1. CS703 - Advanced Operating Systems
By Mr. Farhan Zaidi

2. Lecture No. 35

3. Overview of today’s lecture
Goals of I/O software
Layers of I/O software
Direct Vs memory mapped I/O
Interrupt driven I/O
Polled I/O
Direct Memory Access (DMA)

4. Principles of I/O Software
Goals of I/O Software
Device independence
programs can access any I/O device without specifying device in advance (floppy, hard drive, or CD-ROM)
Uniform naming
name of a file or device a string or an integer not depending on which machine
Error handling
handle as close to the hardware as possible
Synchronous vs. asynchronous transfers
blocked transfers vs. interrupt-driven
Buffering
data coming off a device cannot be stored in final destination immediately

5. Device Controllers mechanical component electronic component
I/O devices have components:
mechanical component
electronic component
The electronic component is the device controller
may be able to handle multiple devices
Controller's tasks
convert serial bit stream to block of bytes
perform error correction as necessary
make available to main memory

6. Layers of the I/O system and the main functions of each layer

7. Memory mapped I/O Vs Direct I/O
Device controller’s registers and internal memory are directly mapped into the processor’s address space
Direct I/O
Device controller’s registers and internal memory is accessible via special instructions in the assembly language instruction set. The arguments to these instructions are usually called ports.

8. Writing a string to the printer using programmed I/O

9. Interrupt-Driven I/O a. Code executed when print system call is made
Writing a string to the printer using interrupt-driven I/O
a. Code executed when print system call is made
b. Interrupt service procedure

10. A Typical Hardware System
CPU chip
register file
ALU
system bus
memory bus
main
memory
bus interface
I/O
bridge
I/O bus
Expansion slots for
other devices such
as network adapters.
USB
controller
graphics
adapter
disk
controller
mouse
keyboard
monitor
disk

11. Reading a Disk Sector: Step 1
CPU chip
CPU initiates a disk read by writing a command, logical block number, and destination memory address to a port (address) associated with disk controller.
register file
ALU
main
memory
bus interface
I/O bus
USB
controller
graphics
adapter
disk
controller
mouse
keyboard
monitor
disk

12. Reading a Disk Sector: Step 2
CPU chip
Disk controller reads the sector and performs a direct memory access (DMA) transfer into main memory.
register file
ALU
main
memory
bus interface
I/O bus
USB
controller
graphics
adapter
disk
controller
mouse
keyboard
monitor
disk

13. Reading a Disk Sector: Step 3
CPU chip
When the DMA transfer completes, the disk controller notifies the CPU with an interrupt (i.e., asserts a special “interrupt” pin on the CPU)
register file
ALU
main
memory
bus interface
I/O bus
USB
controller
graphics
adapter
disk
controller
mouse
keyboard
monitor
disk

14. Direct Memory Access (DMA)
Operation of a DMA transfer

15. I/O Using DMA a. code executed when the print system call is made
Printing a string using DMA
a. code executed when the print system call is made
b. interrupt service procedure