Device Mgmt © 2004, D. J. Foreman

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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