Computer Organization and Architecture

• Protocol = Signals exchanged between a sender and a receiver. Includes command signals ('Printer reset'), status signals ('Tape ready') or data-passing signals ('Here are the bytes you requested').
• Handshake = The acknowledgement the receiver must give to the commands and data sent to it or indicate that it's ready to receive data.
• Durable Storage = Storage medium that doesn't rely on the continued supply of electric currents to retain data. (EX. Magnetic disks/tape and Optical Disks)

I/O Control Methods

• differences in control methods and transmissions modes make is hard to connect I/O devices to a system bus
• I/O modules are used as interfaces between the CPU and peripherals to manage these differences
• modules perform functions such as controlling device action, buffering data, performing error detection, and communication with the CPU.

Programmed I/O

• The simplest way for a CPU to communicate with an I/O device is through programmed I/O (also called: polled I/O or Port I/O)
• CPU monitors a control register associated with each I/O port and once the 'data ready' control bit is set:

• resets the control bit.
• retrieves the byte and processes it according to instructions programmed for that particular port.
• After processing is complete, the CPU resumes polling the control registers

• Benefit: there is control over the behavior of each device (you can adjust the number and types of devices in the system, their polling priority, and intervals)
• Cons: CPU is constantly polling in the 'busy wait' loop before servicing an I/O to process

• Polling frequency: Some CPUs poll faster than others

• Because of this, Programmed I/O is best used for specialized systems (such as teller machines/embedded systems that control or monitor environmental events

Interrupt Driven I/O

• Most common and efficient method
• Opposite of programmed I/O, device tells the CPU when they want data to send. The CPU proceeds with other tasks in the system until a device requests a service, which interrupts the CPU to complete the task.
• communication mostly takes place through an intermediary interrupt controller
• Similar to programmed I/O (can both modify service routines in order to accommodate hardware changes)

Memory Mapped I/O

• I/O devices and main memory share the same address space.
• Each I/O device has is own block of memory, data is transferred to and from the memory address mapped to the device
• Simplifies system design, making it easier to navigate and use
• Best used on smaller systems

Direct Memory Access (DMA)

• CPU offloads the execution of time-consuming I/O instructions.
• CPU provides the DMA controller with the location and number of the bytes to be transferred and the destination device or memory address
• This process usually takes place through special I/O registers on the CPU,
• block oriented; interrupting the CPU only after completion (success or failure) of transferring a group of bytes.

Channel I/O

• used in mainframe computers (becoming more popular on file servers and storage networks)
• One or more I/O processors control various I/O pathways called channel paths
• channel paths are categorized (EX. 'slow devices' like printers/terminals) are combined (multiplexed), allowing for management of several of the devices through only one controller
• I/O channels are driven by small CPUs called Processors (IOPs), optimized for I/O. Unlike DMA circuits, IOPs have the ability to execute programs that include arithmetic-logic and branching instructions
• IOPs execute programs (called Channel Command Words, 'CCW') that are placed in main system memory by the host processor
• Once the I/O program has been placed into memory, the host issues a start sub channel (SSCH) command, which informs the IOP of the location in memory where the program be found.
• After the IOP is done, completion information is put onto memory which interrupts the CPU
• Channel I/O is a type of isolated I/O.

Character I/O vs. Block I/O

• Each key in a keyboard controls a small switch that closes a connection in the matrix of conductors that run horizontally and vertically beneath the keys. When a key is pressed, a scan code is processed by the keyboard circuitry. Then translates the code to a character

• Magnetic disks and tapes store data in blocks; managed in block I/O units and lends itself to DMA or channel I/O processing.

• Blocks can be different sizes (depends on hardware/software/applications)

• Determining the right block size is important in the system to make sure the system is able to handle all information processed.

Serial and Parallel Transmission

Serial Transmission

• Data-bit goes between two computers (bi-direction)
• long distance
• One clock pulse = One bit transferred
• simple circuitry
• cost efficient

Parallel Transmission

• bits flow together simultaneously between two computers
• faster than serial transmission
• short distance
• eight bits transferred at a time (contain a start and stop bit)
• complex circuitry