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Practical Session 12

Practical Session 12. Input &Output (I/O). I/O Device. Input / Output (I/O) devices provide the means to interact with the “outside world”. mouse, screen, disks, printer, etc. CPU-Memory-I/O Architecture. Memory. CPU. I/O module. I/O controller. I/O device. “CPU bus” or “System bus”.

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Practical Session 12

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  1. Practical Session 12 Input &Output (I/O)

  2. I/O Device • Input / Output (I/O) devices provide the means to interact with the “outside world”. • mouse, screen, disks, printer, etc.

  3. CPU-Memory-I/O Architecture Memory CPU I/O module I/O controller I/O device “CPU bus” or “System bus” “Bus interface” “I/O bus”

  4. Types of I/O • Programmed I/O • Interrupt-driven I/O • Direct memory access (DMA)

  5. Programmed I/O • I/O operations are under direct control of software (program) • Software initiates the I/O operation • Disadvantage: • Slow • Uses a lot of CPU resources • Advantage: • Simple

  6. Types of I/O • Programmed I/O • Interrupt-driven I/O • Direct memory access (DMA)

  7. Interrupt-driven I/O • I/O operations are initiated by the device • The device, or its I/O module, includes a signal to interrupt the CPU • When an interrupt occurs (and is accepted), a special routine executes to service the interrupt

  8. Types of I/O • Programmed I/O • Interrupt-driven I/O • Direct memory access (DMA)

  9. Direct memory access (DMA) • Used for high-speed block transfers between a device and memory • During the transfer, the CPU is not involved • The CPU “prepares” the DMA operation by transferring information to a DMA controller (DMAC): • Location of the data on the device • Location of the data in memory • Size of the block to transfer • Direction of the transfer • When the device is ready to transfer data, the DMAC takes control of the system buses

  10. I/O controller to DMA module controller controller

  11. I/O 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.

  12. Operating system

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

  14. I/O device interface to the system

  15. Communication example – character output • Check the status register of I/O controller • e.g. busy, idle, offline • Put data into data register • Put command into command register • e.g. “send the character in the data register to the printer”

  16. I/O Ports • 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

  17. I/O Mapping • Memory mapped I/O • Devices and memory share an address space • I/O looks just like memory read/write • No special commands for I/O • Large selection of memory access commands available • Isolated I/O • Separate address spaces • Need I/O or memory select lines • Special commands for I/O • Limited set

  18. I/O Ports and Instructions • x86 provides 64 KB of isolated I/O address space • x86 provides two instructions to access I/O ports • in instruction is used to read data from an I/O port: in register, port address (direct address) in register, DX (indirect address) • out instruction is used to write data to an I/O port: out port address, register (direct address) out DX, register (indirect address) • register must be AL, AX, or EAX

  19. Example – Keyboard Driver • Use PA input port register at address 60H • PA7 = 0 if a key is depressed • PA7 = 1 if a key is released • PA0–PA6 = key scan code • Use busy wait loop • Waits until a key is pressed i.e., until PA7 = 0. • Scan code is read from PA6 to PA0 • Pressing the ESC key terminates the program

  20. Example – 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: ;loop until a key is pressed i.e., until PA7 = 0 in AL, KB_DATA ; read keyboard status & scan code test AL, 80H ; PA7 = 0? (80H=10000000b) jnz key_up_loop ; if not, loop back (busy wait)

  21. Example – 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 ..Print character in AL to screen.. 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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