1. I/O Memory Interface Topics:
Programmable, Interrupt initiated, DMA, Serial and Parallel Interfaces for data transfer

2. Content Introduction to data transfer techniques
Programmed Data Transfer
DMA Controlled Data Transfer

3. 1. Introduction to Data Transfer Techniques
Data transfer may take place between two devices. For e.g.
Microprocessor and memory
Microprocessor and I/O device
Memory and I/O device
Classification of data transfer techniques
Programmed data transfer
Direct Memory Access (DMA)

4. 2. Programmed Data Transfer
Data is transferred from the I/O device to the microprocessor or memory
Data is transferred under the control of the program stored in the program memory of the microprocessor based system
This technique of data transfer is normally used if the size of data to be transferred is small

5. Programmed data transfer techniques
Classified as
Parallel data transfer
Serial data transfer
Synchronous data transfer
Asynchronous data transfer
Interrupt Initiated data transfer

6. Parallel Data Transfer
All the bits are transferred at the same time
Used for data transfer between nearby devices
E.g microprocessor and program memory
Advantage
Data transfer is very fast

7. Serial data transfer Data is transferred one bit at a time
Suitable for data transfer over long distances
E.g. dial-up internet connections using modems
Data transfer is slow as compared to parallel data transfer technique

8. Comparison of Parallel/ Serial
8 bits of data transferred at a time
9 lines required to connect two devices
Advantageous over small distances
Serial
Only 1 bit of data is transferred at a time
2 lines required to connect two devices
Advantageous over long distances

9. Synchronous data transfer
Used when the I/O device is always in the ready state for data transfer
Microprocessor reads / writes to the I/O device without checking if it is in a ready state
For e.g. data transfer from 8085 microprocessor to a 7-segment display

10. Asynchronous Data Transfer
Microprocessor reads the status of a device to check if it is ready for data transfer
This method is also known as polling method of handling I/O devices
Handshake signals are used for communication between microprocessor and I/O devices
Used to connect slower peripherals with microprocessor

11. Comparison of Asynchronous/ Synchronous Data Transfer techniques
Used to transfer one character at a time
Start and stop bits are used with each character
Speed is less
Transmitter and receiver can use two separate clock inputs
Synchronous
Used to transfer a block of characters at a time
No start/ stop bits are used
Speed is high
Transmitter and receiver share a common clock

12. Syn and asycn transmission
a) Synch format b) asynch format

13. Interrupt Initiated Data Transfer
Microprocessor initiates the interrupt mechanism and starts executing the main program
I/O device informs the microprocessor that it is ready by generating an interrupt signal
Microprocessor services the interrupt by completing the data transfer

14. 3. DMA Controlled Data Transfer
DMA stands for Direct Memory Access
used when large amount of data is to be transferred
Microprocessor does not participate in this type of data transfer
Data is transferred directly between an I/O device and memory or vice-versa
Data transfer is controlled by an I/O device or a DMA controller
DMA data transfer is fast as compared to programmed data transfer

15. Microprocessor
Memory
DMA Controller
I/OPort
I/OPort
I/OPort
External device
External device
External device

16. 8085 signals used for DMA
DMA controlled data transfer uses two pins HOLD (pin 39) and HLDA (pin 38) of 8085
HOLD –
An input signal to 8085 from a DMA controller
It is a request to 8085 to hand over the control of data and address bus to a DMA controller
8085 responds to HOLD request by
Freeing up the buses for use by a DMA device
Sending out Hold acknowledge signal HLDA on pin 38

17. DMA Modes of Operation DMA operates in these two modes Slave Mode
Master Mode
DMA behaves as a I/O peripheral requesting the microprocessor for the control of the buses
DMA plays the role of a data transfer processor to peripherals such as floppy disks

18. DMA Data Transfer Techniques
These are
Burst or Block transfer DMA
Cycle Steal or Single Byte transfer DMA
Transparent or Hidden DMA

19. Burst or Block Transfer
It is the fastest DMA mode
Two or more data bytes are transferred continuously
N number of DMA cycles are added into the machine cycles of the microprocessor
DMA controller sends HOLD signal to 8085 and waits for HLDA
After receiving HLDA signal, DMA controller gains control of buses and executes DMA cycle to transfer 1 byte
Then it increments the memory address, decrements the COUNT and transfers the next byte

20. Cycle Steal/ Single Byte Transfer DMA
Only 1 byte is transferred at a time
Slower than the burst DMA
Only 1 DMA cycle is added between 2 machine cycles of the microprocessor
DMA controller sends HOLD signal to 8085 and waits for HLDA
After receiving HLDA signal, DMA controller gains control of buses and executes DMA cycle to transfer 1 byte
After single byte transfer, it disables the HOLD signal and goes into SLAVE mode again

21. Transparent or Hidden DMA
Microprocessor executes some states during which it floats the address and data buses
During these states the microprocessor is isolated from the system bus
Transparent or Hidden DMA uses these states to transfer data between memory and I/O devices
This type of DMA transfer does not reduce the speed of the microprocessor
This is the slowest type of DMA transfer