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Practical Session No. 12

Practical Session No. 12. Input &Output (I/O). I/O Devices. Input/output (I/O) devices provide the means to interact with the “outside world”. An I/O device can be purely input, purely output, or both an input and output device (e.g. mouse, screen, disks).

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Practical Session No. 12

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  1. Practical Session No. 12 Input &Output (I/O)

  2. I/O Devices • Input/output (I/O) devices provide the means to interact with the “outside world”. • An I/O device can be purely input, purely output, or both an input and output device (e.g. mouse, screen, disks). • Are mostly used to communicate with the outside world, and to store data.

  3. Controller • I/O devices are not directly connected to the system bus. Instead, there is usually an I/O controller that acts as an interface between the system and the I/O device.

  4. Reasons for using an I/O controller • Different devices exhibit different characteristics. The processor would spend a lot of time for interaction.Controller could provide the necessary low-level commands and data for proper operation

  5. Reasons for using an I/O controller • The amount of electrical power used to send signals on the system bus is very low Controllers typically contain driver hardware to send current over long cables

  6. I/O Controller • Typically has 3 internal registers: • Data register • Command register • Status register • Processor interacts with an I/O device via the associated I/O controller

  7. I/O device interface to the system

  8. Communication (character output) • Before the processor sends output character, it has to first check the status register of the associated controller (e.g. busy, idle, offline). • The data register holds the character (e.g. to be printed). • The command register determines the operation requested by the processor (e.g. send the character in the data register to the printer).

  9. Communication (character output) • Sequence of operations: • Wait for the controller to finish the last command; • Place a character to be printed in the data register; • Set the command register to initiate the transfer.

  10. I/O Ports • An I/O port is the address of a register associated with an I/O controller. • Two kinds of mapping: • memory-mapped I/O: writing to an I/O port is similar to writing to a memory address. • I/O address space: separated from the memory address space. • Special I/O instructions are needed for I/O address space map. x86 architecture provides two instructions: in and out,to access I/O ports.

  11. I/O Ports • x86 provides 64 KB of I/O address space. This address space can be used for 8-bit,16-bit, and 32-bit-size I/O ports. • A combination cannot be more than the I/O address space. • For example, we can have 64-K 8-bit ports, 32-K 16-bit ports, 16-K 32-bit ports, or a combination of these that fits the 64-K address space.

  12. Register I/O Instructions • The in instruction is used to read data from an I/O port: • in accumulator, port8 (direct address) • in accumulator, DX (indirect address) • The out instruction to write data to an I/O port: • out port8, accumulator (direct address) • out DX, accumulator (indirect address) • accumulator must be AL, AX, or EAX. • port8 - access the first 256 ports 00..FF.

  13. 8255 Programmable Peripheral Interface (PPI) Chip • Provides three 8-bit registers to interface with I/O devices:Register Port address • PA (input port) 60H • PB (output port) 61H • PC (input port) 62H • Command register 63H

  14. 8255 Programmable Peripheral Interface (PPI) Chip • Keyboard interface is provided by ports PA and PB7 (MSB bit of PB). • Keyboard sends an interrupt (to other interrupt controller) whenever a change occurs (e.g, key is pressed). • A scan code of the key whose state has changed is written in PA. • Keyboard waits for an acknowledgement signal from CPU in PB7

  15. Register Bit Map of 8255 • Keyboard scan code if PB7 = 0 • PA7 = 0 if a key is depressed • PA7 = 1 if a key is released • PA0–PA6 = key scan code • Configuration switch 1 if PB7 = 1 • PB7 — selects source for PA input • 0— keyboard scan code • 1— configuration switch 1 • Also, 1 is used as keyboard acknowledge

  16. Keyboard Driver • Uses busy wait loop. • Pressing the esc key terminates the program. • Waits for the PA7 bit to go low to indicate that a key is depressed. • Scan code is read from PA6 to PA0

  17. Keyboard Driver section .data ESC_KEY EQU 1Bh ; ASCII code for ESC key KB_DATA EQU 60h ; 8255 port PA section .text global _start _start: key_up_loop: ;Loops until a key is pressed i.e., until PA7 = 0. ; PA7 = 1 if a key is up. in AL, KB_DATA ; read keyboard status & scan code test AL, 80H ; PA7 = 0? jnz key_up_loop ; if not, loop back

  18. Keyboard Driver and AL,7FH ; isolate the scan code ..Translate scan code to ASCII code in AL.. cmp AL,0 ; ASCII code of 0 => non-interesting key je key_down_loop cmp AL,ESC_KEY ; ESC key---terminate program je done display_ch: ; char is now in AL ..Print character AL to screen..

  19. Keyboard Driver key_down_loop: in AL,KB_DATA test AL, 80H ; PA7 = 1? jz key_down_loop ; if not, loop back jmp key_up_loop done: ..Exit Program..

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