7 IO & Summary

Input/Output Devices

Bus: Common set of wires for communication among hardware devices plus protocols for carrying out data transfer transactions

• Operations: e.g., Read, Write
• Control lines, Address lines, Data lines

Two types of Disks:

• Magnetic Drives: Information stored by magnetizing ferrite material on surface of rotating disk
• Flash Drives(Solid State Drives and Thumb Drives): Information stored by trapping charge in a semiconductor and MOSFET based dualgate transistor

HDD

Actuator moves head (end of arm) over track ("seek"), wait for sector to rotate under head, then read or write

Disk Access Time = Seek Time + Rotation Time + Transfer Time + Controller Overhead

• Seek Time: time to position head to correct track
  • Average Seek Time = 1/3 time for the head to cross all tracks
• Rotation Time: time for disk to rotate to proper sector
  • Average rotation time = 1/2 time of a rotation
• Transfer Time: time for data to rotate under the head

HDD Scheduling

• FIFO
  • Pro: Fair among requesters
  • Con: order of arrival may be to random spots on the disk(Very long seeks)
• SSTF(Shortest seek time first): Pick the request that’s closest to head
  • Pro: Reduce seek time, throughput increases
  • Con: may lead to starvation
• SCAN(aka elevator): Take the closest request in a particular direction. When reaches the end, it reverses direction and follow the closest request.
  • Pro: no starvation
  • Con: Long waiting time for locations just visited by disk arm
• C(Circular)-SCAN: only goes in one direction, skips any requests on the way back
  • Pro: A bit more fair to inner and outer tracks

SSD

Pros (vs. hard disk drives):

• Low latency, high throughput (eliminate seek/rotational delay)
• No moving parts: Very light weight, low power, silent, very shock insensitive
• Read at memory speeds (limited by controller and I/O bus)

Cons

• expensive (3-20x disk)
• Limited drive lifetime

IO Communicating

Processor accesses registers in two ways:

• Port-Mapped I/O: in/out instructions
• Memory-mapped I/O: load/store instructions

Here are 3 types of Memory-mapped I/O:

Polling

Path to a device generally has 2 registers:

• Control Register says it’s OK to read/write (I/O ready)
• Data Register contains data

Polling:

(1)Processor reads from control register in a loop, waiting for device to set Ready bit (0 → 1)

(2)Processor then loads from (input) or writes to (output) data register, resets Ready bit of control register (1 → 0)

Pro: low overhead

Con: may waste many cycles on polling if infrequent or unpredictable I/O operations

Interrupt

Device generates an interrupt whenever it needs service

• Pro: handles unpredictable events well, asynchronous to current program
• Con: interrupts relatively high overhead

Polling vs. Interrupts

Low data rate (e.g. mouse, keyboard)

• Use interrupts to avoid "waiting" for data
• Overhead of interrupts ends up being low

High data rate (e.g. network, disk)

• Start with interrupts
• Once you start getting data, switch to polling. Keep grabbing data until empty

DMA

Directed Memory Access:

• Device controller transfers data directly to/from memory without involving the processor
• Only interrupts once per page once transfer is done

Mechanism

1. For incoming data, CPU receive interrupt from device; For outgoing data, CPU need to confirm that external device is ready
2. CPU instructs DMA engine/device that data is available @ certain address
3. Device/DMA engine handle the transfer, CPU is free to execute other thing
4. Upon completion, Device/DMA engine interrupt the CPU again

Performance

Efficiency

• CPI: Clock cycles per Instruction
• MIPS: Millions of Instructions per Second

cpu time = Instructions CPI Clock cycle time

= Instructions * CPI / Clock rate

= Instructions / MIPS*10^6

Power and Energy

Total power = dynamic power + leakage power

Dynamic power $\propto$ activity $\times$ capaictance $\times$ voltage$^2$ $\times$ frequency]

Leakage power $\propto$ voltage

Energy = power $\times$ time

Five Great Ideas in Computer Architecture