1. Device Mgmt
© 2004, D. J. Foreman

2. Device Management Organization
Multiple layers
Application
Operating System
Driver
Controller
Device
Tradeoffs of layering
Loss of efficiency
Increased generalization and flexibility
Reduced cost of maintenance and development for all (driver writers, O/S writers, app writers and users)
© 2004, D. J. Foreman

3. Device Management Organization-2
read, write
read, write
read, write
Applications
API
Device independent
O/S
Device Driver
Device Driver
Device Driver
Device dependent
Controller Firmware
Devices
© 2004, D. J. Foreman

4. The API Simple set of abstract operations Direct vs Sequential
read, write, seek, control
Direct vs Sequential
Disks vs tapes, printers, etc
Block vs Character
Disk vs keyboard
Blocking vs Non-blocking
© 2004, D. J. Foreman

5. Handling I/O Polling Interrupt driven
Program loops until device signals completion
No other program can run
CPU runs, but no work gets done!
Interrupt driven
Program starts I/O
Program decides to wait or not
O/S switches to another thread or process
© 2004, D. J. Foreman

6. Key concepts CPU cycles are wasted during a wait
Devices are independent
Multitasking (or threading) is possible
Why not overlap I/O with CPU
Threads can decide when to wait
Non-thread programs can also decide
System throughput is increased
© 2004, D. J. Foreman

7. Driver/Kernel Interface
Drivers merged with kernel
Kernel makes function calls to drivers
Kernel functions used by drivers:
Device allocation
Resource (e.g., memory) allocation
Scheduling
Others: depends on O/S
© 2004, D. J. Foreman

8. System bottlenecks Compute bound processes I/O bound processes
no devices get started until time-slice used up
Only one process/thread gets service
I/O bound processes
CPU under-utilized
Solution:
"Good" mix of applications
Pre-emptive scheduling (more later)
© 2004, D. J. Foreman

9. Overlapped I/O Apps t1 t2 t3 t4 t5 t6 t7 t8 t9 P1 th1 th1 th1 th1 th1
Device t1 t2 t3 t4 t5 t6 t7 t8 t9
Non-Blocking I/O
Blocking I/O
© 2004, D. J. Foreman

10. Buffering Provides a means for speed-matching Many methods Problems
Lists
1 way, 2 way, circular
Arrays
Fixed size
Variable size
Circular
Problems
Overflow
Protected access
Synchronization – more in another chapter
© 2004, D. J. Foreman

11. Example of using buffers
Customer Office
Water Company
Returning the Empties
Water Producer
Water Consumers
Production qty?
Delivering Water
Consumption qty?
Inventory
Classic "Producer-Consumer"
© 2004, D. J. Foreman

12. Hardware Buffering In the Device controller
In the Device itself = "double-buffering"
Reduces system overhead from:
Buffer mgmt
Buffer content
Synchronizing
© 2004, D. J. Foreman

13. Disk I/O Goal – minimize access time = (X + Y*K) + latency + transfer
Mixed solution: h/w & s/w
= (X + Y*K) + latency + transfer
Seek time: head movement delay
for 1 cylinder (X) ≈ 10 ms (device dependent)
For Y cylinders = Y * K (K is device dependent)
Latency: rotational delay
5400R/M*1M/60s= 90R/s  .01 s/R = 10ms/R
7200 R/M*1M/60s=120R/s  .008 s/R= 8ms/R
Transfer time: delay between disk and RAM
Bus speed (currently) 400 Mhz or 800 Mhz
Access Time = seek + latency + transfer
© 2004, D. J. Foreman

14. Access Algorithms FCFS SSTF – min seek from current position Scan/Look
No optimization
SSTF – min seek from current position
Starvation can occur (from local minimization)
Scan/Look
No starvation
Requests may wait a full scan (0-n-0)
Circular Scan/Look
Requires fast-reposition to 0
© 2004, D. J. Foreman

15. Serial I/O Must convert And back again at the other end
Parallel (bytes) to serial (bit-by-bit)
Digital (1/0) to analog (+/- voltages)
And back again at the other end
Protocol: RS-232
© 2004, D. J. Foreman

16. MS-bootable disk layout
0x00-0x02 jump inst to 0x1e
0x03-0x0a PC manufacturer name
0x0b-0x0c sectors/cluster
0x0d-0x0f reserved for boot record
0x10-0x10 # of FAT's
0x11-0x12 # root directory entries
0x13-0x14 # logical sectors
0x15-0x15 media descriptor
0x16-0x17 sectors/FAT
0x18-0x19 sectors/track
0x1a-0x10b # surfaces (heads)
0x1c-0x1d # hidden sectors
0x1e-… boot program
© 2004, D. J. Foreman

17. Mainframe Device Mgmt CPU Channel Controller Hardware Devices
© 2004, D. J. Foreman

18. Channel programming SIO devaddr Channel Program Seek Search Read/write
Channel Address Word
Device
© 2004, D. J. Foreman