course CS Primer computer science

Computer Systems

• Also called computer architecture
  • How the machine expects data to be encoded
• Understanding it enables us to write faster, more more-efficient/secure code
• Recommended textbook: Computer Systems: A Programmer’s Perspective

Topics

• Bits and bytes
• Intro to C
• Low-level programming
• CPU microarchitecture and low-level performance
• The memory hierarchy (CPU caches)

Bits and bytes

• Develop skills for working with binary encodings of data
  • Useful for networking, databases, etc.
• Key concepts:
  • Fixed width integer encodings and byte ordering
  • Signed integer encodings
  • IEEE-754 floating point scheme
  • Unicode, UTF-8, and other encodings

Protobuf varint

• Implement Base 128 Varint encoding using Protocol Buffers
  • Base 128 Varints
• Can we come up with a variable length encoding to save space?

Explainers

What does it mean for something to "be" a certain number of bits?

• We have a fixed amount of bits to store data
  • Memory is allocated for a value
• The amount of memory limits determines can be stored there
  • eg. 4 bits can only store 0000 --> 1111
  • Adding 1 to 1111 results in 0000 (overflow)
• The "width" is how many bits something takes up
• Python solves overflows in software

What is the effect of shifting bits?

• Shifting to the left is basically multiplying by a power of 2
  • Eg. 1 << 3 = 1 * 2^3 = 8
  • 6 << 1 = 12
    • 0110 (6) --> 1100 (12)
• Shifting to the right is basically dividing by a power of 2
• When you bit shift left or right, you truncate

Why does a byte have values 0-255?

• A byte has 8 places (each can have a 0 or a 1)
• 11111111 --> 255

Why do I sometimes see the sequence \r\n for a newline?

• It's a carriage return plus a line feed
• The carriage return (\r) would originally return the carriage of a typewriter to the start of the current line
  • Need \n to get to the next line
• Many modern systems treat \n with an implicit carriage return