I/O and Secondary Storage

Posted on Jun 8, 2022

I/O

Programmed I/O: CPU does all the work. 2 forms:
- Memory mapped I/O: I/O devices treated like memory, and sending / receiving data between them uses the same instructions as writing / reading to memory. Requires reserving memory space for I/O devices.
- Isolated I/O: Bus has seperate I/O read / write lines. Instructions specify whether an address is for I/O or main memory.

Waiting for I/O devices to be ready takes a long time.

Polled I/O: Simply check if the device is ready every now and again.
Interrupt driven I/O: When a device is ready, it sends an interrupt to the CPU, which then services the device.
- Non-maskable Interrupts (NMIs) must be serviced by the CPU, whereas normal IRQs can be masked out.

Direct Memory Access (DMA): Used for when I/O devices need to quickly transfer a lot of data. Gives the I/O device direct access to memory, bypassing the CPU. Uses a DMA Controller, located on the bus.
Single bus, detatched DMA: All modules share a bus, DMA mimicks the CPU to exchange data between I/O and main memory.
Single bus, intergrated DMA: I/O devices connected via a DMA to the bus.
I/O bus: Seperate I/O bus is connected to the main bus via the DMA.

Operation modes:

Cycle stealing: Uses the system bus in cycles where the CPU isn’t using it.
Burst mode: Locks the CPU out of the bus for a short amount of time, where DMA has full access.

RAID 0: Non redundant striping. Data striped across disks for better read and write performance. Any disk failure will result in data loss (no redundancy).
RAID 1: Mirrored. 2 copies of data on 2 separate disks. All data gets written twice. Slight improvement in read speed - allows it to read from either disk.
RAID 10: Mirrored striped: Have 2 striped systems mirroring eachother.
RAID 2: Redundancy through hamming codes. Redundant disk stores hamming codes for all words, which allows for single bit correction. Pretty overkill. Redundant disks proportional to log of number of data disks.
RAID 3: Bit-interleaved parity: Parity bit calculated for all bits in the same position on each data disk - so needs 1 parity disk only. Easy to implement, good performance.

For RAID 4-6, I/O requests can be satisfied in parallel, since each member disk operates independently.

RAID 4: Block-interleaved parity. Bit-by-bit parity strip calculated across corresponding strips on each disk. Parity bits stored in corresponding strips in parity disk.
RAID 5: Block-interleaved distributed parity. Instead of having dedicated parity disk, parity is uniformly distributed over all disks, eliminating bottleneck.
RAID 6: Dual redundancy. 2 disk fault tolerance - extends RAID 5 to have 2 parity stripes.

Infrastructure as as Service (IaaS): Cloud provider offers machines and other infrastructure, eg. Azure.
Platform as a Service (PaaS): Cloud providers makign a platform available, consisting of eg. an operating system, programming language execution support, a database system, and a web server.
Software as a Service (SaaS): Allowing users to access software via a client eg. Office 365, Google apps.
Network as a Service (NaaS): Providers allowing their infrastructure to be used as a network connectivity layer.