range of timescales

• w/ different capabilities, control-bit def., protocol to interact

• involving abstraction, encapsulation, and software layering

• each: accessed through interface: standardized set of functions

• abstraction & encapsulation!

• memory management of IO
  • buffering: storing data temporarily while being transferred
  • caching: storing parts of data in faster storage for performance
  • spooling: overlapping / queuing of one job's output as input of other jobs
    • typically: printers
• general device-driver interface
• driver for specific hardware devices

• moving byte one-by-one
• between memory & slow devices (keyboard & mouse)

• instead, use direct memory access (DMA) controller
• idea: let IO device and memory transfer data directly
  • bypass CPU!
• CPU / OS initialize DMA controller, and it's now responsible for moving data

• preparation
  • source & destination addresses
  • read || write mode
  • no. of bytes to be transferred
• execution
  • write location of command block to DMA controller
  • bus mastering of DMA controller
    • i.e. grab bus from CPA
• send interrupt to CPU for signaling completion

• core: executes instructions & registers for storing data locally
• processing core / CPU: capable of executing a general-purpose instruction set

• e.g. device-specific processors (for disk) and graphic controllers (GPU)
• those: w/ limited instruction set
  • not executing instructions from user processes

• traditionally: 2+ processors and each w/ single-core CPU
• speedup ratio: less than no. of processors
  • due to overhead - contention for shared resources
  • e.g. bus / memory

• increased throughput: more computing capability
• economy of scale: share other devices (e.g. IO devices)
• increased reliability: graceful degradation / fault tolerance

• asymmetric multiprocessing
  • master-slave manner
  • master processor: assign specific tasks to slaves
    • master: handles IO
• symmetric multiprocessing
  • each processor: performs all tasks
    • including OS functions & user processes
  • aka SMP
  • each processor: w/ own set of registers and private || local cache
    • but all processors: share physical memory via system bus
  • 

• faster intra-chip communication than inter-chip
• using significantly less power: important for mobile / laptops
• 

• as system bus can be a bottleneck

• accessed via small & fast local bus

• all CPU: share one physical memory address space

• increased latency when CPU must access remote memory across system interconnect
  • e.g. memory of another CPU?
• scheduling & memory management implication

• or: component executing instructions / registers for storing data locally

• aka batch system
• single program: can't keep CPU & I/O busy
  • thus multi-program: organize multiple jobs
  • s.t. CPU will not stay idle
• in mainframe PC: jobs being submitted remotely & queued
  • selected & ran via jab scheduling: load into the memory

• aka multitasking
• CPU: switches frequently between jobs
  • s.t. users: can interact w/ each job while running
  • enables: interactive computing
• characteristics
  • response time: $<1s$
  • each user: w/ at least one program executing in memory (= process)
  • if several jobs ready to run: CPU scheduling needed
  • if: processes don't fit in memory
    • swapping technique required
    • moving some program in and out of memory during execution
  • virtual memory: allows execution of processes that are not completely in memory

• into multiple different execution environments
  • creating illusion: each user / program is running on their "private computer"
  • creates: virtual system = virtual machine (VM) to run OS / applications over
    • allows: an OS to run as an app within another OS
      • 👨‍🏫 growing industry!

• host: underlying hardware system
• virtual machine manager (VMM): creates & runs VM by providing interface
  • s.t. it's identical to the host
  • aka hypervisor
• guest: process provided w/ virtual copy of host
  • i.e. program. often: an OS (guest OS)

• each on their own VM

• originally: designed in IBM mainframes (1972)
  • for: allowing multiple users to run tasks concurrently
    • under: system designed for a single use
  • or: sharing batch-oriented system
• VMware: runs one or more guest copies of Windows on Intel x86 CPU
• virtual machine manager (VMM): provides environment for programs
  • that is: essentially identical to original machine (interface)
• programs on such environment: only experience minor performance decrease (due to extra layers)
• VMM: in complete control of system resources

• technology: treated by $Xen,VMware$, which are still relevant today
• virtualization: expended to many OSes, CPUs, VMMs