What is an OS?

• Originally, hardware is expensive, people are cheap
  • Goal: maximize hardware usage
• Now, we have cheap hardware, people are expensive
  • Optimize for making things easier to use for people
  • Provides a common way of doing things / interface

An OS is a standardized abstraction: a VM implemented on the underlying machine

What do you need in a computer?

• One or more CPUs
• memory
• I/O modules
• timer(s)
• interrupt controller
• system bus connecting them

Registers

User-visible registers

• Enable programmer to minimize references to memory
• available to all programs
• depends on computer architecture
• maybe be referenced by machine language
• Usually controlled by the compiler
• Two types:
  • Data registers: stores data (results from calculations, etc)
  • Address registers: points to memory
    • Indexed addressing (Add an index to a base value to get an effective address)
    • Segmented pointer (when memory is divided into segments, memory is referenced with a segment + offset)
    • Stack pointer

Control and status registers

• Invisible to the user on most architectures
• used by processor to control operating of computer
• Used by privileged OS routines to control execution

Condition codes and Flags

• Part of Program Status Word (PSW)
• Bits set by processor hardware as a result of operations
• e.g. positive, negative, zero, overflow as results of arithmetic
• Conditional branching uses this
• Only implicitly accessible on most architectures

Instruction execution

Simple:

1. Processor fetches from memory
2. Processor executes
3. Goto 1

Reality:

• Pipeline
• Very Long Instruction Word
• superscalar architectures

Instruction Register

• Fetched instruction put in IR
• action category dictated by instruction

Interrupts

• Most IO devices are slower than processor by a LOT
  • Processor must pause to wait
• Interrupts help improve processor utilization
• Changes normal processor sequence
• types:
  • program (condition that occurs as a result of an instruction execution, e.g. division by zero)
  • timer (from timer in processor)
  • IO (from IO controller)
  • hardware failure (e.g. power failure)

New processor flow

• Processor checks for interrupts
• If interrupt:
  • suspend program execution (store snapshot)
  • execute interrupt handler routine
  • resume execution (restore snapshot)

This is a simplified version, does not account for nested interrupts, interrupt priorities, counting interrupts, etc

Direct Memory Access (DMA)

• allows hardware subsystems to access RAM without going through the CPU
• transfers block of memory directly to device

Memory Hierarchy

Traversing heirarchy:

• Decreasing cost per bit
• increasing capacity
• increasing access time
• decreasing frequency of access

Cache memory

• typically invisible to OS
• Processor accesses memory at least once per instruction cycle
• Executing is limited by memory cycle time
  • hint: figure out what flash wait states are for your project MCU
• Use locality for small, fast memory
  • memory accesses close in time typically are close in space
• Caches contain a copy of a portion of main memory
• Processor first checks cache, and if not found, block is read into cache
• Has a replacement algorithm (e.g. least recently used, least frequently used, etc)
• Has a write policy (e.g. write back, write through)

Symmetric Multiprocessing (SMP)

• Processors share:
  • same main memory
  • access to IO
• connected by bus
• integrated operating system provides interaction between processors and their programs