8
Hard Disks
hard disk installed
in system unit
What is a hard disk?
High-capacity storage
Consists of several
inflexible, circular
platters that store items
electronically
Components enclosed in
airtight, sealed case for
protection



HARD
DISKS
p. 7.09 Fig.
7-12
Next
Fixed Hard Disks (Internal)
 consist of several 3.5 inch disks, called platters, made of aluminum or glass and coated with magnetic
oxide. Both sides of each platter is a recordable surface, so 4 platters = 8 recordable surfaces. Each
surface consists of tracks and sectors. There are also cylinders (a few hundred to a couple of
thousand). Cylinders are a vertical stack of tracks. To find a file, the computer must know which
platter, cylinder, and sector.
 In 1954, IBM invented the first hard disk – capacity was a mere 5 MB stored across 50 platters that were
each 24-inches in diameter.

The magnetic field on hard disks is much more compact (than floppy disks) - more data in a smaller
area. Hard disks offer much more capacity (today, anywhere from 100-300+ GB; 20 GB = 13,000
traditional floppies).
9
Hard Disks
What is a head crash?


Occurs when read/write head touches platter surface
Spinning creates cushion of air that floats read/write head above platter


Clearance between head and platter is approximately two-millionths of an inch
A smoke particle, dust particle, or human hair could render drive unusable
hair
read/write head
dust
clearance
smoke
platter
p. 7.12 Fig. 7-16
Next

Hard disks are continually spinning, and they spin faster than other disks, so they have faster accesses
than other disks.
 Access times (seek time): 5-20 milliseconds – the amount of time between when the CPU requests a
file and when the first byte of the file is sent to the CPU (.005 - .020 or 1/200th – 1/50th of a second)
 Data rate: 5-40 MBps – the number of bytes per second that the drive can deliver to the CPU
HARD DRIVE INTERFACES (HARD DRIVE CONTROLLERS)


Most hard disks come with a circuit board attached to them – the controller (also called hard disk
interface). The controller is what makes the internal connection to the computer and controls the way
the hard drive interfaces with the rest of the computer. The newer hard drives and controllers offer
better performance, higher density, faster access, and better interfacing with other hardware. (External
interfaces include USB and Firewire).
These refer to the type of connection, or port/bus, that the hard drive and the other drives have to the
motherboard. We don’t usually consider the connection the hard drive has to the motherboard a port,
though it really is; we just refer to it as the hard drive interface. Originally, personal computers knew
how to handle only a few types of hard drives. The information on how to control each of them was
embedded in the PC’s CMOS memory. When a hard drive was installed, its number of tracks, sectors,
platters (i.e. it’s geography) had to match one of the configurations the CMOS could recognize. The
BIOS uses the CMOS information to direct the movement of the read/write head and to route data
between the processor and the drive. The drive controller expansion card passed the signals to the drives
and handled the transfer of data between the processor and the drive. To overcome this limitation in the
evolution of hard drives, IDE (Integrated Drive Electronics) was developed to replace both the disk
controller card and the CMOS-based knowledge of every possible drive’s physical geometry. Today,
virtually all PCs come with two IDE or EIDE controllers built into the motherboard. With two IDE
connectors on the motherboard, a PC can have up to 4 hard drives. EIDE, which is nearly universal on
new PCs, can control floppy drives, CD-ROM drives, and tape drives that are IDE compatible.
Another type of hard drive interface is called SCSI. If the hard drive in your computer has a SCSI type
of interface, then your motherboard contains SCSI buses on it, and the computer probably has a SCSI
port on the back of the computer into which you can connect peripheral devices, such as CD-writers,
scanners, etc. This type of interface is used often in workstations or servers. The peripheral devices that
are SCSI enabled also cost more than the non-SCSI devices.
The newest type is called SATA. This is a fast type of interface, and is becoming very common in new
computer systems today. SATA connections can also control CD and DVD drives.
So, hard drive interfaces are typically either some type of IDE, or some type of SCSI, or SATA.
1. Various IDE (or ATA) technologies:
 IDE (Integrated Drive Electronics) is a commonly used one. It is also called ATA (Advanced
Technology Attachment)
 EIDE (Enhanced Integrated Drive Electronics) is an improved IDE. It is prevalent because it is
an inexpensive, dependable technology. It is also called Fast IDE, or Fast ATA.
 Ultra ATA (also called Ultra DMA (Direct Memory Access))
 uses parallel signals
 can support up to 4 hard disks at 137GB per disk
 data transfer rates: up to 100 MBps (800 Mbps)
 can connect CD and DVD and tape drives
2. Various SCSI technologies:
 Wide SCSI, Fast SCSI, Ultra SCSI, Ultra Wide SCSI, Ultra 2 SCSI, Ultra2 Wide SCSI,
Ultra3 SCSI
 also uses parallel signals
 can connect up to 8 or 15 peripheral devices, such as CD, DVD, tape, printers, scanners, etc.
 can have built in SCSI interface (more expensive computer system) or can add that with an
adapter card
 data transfer rates: up to 320 MBps (2560 Mbps)
3. the newer SATA
 newest type
 uses serial signals to transfer data, instructions, and information
 primary advantage: cables are thinner, longer, more flexible, less susceptible to interference
 data transfer rates: up to 300 MBps (2400 Mbps)
 can be used also for CD and DVD drives
 for comparison, data transfer rates for USB 2.0 are up to 480 Mbps and Firewire up to 800 Mbps

What’s important to look for?
 Capacity: typical capacities today are at least 80 GB
 Spindle speed: sometimes it’s a trade-off between capacity and speed – if money is limited, the
advice is to go with a slightly lower capacity and a higher speed. For example, it’s better to get a
300 GB 7200 rpm hard drive, than it is to get a 320 GB 5400 rpm hard drive. The speeds you’ll see
are 5400 rpm or the 7200 rpm, though there are higher speeds (10,000 rpm and 15,000 rpm), but
they are not the common ones you’ll see.
External Hard Disks


You can get an external hard disk, which are becoming popular. They connect via the USB port (or
FireWire). Prices, capacities, and interfaces vary. SeaGate and Western Digital are popular hard drive
manufacturers for both internal and external types of hard drives.
Reviews of latest hard drive storage: http://www.storagereview.com/
MAIN MEMORY (RAM)
 Electronic components that store instructions, data, and results
 Consists of one or more chips on motherboard or other circuit board
 Each byte stored in unique location called an address
 RAM is volatile
 The memory a computer has, the faster it responds
 The contents are constantly changing
How is memory measured?
 By number of bytes available for storage
DRAM – “main memory”
o made up of capacitor-transistor pairs; the charges in the capacitors need to be refreshed thousands of
times per second so they don’t lose their contents, thus the name Dynamic RAM
o http://computer.howstuffworks.com/ram1.htm - this illustrates how the charge leaks and has to be
“refilled”
o when you start needing to upgrade your computer, main memory is usually the first place you start – if
you don’t have enough, this is usually the quickest, easiest, cheapest upgrade to do to improve the
performance of your computer
o how much is enough? – depends on the types of applications you run…
 bare minimum that you should have today: 256 MB
 a better minimum amount is 512 MB
 the more you have, the better, especially if you’re running CAD or using graphic intensive
applications or other demanding application; if you don’t have enough, you will have problems
multitasking smoothly or maybe have problems running certain programs
o types of DRAM:
 SDRAM – synchronized to the system clock
 DDR SDRAM – double data rate – transfers data twice for each clock cycle, so twice as
fast as the SDRAM; the common one today is DDR2 SDRAM and soon DDR3
SDRAM
 Rambus DRAM or RDRAM – uses pipelining techniques; the early P4 systems could
only use this type of RAM – was all the rage – supposedly way faster – much more
expensive; Intel introduced support for DDR because the RDRAM was so expensive and
so the DDR SDRAM has prevailed making the RDRAM become obsolete starting around
2003; RDRAM still used in the Sony PS2. The Sony PS3 expected to come out in 2006
is going to use Rambus’ new type of memory called XDR, which is a high performance
RAM
Other types of RAM
 SRAM – faster, more reliable, more expensive; does not need to be recharged, thus the name “static RAM”;
used for more speed critical parts of the computer, such as cache memory
 MRAM – uses magnetic charges instead of electrical charges; non-volatile; supposedly has higher
capacities, consumes less power, and has faster accesses times; in the future, may replace DRAM and
SRAM for main memory
 Other types of RAM:
o VRAM – video RAM – used for video (graphics) cards
o SGRAM – specialized form of SDRAM for graphics cards – is becoming more popular and replacing
the VRAM
 Memory modules:
o SIMMs – Single In-line Memory Modules – the pins are on one side; used with the Pentium and older
PowerPC processors
o DIMMs – Dual In-line Memory Modules – pins on both sides; started with the Pentium II and newer
PowerPC processors.
 168-pin DIMMs - - > SDRAM
 184-pin DIMMs - - > DDR SDRAM
 240-pin DIMMs - - > DDR2 SDRAM
o RIMMs – actually DIMMs but only fit into propriety RIMM slots; used for the RDRAM