Input/Output. A computer system has three major components: CPU Memories (primary and secondary) I/O ( Input/Output ) equipment printers scanners modems etc. Buses. Physically, most PCs have a structure similar to that shown on the next slide.
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The bus is used by both I/O controllers and the CPU. When both want to use the bus, a bus arbiter decides who will go next. In general, I/O devices are given precedence since disks and other moving devices cannot be stopped without losing data.
When no I/O is in progress, the CPU has all the bus cycles for itself to reference memory. When some I/O device is also running, it will request and be granted the bus. This is called cycle stealing and it slows down the computer.
As CPUs, memories and I/O devices became faster, the bus could no longer handle the load.
Replacing the bus every time it became overloaded with a higher capacity one would cause the old peripherals to become useless.
Therefore many manufacturers stayed with the old ISA (Industry Standard Architecture) bus. Eventually companies started developing computers with multiple buses including the ISA, the backward-compatible EISA (Extended ISA), and now PCI (Peripheral Component Interface).
To lessen this requirement, some computers use an 8-bit number to indicate the desired color. This number is then used as an index into a hardware table called the color palette that contains 256 entries, each holding a 24-bit RGB value. This is called indexed color. It reduces the required RAM by 2/3, but allows only 256 colors.
Usually each window on the screen has its own mapping. The palette is changed when a new window gains focus.
To display full-screen full-color multimedia on a 1024x768 display requires copying 2.3 MB of data to the video RAM for every frame. For full-motion video, 25 frame/sec is needed for a total data rate of 57.6 MB/sec.
This is too much for an (E)ISA bus, so high-performance video cards need to be PCI cards.
To allow terminals to be used with many computer systems, a standard computer-terminal interface called RS-232-C has been developed.
When the computer and terminal are far apart, the only practical way to connect them is over the telephone system. The telephone system, however, is not capable of transmitting the signals required by RS-232-C, so a device known as a modem (modulator-demodulator) has to be used.
To communicate, the terminal and computer each contain a chip called a UART (Universal Asynchronous Receiver Transmitter), as well logic to access the bus.
To display a character, the computer takes a word from memory and presents it to the UART which shifts it onto the RS-232-C cable 1 bit at a time. It also adds a start bit and a stop bit to each character to delimit the character.
A mouse is used as a pointing device. The mouse can have one, two, or three buttons.
Three kinds of mice have been produced:
The optical mouse is used on top of a special plastic pad containing a rectangular grid of lines.
A common arrangement is to have the mouse send a sequence of three bytes every time the mouse moves a certain minimum distance (e.g. 0.01 inch). Byte 1 gives movement in the x direction, 2 in the y direction and 3 gives the state of the mouse buttons.
The color laser printer has four different toners (for C, M, Y, and K) on which separate images are laid down.
The wax printer has a wide ribbon of of four-color wax that is segmented into page-size bands. Thousands of heating elements melt the wax as the paper moves under it. The wax is fused to the paper.
In dye sublimation printers, a carrier containing the CMYK dyes passes over the thermal print head containing thousands of programmable heating elements. The dyes are vaporized instantly and are absorbed by a special paper close by.