IO

Protocol Variants

Mist devices have a status register (if it's doing a transfer.

status register
command register
data register

Usually interrupts will be more efficient

PIO vs. DMA

Programmed IO - PIO

For every word or byte of data that gets transferred the operating system has to put in an instruction for each. Keyboards will usually used PIO since not much data is necessary to transfer.

DMA

Large sizes of data being transferred. For large magnetic disk drives DMA is always used. A block is typically 512 bytes. An interrupt will be raised to let the operating system know that the operation was completed.

Special Instructions vs Memory Mapped IO

Special Instructions

There are instructions in the instruction set architecture (ISA). There are some ISA that have special set of instructions for IO.

Memory Mapped IO

All devices on the system are mapped to memory in the proccesor's address space. Thus special IO instructions are not needed. Thus X86 instructions for moving can be used to move data from a register to a devices register.

It doesn't really matter which is done in a given architecture. They both can work equally well.

Device Driver

Suffices to create an abstraction for the variety of devices and allow data to be sent and received by the device without the operation system adding extra code. 70% of an OS is device drivers.

Basic Interface

Magnetic drives have moving parts. One or more platter inside the drive, that are covered with magnetic films. A spindle is in the middle and the platter rotates around the spindle. Each surface is divided into rings called tracks. The tracks are divided into numbered sectors numbered from the inner most track to the outer most track.

Positioning

The platters are not perfectly align or perfectly concentric. It is difficult to put the head over the track.

Time To Read

Seek Time

Time it takes to position the head over the track. It has to be accelerated nad allowed to settle. Takes typically 4-10 ms.

Rotation Delay

depends on how fast the drive spins
typically 7200 rpm ➡︎ 0.83 milliseconds for a rotation

Transfer Time

quite fast (faster of the three delays)
depends on how fast the platter is rotating
100 MB per second is the maximum
5ms to transfer data on overage

Skew

When consecutive sectors are being read but the sectors are on different tracks there would be a delay even though they are next to each other. Thus the tracks are skewed or rotated as you go out from the middle

Zones

The data stored into each sector has a higher capacity as you moved towards outside tracks. Thus they are separated into zones where outside tracks are split into more sectors. This ill allow more data to be store on the disk with better performance.

Cache

Both average read and write time gets faster
For reads the disk will read the track into cache and
For writes the drive won't immediately write the data but save it in cache

I/O Schedulers

FCFS (First Come First Serve)

For each request we get the whole latency
If the requests are ordered so the sections are hit back to back then the latency to move the head to the next section is removed
Performance: depends on how the requests come in

SPTF (Shortest Positioning Time First)

Do the transfer that's closest to the head's current position.
Can be implemented in the disk
The OS can't do it so if the operating system tries to it will become SST (Shortest Seek Time) instead.

SSTF (Shortest Seek Time First)

Closer sections get serviced first
However starvation is an issue with this.

SCAN (Elevator Algorithm)

Has one direction of movement and will service requests in that direction until it hits the inner or outer radius. Then it will reverse directions
Eliminates starvation but fairness is an issue for things on the edges that will only get called once while things in the middle will get called twice.

Circular Scan

Instead of reversing direction go back to the first section so fairness is distributed

LOOK

Works just like SCAN but as the head moves in one direction it will reverse direction if there is no other requests in that direction.

IO