Definitions for Common Acronyms
copyright (c) 1992 -1998
Nicholas Majors & ActionFront Data Recovery Labs Inc.
IRQ (Interrupt Request) - Lines on the bus used to signal hardware interrupts.
I/O (Input Output) - Peripherals accessible by the CPU through registers
at specific I/O addresses (or I/O ports).
PIO (Programmed Input Output) - Exchange of data between memory and peripherals
by means of Input Output commands.
DMA (Dynamic Memory Access) - Transferring data directly between memory and
peripherals without going through the CPU.
BUS ARCHITECTURES:
ISA (Industry Standard Architecture) - 8 bit and 16 bit expansion slots used
by PC, XT, and AT designs. Often called IBM Standard Architecture.
EISA (Extended Industry Standard Architecture) - Developed by several independent
manufacturers (Compaq, AST, Zenith, Tandy, etc.) to standardize 32 bit operation
and combat IBM's MCA.
MCA (Micro Channel Architecture) - Expansion bus introduced by IBM in 1987,
used by some (but not all) PS/2 models.
PCI (Peripheral Component Interconnect) - High speed bus developed by Intel
to support the demands of Pentium and 486 based computers.
VLB (VESA Local Bus) - High speed, 32 bit extension to the ISA bus promoted
by the VESA (Video Electronics Standards Association).
DRIVE INTERFACES:
ST506/412 - Standard interface used on XT and AT drives and controllers.
Originally developed by Seagate Technologies to support their ST506 (5 MB) and ST412
(10 MB) drives. The entire controller mechanism is located on a controller card
and communications between the drive and controller flow over 2 ribbon cables -
one for drive control and one for data.
ESDI (Enhanced Small Device Interface) - Developed by Maxtor in the early
1980's as an upgrade and improvement to the ST506 design. While the drive does not
have an embedded controller, one of the most critical functions ,encoding-decoding,
is performed on the drive. This allows for faster communications and higher drive
capacities. Uses the same cabling as ST506 interface, but carries different signals
on each line.
SCSI (Small Computer System Interface) - Based on an original design by Shugart
Associates, SCSI is not specifically a drive interface, but a method of allowing
different devices to communicate with a PC. For hard drives the entire controller
is built onto the drive PCB, allowing for very high speed transfers to and from
the drive. Fully interpreted, parallel data is then transferred to and from the
PC by way of a single cable through a bus interface that has configured the device
as a hard drive.
IDE (Integrated Drive Electronics) - A technology pioneered by Compaq and
Conner that embedded a controller onto the hard disk PCB while maintaining compatibility
with the register level commands sent by the computer's INT 13 routines. IDE drives
are configured and appear to the computer like standard ST506 drives.
ATA (AT Attachment) - Implementation of the IDE design with a 16 bit AT style
controller on board the drive.
XTA (XT Attachment) - Rarely used implementation of IDE with an integrated
8 bit XT controller.
ATA-2 - Enhancement to the AT Attachment standard to provide for considerable
performance improvement and more sophisticated drive identification.
EIDE (Enhanced IDE) and FAST-ATA - Various implementations of the ATA-2 standard
as marketed by Western Digital (EIDE) and Seagate/Quantum (FAST-ATA).
DATA ENCODING SCHEMES:
MFM (Modified Frequency Modulation) - Common technique used to encode the
magnetic fluxes recorded on a drive into data. Still used on floppy drives and most
original XT and AT systems. Notice that most drive types supported in CMOS have
17 sectors per track. This is the standard density for MFM encoding.
RLL (Run Length Limited) - Encoding method that allows 50% more information
to be recorded on a track than MFM. Actually accomplished by recording more fluxes
for every byte, but packing them more tightly onto the surface. Often called 2,7
RLL because the recording scheme involves patterns with no more than 7 successive
zeros and no less than two.
ARLL (Advanced Run Length Limited) - More complex yet powerful derivatives
of the RLL scheme. Include 1,7 and 3,9 encoding.