An overview of common devices

Terminals

• Connect to computers via serial lines (RS-232) : 9600 baud (bits/sec) 900 bytes/sec
• Although small rate of transfer - can swamp CPU die to overhead of handling each byte
• Say 10 users (running vi or emacs) = 900 interrupts per user : overhead of handling interrupt 100 microseconds
• Direct Memory Access : Instead of interrupt on every block - do a block transfer - then interrupt (overall - faster transfer rates)
• Redirection : Bitmap displays : CPU writes to video memory - display then reads video memory

Disks

• Options : choice of sector size (tradeoff between space and time efficiency), usually big blocks mean greater transfer rate (time efficiency) as time to find data on disk is minimised

Trends in Disk Technology
for a typical disk today :

• Disk capacity : 2.1 GB
• Number of surfaces per pack : 18
• Number of tracks per surface : 2048
• Number of sectors per track : 32
• Number of bytes per sector : 1 KB
• Number of revolutions per minute : 7200

Transfer rate = 1 KB bytes/sector * 32 sectors/cycle * 120 cycles/second = 4 MB/sec