CDA-4101 Lecture 6 Notes

Final topic under computer organization We've talked about processing data and storing data Now we discuss how the data gets in and out of the processor and memories. short on details, but it is important to have a general idea of how these devices work

Buses

• buses connect the processor to memory, disk drives and other system components.
• All this is part of the motherboard
• most machines have multiple buses
  • legacy buses to support older peripherals (e.g., ISA)
  • special purpose buses (e.g., AGP)
  • dedicated memory buses for extra speed
  • "bridges" between buses

I/O Device has the controller (special purpose processor) and the actual hardware component itself. Its jobs: accessing the bus (dataa and control signals) operate the hardware (service requests) At the lowest level it needs to put bits on the wire and read bits from the wire (i.e., bus). The controller is either: on the device itself a card plugged into the motherboard where device is plugged into the controller card built into the motherboard

• The CPU commands the devices.
• Since devices are so much slower than memory, you do not want the CPU waiting around for the I/O devices to do their job.
  • DMA
  • Interrupts

• instead of the processor being involved with the transfer of every byte from the device to memory, the device can access memory itself.
• the CPU only needs to issue the read/write command specifying the amount of data to transfer and it can turn its attention to processing other data.

the CPU wants to know when an I/o device is finished. it should not have to wait for the I/O device to read the data since it is wasting a lot of CPU cycles it also would be a waste to even periodically "poll" the device to see if it is done. interrupts are implemented in hardware as a way to interrupt the processor to make it temporarily handle an event (e.g., I/O complete or data is ready) IRQ settings in PC world

• an interrupt handler is a short program whose job is to do the necessary tasks require when an interrupt occurs
• typically, there are different kinds of interrupts with a separate handler for each.
• handler should take a minimum amount of time and then return control to the place that the CPU was interrupted

as discussed, there are many devices sitting on the same bus the bus can only carry one piece of data at a time need to make sure devices do not conflict in their use of the bus a bus arbiter is a chip that sits on the bus and acts as the judge about who gets to use the bus I/O devices givcen preferrences since they are more timing dependent the CPU can operate while the bus is being used by devices, but it it needs the bus it may waste cycles waiting for the I/O device to finish using it

Bus Evolution

• with a bus arbiter and multiple devices sharing the bus, you need a "standard" that defines how the bus should be controlled and accessed.
• need to know how to work with the bus arbiter and trasfer the data to other devices on the bus
• Each time a bus standard is defined, the advances in technology make it obsolete and a faster, better bus protocol is designed.
• most buses are synchronous (i.e., clocked) and increasing the clock speed of the bus has helped to increase the bandwidth)
• On the PC
  • ISA (industry standard architecture) (original IBM)
  • EISA (extended ISA)
  • PCI (peripheral component interconnect)
  • AGP (Accelerated Graphics Port) video needed speed (~4X PCI)

Digital Cameras Keyboards Mice Modems Modems Printers Punch Card Readers Scanners Speakers Terminals Joysticks Steering Wheels Network Interface Cards (NICs) MIDI instruments/devices Touch Screens Video Monitors And many, many more...

two parts: something to input the data something to show the data input is typically a keyboard output is typical some form of Monitor When these are integrrated together an hooked to a computer it is usually called a terminal. In the PC world, keyboard and monitor are separate (and mouse adds more input) the terminal I used was a keyboard hooked up to a dot matrix printer for the output

Physical results of pressing a key: complete the circuit of a swith (mechanical) pass magnetic field through a coil (electromechanical) Logical results of pressing a key an interrupt is generated, specifically a keyboard interrupt interrupt handler will read a register from the keyboard controller which indicates which key was pressed multi-key sequences handled by interrupt handler

CRT = cathode ray tube (cathode is a source of electrons) deflection plates are copper windings that create a magnetic field when current passes through them electric fields bend in the presence of a magnetic field (physics) putting more or les current through deflection plate windings alter the amount of deflection and "aims" the electron beam inside surface is coated with a material that gives off visible light when hit with electrons moving the beam left-to-right, top-to-bottom paints the screen repeat many times each second and it looks like continuous motion

LCD (liquid crystal display)

a liquid material whose molecular orientation defines its reflective properties and whose molecules can be aligned by passing a current through it. thus, an electrical signal can control whether the liquid is opaque or transparent (translucent?) need polarized light and a whole mess of intricate electronics, wire meshes, etc to get three visible light colors some details in book, many more on the web

• The book is defining these terminal type based on the way information is is passed to these devices.
• A single piece of hardware can, and often does, support operating as more than one terminal type.
• Types
  • character mapped
  • bit mapped
  • RS-232-C

Information is passed in a simplified character-based protocol One byte for the character attribute and one byte for the character itself. attribute can be: color, underline, blinking, bold, etc. typical 25x80 character screen then needs 25x80x2= 4,000 bytes the video controller is responsible for fetching the characters and attributes from video memory and converting this to the right signals to send to the CRT The font is decided by the video controller (it can be changed, but only on a global basis)

instead of organizing the screen as a matrix of characters, it is simple a matrix of dots, a.k.a. pixels typical matrix sizes: 640x480, 800x600, 1024x768,1280x960,1600x1200 aspect ratios 4:3 match television sizes Recently: Viewsonic: 22.2 inch, 3840x2400 for $8K

• To run a windowing environment you need a bit map terminal
• video cards can operate in either character or bit map modes and the clicking and flash you hear when booting up is the change between those modes
• Allows the software programs complete control over appearances and thus can mix and match fonts and show arbitrary graphics
• Each pixel needs some number of bits to define the color.
  • if monochrone then 1 bit suffices
  • if color, number of bits varies from 8, 16, 24, 32, called the "color depth"
  • "true color" assigns 8 bit per color: Red, Green, Blue (RGB)
  • in the 8 bit days the 8 bits would be an index into a color map allowing any colors, but only 256 at a time.

• At home I run a 1600x1200 display with 24 bit color depth. This requires 3 bytes for each of the 1600x1200 pixels: 5,760,000 = 5.7 MB of video memory.
• Once in video RAM, data can be used to paint CRT without affecting other components/buses.
• However, if a program wants to have full control over the entire bit-mapped display many times a second, then it need to send the data over a bus from the processor to the video memory.
• Even 24 bit color at 1024x768 needs 2.3 MB for each "frame" displayed.
• If you want to do motion video, need >= 25 frames-per second which is 2.3x25=57.5 MB/sec over a bus. PCI barely can handle this, sot eh AGP bus was developed specifically for video cards.

standard defined to let terminals interoperate with any computer defines the physical characteristics of the connector, cable, voltages and the meanings of each pin and voltage signal since telephone lines predate the RS-232-C standard, and it was a desired way to connect to remote computers, a modem is used between the computer and terminal

The command line approach was fine for programmers and sophistictaed users, but as more novice users started to interface with computers a there was a need to present a more intuitive interface. pointing at images and words would help, but need a way to point mice technologies mechanical optical optomechanical when mouse moves some minimum distance a signal (interrupt) is sent to the computer about how far it moved, in what direction and which if any buttons were pressed

• dot matrix

  slow, noisy, best for text number of needles defined the quality of text multiple passes can increase quality, but costs time still cheap and reliable and useful for odd-sized paper, carbon copies

• inkjet

  movable head while nozzle sprays ink limited speed cheap, quiet, passable quality

• laser printer

  fast, high quality, semi-reasonable cost just like a photocopier book explains how it works which is worth knowing usually have a full processor and sizable RAM for storing pages to be printed.

scanned image will have dots and intensity (e.g., gray-scale values) b/w printer only has the presence of ink or the absence of ink for each dot. need to simulate greys through halftoning

monitor colors are transmitted light (RGB: additive primary colors) printer colors are reflective light In theory, all colors can be constructed with Cyan, Yellow and Magenta (subtractive primary colors), in reality, exact pigments and mixtures are not ideal, especially causes problems producing black. most decent color printers will have the three primary subtractive colors and a separate dedicated black color (CMYK) producing exact color is very tricky no device can produce all the real-world colors the set of RGB and CMYK colors are different color printers come in ink-jet and laser printer versions as well as a few other technologies

phone lines have physical characteristics that make make the transmission of a two level signal impossible to achieve reliably (e.g., +0 volts and +5 volts) phone lines do allow reliable transmission of a pure sine wave signal in the 1000 to 2000 Hz range. Need a way to "encode" the 0's and 1' (two voltage levels) within this sine wave signal. The pure sine wave is the base line and is the equivalent of no data or complete silence.

Data is encoded by altering the sine wave, which there are three possible things to be altered: frequency (pitch) e.g., FM Radio amplitude (volume) e.g., AM Radio phase baud rate is the number of number of time intervals in which the phase can be shifted can encode more than one bit of information per time interval by shifting wave different amounts to represent different values

The more wires we have the more data we can get through (parallel transmissions) phone lines have only one wire for the data (two total), so we have to sequence the bits over the wire (serial transmission) the modem is what converts the computer data into a phone line signal and what converts the phone signal into data. transmission rates on phone lines are 28Kb/s to 57Kb/s (POTS line = plain old telephone service) note that these transmission limits have more to do with the hardware on either ends of the wire trhan with the wire itself. full duplex: data both ways at the same time (using different frequencies) half duplex: only one way at a time simplex: only one way

Faster Than a Speeding POTS Line ISDN - integrated services digital network DSL - digital subscriber line ADSL - asymetric DSL, higher bandwidth downloads than uploads CATV - using the cable TV companies existing coaxial (or satellite) Note that there is still a modem device that converts the data to be carried along the wire. Character Codes Just like the transmission from a big-endian to little endian machine requires translation of some form, transmission of characters also have to worry about the same problem. historically, ASCII became a standard that most used and this presented little problem. the older schemes (e.g., EBCDIC) faded away ASCII was extended from 128 chars to 256 chars called Latin-1 Unicode However, ASCII is English-centric, but the world is not Unicode is attempting to become the standard each character is 16 bits, for 56K possible chars used Latin-1 as first 256 to be backward compatible there are more than 200K chars in all languages though, so some chars will not make it