits are in a virtual page number?
• Q: How many physical pages are there?
• Suppose a process's page table is a contiguous array of 72-bit entries (64 for the physical page number, and 8 for miscellaneous flags). How much space does the page table require?

Two-level page table structure

• Pretend all of the above
• Pretend there's a top-level page table with 4K entries (2^12 = 4096). Each of those entries points to a second-level page table.
• Q: How many second-level page tables are there?
• Q: How many entries does each second-level page table have?
• Q: How much virtual address space does each second-level page table represent?
• Q: Assuming each second-level page table's entries are 72 bits long, how much space does each second-level page table take up?
• Q: Given the address ADDR = 0x5678FEDC1234ABCD
  • what virtual page number is this address on?
  • what index in the top-level page table does ADDR refer to?
  • what index in the second-level page table does ADDR refer to?

Revisiting the layers of abstraction

• Where have we spent our time?
• What would be next?
  • Deeper into the OS
  • Digital logic
  • Hardware

Operating system notes

• Where does the code live?
• Interrupts and interrupt handlers
• Kernel mode
• Scheduler
• System call table

Wednesday & Friday

• A quick spin through digital logic and CPU architecture