1. Chapter 5: I/O Systems

2. How complex is the OS’s job?
Let’s look at one of the resources managed by the OS: I/O devices
Fall 2018
CS/COE 1550 – Operating Systems – Sherif Khattab

3. CS/COE 1550 – Operating Systems – Sherif Khattab
Device controllers
Electronic component controls the device
May be able to handle multiple devices
May be more than one controller per mechanical component (example: hard drive)
Controller's tasks
Convert serial bit stream to block of bytes
Perform error correction as necessary
Make available to main memory
Block Devices
A device that stores data in fixed‐sized blocks, each uniquely addressed, and can be randomly accessed
Character Devices
Device that delivers or accepts a stream of characters
Fall 2018
CS/COE 1550 – Operating Systems – Sherif Khattab

4. Memory-Mapped I/O Memory I/O ports Separate I/O & memory space
0xFFF…
I/O ports
Separate I/O & memory space
Memory-mapped I/O
Hybrid: both memory-mapped & separate spaces
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

5. Dynamic Frequency on XScale
Fall 2018
CS/COE 1550 – Operating Systems – Sherif Khattab

6. How is memory-mapped I/O done?
Single-bus
All memory accesses go over a shared bus
I/O and RAM accesses compete for bandwidth
Dual-bus
RAM access over high-speed bus
I/O access over lower-speed bus
Less competition
More hardware (more expensive…)
CPU
Memory
I/O
CPU
Memory
I/O
This port allows I/O devices access into memory
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

7. Direct Memory Access (DMA) operation
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Chapter 5

8. Hardware’s view of interrupts
Bus
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Chapter 5

9. I/O software: goals Device independence Uniform naming Error handling
Programs can access any I/O device
No need to specify device in advance
Uniform naming
Name of a file or device is a string or an integer
Doesn’t depend on the machine (underlying hardware)
Error handling
Done as close to the hardware as possible
Isolate higher-level software
Synchronous vs. asynchronous transfers
Blocked transfers vs. interrupt-driven
Buffering
Data coming off a device cannot be stored in final destination
Sharable vs. dedicated devices
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

10. Programmed I/O: printing a page
Printed page
Printed page
Printed page
User
ABCD EFGH
ABCD EFGH
ABCD EFGH A AB
ABCD EFGH
ABCD EFGH
Kernel
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

11. Polling copy_from_user (buffer, p, count); // copy into kernel buffer
for (j = 0; j < count; j++) { // loop for each char
while (*printer_status_reg != READY)
; // wait for printer to be ready
*printer_data_reg = p[j]; // output a single character }
return_to_user();
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

12. Interrupt-driven I/O Code run by system call
copy_from_user (buffer, p, count);
j = 0;
enable_interrupts();
while (*printer_status_reg != READY) ;
*printer_data_reg = p[0];
scheduler(); // and block user
Code run by system call
if (count == 0) {
unblock_user();
} else {
*printer_data_reg = p[j];
count--;
j++; }
acknowledge_interrupt();
return_from_interrupt();
Code run at interrupt time
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

13. I/O using DMA Code run by system call Code run at interrupt time
copy_from_user (buffer, p, count);
set_up_DMA_controller();
scheduler(); // and block user
Code run by system call
acknowledge_interrupt();
unblock_user();
return_from_interrupt();
Code run at interrupt time
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

14. Layers of I/O software User User-level I/O software & libraries
Device-independent OS software
Operating system (kernel)
Device drivers
Interrupt handlers
Hardware
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

15. Interrupt handlers Interrupt handlers are “under the hood”
Driver starts an I/O operation
If it blocks, it calls scheduler for the next process
When devide ends, interrupt notifies CPU of completion
Interrupt service routine bottom half
Perform minimal actions at interrupt time
Save regs not already saved by interrupt hardware
Set up context for rest of interrupt handler
Then unblocks driver that started it
Some of the functionality can be done by the driver after it is unblocked (top half)
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

16. What happens on an interrupt
Set up stack for interrupt service procedure
Ack interrupt controller, reenable interrupts
Copy registers from where saved
Run service procedure
(optional) Pick a new process to run next
Set up MMU context for process to run next
Load new process' registers
Start running the new process
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

17. Device drivers
User space
Device drivers go between device controllers and rest of OS
Drivers standardize interface to widely varied devices
Device drivers communicate with controllers over bus
Controllers communicate with devices themselves
User program
Kernel space
Rest of the OS
Keyboard driver
Disk driver
Keyboard controller
Disk controller
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

18. Why a standard driver interface?
Non-standard driver interfaces
Standard driver interfaces
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

19. Buffering device input
User space
User space
User space
User space
Kernel space
Kernel space
Kernel space
Kernel space 2 2 1 1 3
Unbuffered input
Buffering in user space
Buffer in kernel Copy to user space
Double buffer in kernel
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5

20. Anatomy of an I/O request
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Chapter 5

21. Device-independent I/O software
Device-independent I/O software provides common “library” routines for I/O software
Helps drivers maintain a standard appearance to the rest of the OS
Uniform interface for many device drivers for
Buffering
Error reporting
Allocating and releasing dedicated devices
Suspending and resuming processes
Common resource pool
Device-independent block size (keep track of blocks)
Other device driver resources
CS 1550, cs.pitt.edu (originaly modified by Ethan L. Miller and Scott A. Brandt)
Chapter 5