1. Accessing I/O Devices
Processor
Memory
BUS
I/O Device 1
I/O Device 2

2. Parallel I/O I/O devices connect to processor through PORTS Ports are:
registers (part of the I/O interface)
8, 16, or 32 bits wide
Addressed in the range 0000-FFFFh
Accessed with 2 instructions – IN, OUT

3. Modes of I/O Instructions
Direct I/O – the port address is one of the operands.
Address must be 00-FFh.
IN AL, 27h
Data flows through the accumulator
MOV AX, BX
OUT 26h, AX ; move 16-bit data from AX to port ; 26h (AL to 26h and AH to 27h)
Indirect I/O – the port address is preloaded into DX
Address can be 0000-FFFFh
String I/O – allows data to pass directly through the accumulator (from I/O device to memory)

4. 80x86 I/O Instructions

5. Ways to Differentiate I/O
Memory-Mapped versus I/O-Mapped Ports
address of 80x86 processors is divided into 1M, 4GB, or 64GB of memory space and 64K of I/O space.
With memory-mapped I/O, because I/O ports are mapped to a memory address, any of the memory read/write instructions are available to use. Address can be computed using any of the addressing modes.
With I/O mapped ports, restricted to simple IN/OUT instructions. Address must be in DX.
Memory Space
20-, 32-, or 36-bit
address
I/O Space
16-bit
address
M/IO’ = 0
M/IO’ = 1

6. Memory-Mapped I/O I/O Devices and memory share the same address space.
Each I/O Device is assigned a unique set of addresses.
When the processor places a particular address on the address lines, the device recognizing this address responds to the commands on the control lines.
The processor requests either a read or a write operation, and the requested data is transferred over the data lines.
Any machine instruction that can access memory can be used to transfer data to/from I/O devices.
Mov datain, R0

7. Intel Architecture
Intel processors have a separate 16-bit address bus for I/O devices
Designer has the option of
connecting I/O devices to use special address space
or simply incorporating them as part of the memory address space.
The later approach leads to simpler software.
One advantage of a separate address bus for I/O is reduced number of address lines needed for I/O devices.
Not physically separate address lines on processor. Special signal (I/OR or I/OW, MemR or MemW)

8. Ways to Drive Hardware Devices using Parallel Buses
Programmed I/O through I/O ports
Interrupt I/O using (hardware) interrupts
Direct Memory Access

9. Programmed I/O (driving Hardware devices through I/O ports)
External devices are almost always connected not directly to the system bus but to an INTERFACE.
Registers in the interface allow for a wide range of possibilities for the designer to determine how it is to interface to the bus.
TO avoid confusion with the main registers in the 8086, peripheral interface chip registers are usually referred to as PORTS.

10. Interface Ports Typically consists of three registers
Control Port - the setting of which will determine if the interface is to send or receive.
Data Port – for the data element to be transmitted or to hold a data element received.
Status Port – used to obtain information such as “printer out of paper, don’t send any more data” or, for a serial transmission, “all the bits of the data element haven’t yet been received”
Simple interfaces may have status and control combined into one port; sophisticated ports may have multiple control and status ports.

11. I/O Interface for an Input Device
Address Lines
Data Lines
Control Lines
Address
Decoder
Control
Circuits
Data and
Status Registers
I/O Interface
Input Device

12. I/O Device Speeds
Processors can operate at speeds that are vastly different than I/O speeds.
When a human is entering characters on a keyboard, the processor can execute millions of instructions between successive character entries.
So, how does the processor handle I/O inputs…..

13. Three types of I/O Strategies
Polled I/O
Interrupt Driven I/O
DMA I/O

14. Polled IO versus Interrupt Driven I/O
Polled IO – processor continually checks IO device to see if it is ready for data transfer
Inefficient, processor wastes time checking for ready condition
Interrupt Driven IO – IO device interrupts processor when it is ready for data transfer
Processor can be doing other tasks while waiting for last data transfer to complete – very efficient.

15. I/O Interfacing
A lot of handshaking is required between the CPU and most I/O devices.
All I/O devices operate asynchronously with respect to the CPU. The events that determine when an input device has data available or when an output device needs data are independent of the CPU.

16. Three I/O strategies
Must be capable of data rates fast enough to keep up with the demands of the device, but must not be allowed to transfer data faster than the device can process it.
Polled waiting loops
Interrupt-driven I/O
Direct memory Access (DMA)

17. Synchronization
The CPU must have some way of checking the status of the device and waiting until it is ready to transfer

18. Transfer Rate
A measure of the number of bytes per second transferred between the CPU and an external device.
Maximum transfer rate – a measure of the bandwidth capability of a particular method of doing I/O.

19. Comparison of transfer rates
Polled waiting loops provide data rates that are a bit slower, but still quite reasonable.
Interrupt-driven I/O requires overhead of saving and restoring the machine state (significantly degrades data rates unless more than one byte can be transferred per interrupt.
DMA has fastest transfer rates (additional hardware complexity needed.

20. Latency
Measure of the delay from the instant that the device is ready until the time the first data byte is transferred. Latency is equivalent to the “response time”

21. Comparison of Latency
Polled Waiting Loops – latency can be very high (the computer may not even be checking the device for new data when it arrives).
Interrupt-driven I/O – dramatically lower than polled, but still imposes a software overhead.
DMA – very low (lower than the others)

22. Polled Waiting I/O
Use software to test the status of a device,before transferring each data byte.
Continuously checking the peripheral’s BUSY/READY flag
Ties up the CPU – no other tasks can be performed.