Plan for Today

Reminder from last time

What happens when we press run?

Reminder: Machine Code

Architecture refers to the instructions sent by software that hardware can understand.

• Reminder: computer runs code instructions
• Each line of machine code is one operation
• Javascript is higher-level language
  • Have to translate Javascript to machine code
• Computer runs the machine code (binary -- more on this today)
• Machine-specific

Reminder: Assembly

mov    %esp,%ebp
sub    $0x28,%esp
mov    0x804d300,%eax
add    $0x1,%eax
mov    %eax,0x804d300
mov    0x804d300,%eax
cmp    0x8(%ebp),%eax

• Assembly is converted to machine code
• %esp, %ebp, etc. are in temporary storage

Programs and Applications

• Millions of lines of machine code per program
  • Firefox.exe is about 20 million machine code instructions
• All the machine code is in the .exe (Windows) or .app (Mac) file

Machine code

• What is this machine code we keep speaking of?
• Humans write programs that are human-readable. A machine needs code that is machine-readable (aka binary)
• Our code, like Javascript, is compiled and interpreted to binary (1s and 0s) instructions that machines can understand
• Something in the computer must take and understand these instructions...what component is it?

CPU

• "Brain" of a computer
• Receives input data and performs the binary instructions given to it
• Communicates with I/O (input-output) devices, such as:
  • Keyboard
  • Mouse
• Contains registers that house information, and performs operations (i.e. assembly instructions) on values in these registers

Transistors

• Companies thrive off selling CPUs, which consist of billions of tiny transistors on a computer chip
• Transistors are tiny switches that are triggered by electrical signals. Like a switch, but instead of connecting or disconnecting, transistors conduct or insulate.
• Transistors make arithmetic calculations on the chip possible
• These days, companies are trying to make them smaller and smaller (more profit)
• Moore's law: the number of transistors on a chip will double every two years -- has been true from 1965 to 2015
• Popular CPU makers: Intel, AMD, Apple, NVIDIA, Qualcomm
• Is having a fast CPU enough to have a fast computer?

Memory

• CPUs must get their instructions and data from somewhere
• Memory: computer hardware components that store data
• Two types of memory:
  • RAM (random access memory)
  • Hard drive (also known as disk)
• Any program you are currently running (in foreground or background) has memory stored in RAM. CPU gets data directly from RAM.

RAM

• Anything you are running is stored in RAM
• Very easily accessible (can find something in RAM quickly)
• When computer is turned off (or power cut), everything in RAM is wiped
• (This is why you should save documents before shutting down!)

Hard Drive

• Slow to access memory in hard drive
• When you save a file, it is saved to "disk" or stored in the hard drive
• Typically lots of storage (256GB to 1TB) compared to RAM (4 GB to 16 GB)

What about "flash storage?"

• Flash storage is high-speed memory that doesn't require power like RAM (also known as SSD, or solid state drive)
• Fast compared to traditional hard drives, which rely on spinning disks, motors, and magnetism
• Drawbacks of flash storage: very pricey, rewrite limitations

Circuits

• Basically, hardware is circuits
• Circuits are composed many digital logic gates
• Digital logic gates can take in multiple inputs and redirect to one output
• Logic gates are made up of transistors

AND Gate

• Takes in 2 inputs, each either 0 or 1
• Outputs 1 only if both inputs are 1

OR Gate

• Takes in 2 inputs, each either 0 or 1
• Outputs 1 only if at least input is 1

XOR Gate

• Takes in 2 inputs, each either 0 or 1
• Outputs 1 only if both inputs are 1 OR both inputs are 0
• Stands for "exclusive or"

NOT Gate

• Takes in 1 input, each either 0 or 1
• Outputs the "flipped" input
• ~1 is 0; ~0 is 1

The Operating System: Full Picture

• Name types of operating systems?
• Starts running when the computer "boots up"
• Manager/supervisor
  • Starting/stopping programs
  • Manages RAM, persistent storage, and other shared resources between programs
• Can run multiple programs at once ("sandboxing")
• Allows a computer to change over time (updates)
• Other responsibilities:
  • File system
  • Manages windows
  • Some basic programs

OS: Starting a Program: Full Picture

• Load the program file from the file system (persistent storage)
• Each program gets its own RAM
  • Program's machine code
  • Data the program manipulates (like file being edited)
• Copy machine code from persistent storage to RAM

OS: Running programs: Full Picture

• CPU "fetch/execute cycle"
  • Fetch one instruction
  • Execute (run) that instruction
  • Repeat
• Machine code is loaded into RAM
• CPU begins at the top
  • Instructions like "return to step 5" keep the program running

OS: Program runs and exits normally: Full Picture

• Operating system starts and stops programs
• Each program has its own separate area in RAM: its instructions + data
• CPU "round robin"
• Persistent storage is organized as a file system, programs can read and write data here
• User quits the program
• Operating system reclaims shared resources

OS: Abnormal Exit: Full Picture

• Program "not responding"
• The operating system stops running that program - involuntary vs. normal-exit
• The operating system reclaims the program's area of RAM
• Use Activity Monitor (Mac)/Task Manager (Windows)

OS: Running Out Of Memory: Full Picture

• A program requests more RAM from the operating system
  e.g. to hold an image, but there's not enough RAM available
• The operating system refuses the request, the program gives an error message

OS: Memory Access Error: Full Picture

• A program tries to access the memory of another program
  • Maybe because of a bug (common)
  • Maybe on purpose because it is malware
• The operating system blocks the access (ideally)
• Maybe kills the offending program too

OS: Reboot: Full Picture

• Why does this fix anything?
• Sometimes some operating-system managed bytes in RAM is not quite right
  • a bug in the operating system, or perhaps a hardware error
• A reboot wipes all the data from RAM
• Starts up the operating system fresh
• This should never be necessary

Conclusion