beneath-apple-dos/appendixC.txt

# APPENDIX C - GLOSSARY

ACCESS TIME
The time required to locate and read or write data on a direct access storage
device, such as a diskette drive.

ADDRESS
The numeric location of a piece of data in memory.  Usually given as a
hexadecimal number from $0000 to $FFFF (65535 decimal).  A disk address is the
location of a data sector, expressed in terms of its track and sector numbers.

ALGORITHM
A sequence of steps which may be performed by a program or other process, which
will produce a given result.

ALPHANUMERIC
An alphabetic character (A-Z) or a numeric digit (0-9).  The term used to refer
to the class of all characters and digits.

ANALOG
As opposed to digital.  Having a value which is continuous, such as a voltage or
electrical resistance.

AND
The logical process of determining whether two bits are both ones.  0 AND 1
results in 0 (false), 1 AND 1 results in 1 (true).

ARM
The portion of a disk drive which suspends the read/write head over the disk's
surface.  The arm can be moved radially to allow access to different tracks.

ASCII
American Standard Code for Information Interchange.  A hexadecimal to character
conversion code assignment, such that the 256 possible values of a single byte
may each represent a alphabetic, numeric, special, or control character.  ASCII
is used when interfacing to peripherals, such as keyboards, printers, or video
text displays.

ASSEMBLY LANGUAGE
Also known as MACHINE LANGUAGE.  The native programming language of the
individual computer.  Assembly language is oriented to the machine, and is not
humanized, as is BASIC, PASCAL, or FORTRAN.  An assembler is used to convert
assembly language statements to an executable program.

BACKUP
The process of making a copy of a program or data against the possibility of its
accidental loss or destruction.

BASE
The number system in use.  Decimal is base 10, since each digit represents a
power of 10 (1,10,100,...).  Hexadecimal is base 16 (1,16,256,...).  Binary is
base 2 (1,2,4,8,...).

BINARY
A number system based upon powers of 2.  Only the digits 0 and 1 are used.  101
in binary, for example, is 1 units digit, 0 twos, and 1 fours, or 5 in decimal.

BIT
A single binary digit (a 1 or a 0).  A bit is the smallest unit of storage or
information in a computer.

BIT CELL
The space on a diskette, between two clock pulses, which can hold the value of a
single binary 0 or 1 (bit).

BIT SLIP MARKS
The epilogue of a disk field.  Used to double check that the disk head is still
in read sync and the sector has not been damaged.

BOOT/BOOTSTRAP
The process of loading a very large program into memory by loading successively
larger pieces, each of which loads its successor.  The program loads itself by
"pulling itself up by its bootstraps".

BRK
BREAK.  An assembly langauge instruction which can be used to force an interrupt
and immediate suspension of execution of a program.

BUFFER
An area of memory used to temporarily hold data as it is being transferred to or
from a peripheral, such as a disk drive.

BUG
A programming error.  Faulty operation of a program.

BYTE
The smallest unit of addressable memory in a computer.  A byte usually consists
of 8 bits and can contain a decimal number ranging from 0 to 255 or a single
alphanumeric character.

CARRIAGE RETURN
A control character which instructs the printer to end one line and begin
another.  When printing a carriage return is usually followed by a line feed.

CARRY FLAG
A 6502 processor flag which indicates that a previous addition resulted in a
carry.  Also used as an error indicator by many system programs.

CATALOG
A directory of the files on a diskette.  See DIRECTORY.

CHAIN
A linked list of data elements.  Data is chained if its elements need not be
contiguous in storage and each element can be found from its predecessor via an
address pointer.

CHECKSUM/CRC
A method for verifying that data has not been damaged.  When data is written,
the sum of all its constituent bytes is stored with it.  If, when the data is
later read, its sum no longer matches the checksum, it has been damaged.

CLOBBERED
Damaged or destroyed.  A clobbered sector is one which has been overwritten such
that it is unrecoverable.

CODE
Executable instructions to the computer, usually in machine language.

COLDSTART
A restart of a program which reinitializes all of its parameters, usually
erasing any work which was in progress at the time of the restart.  A DOS
coldstart erases the BASIC program in memory.

CONTIGUOUS
Physically next to.  Two bytes are contiguous if they are adjoining each other
in memory or on the disk.

CONTROL BLOCK
A collection of data which is used by the operating system to manage resources.
Examples of a control block used by DOS are the file buffers.

CONTROL CHARACTER
A special ASCII code which is used to perform a unique function on a peripheral,
but does not generate a printable character.  Carriage return, line feed, form
feed, and bell are all control characters.

CONTROLLER CARD
A hardware circuit board which is plugged into an APPLE connector which allows
communication with a peripheral device, such as a disk or printer.  A controller
card usually contains a small driver program in ROM.

CSWL
A vector in zero-page through which output data is passed for display on the CRT
or for printing.

CYCLE
The smallest unit of time within the central processor of the computer.  Each
machine language instruction requires two or more cycles to complete.  One cycle
(on the APPLE) is one micro-second or one millionth of a second.

DATA
Units of information.

DATA SECTOR BUFFER
On the APPLE, a 256 byte buffer used by DOS to hold the image of any given
sector on the diskette.  As information is read from the file, data is extracted
from the data sector buffer until it is exhausted, at which time it is refilled
with the next sector image.

DATA TYPE
The type of information stored in a byte.  A byte might contain a printable
ASCII character, binary numeric data, or a machine language instruction.

DCT
Device Characteristics Table.  Used as an input parameter table to Read/Write
Track/Sector (RWTS) to describe the hardware characteristics of the diskette
drive.

DECIMAL
A number system based upon powers of 10.  Digits range from 0 to 9.

DEFERRED COMMANDS
DOS commands which may (or must) be invoked from within an executing BASIC
program.  OPEN, READ, WRITE, and CLOSE are all examples of deferred commands.

DIGITAL
As opposed to analog.  Discrete values as opposed to continuous ones.  Only
digital values may be stored in a computer.  Analog measurements from the real
world, such as a voltage or the level of light outside, must be converted into a
numerical value which, of necessity, must be "rounded off" to a discrete value.

DIRECT ACCESS
Peripheral storage allowing rapid access of any piece of data, regardless of its
placement on the medium.  Magnetic tape is generally not considered direct
access, since the entire tape must be read to locate the last byte.  A diskette
is direct access, since the arm may be rapidly moved to any track and sector.

DIRECTORY
A catalog of all files stored on a diskette.  The directory must contain each
file's name and its location on the disk as well as other information regarding
the type of data stored there.

DISK INITIALIZATION
The process which places track formatting information, including sectors and
gaps, on a blank diskette.  During disk initialization, DOS also places a VTOC
and directory on the newly formatted disk, as well as saving the HELLO program.

DISPLACEMENT
The distance from the beginning of a block of data to a particular byte or
field.  Displacements are usually given beginning with 0, for the first byte, 1
for the second, etc.  Also known as an offset.

DRIVER
A program which provides an input stream to another program or an output device.
A printer driver accepts input from a user program in the form of lines to be
printed, and sends them to the printer.

DUMP
An unformatted or partially formatted listing of the contents of memory or a
diskette in hexadecimal.  Used for diagnostic purposes.

ENCODE
To translate data from one form to another for any of a number of reasons.  In
DOS 3.3, Data is encoded from 8 bit bytes to 6 bit bytes for storage on a DISK
II.

ENTRY POINT (EPA)
The entry point address is the location within a program where execution is to
start.  This is not necessarily the same as the load point (or lowest memory
address in the program).

EOF
End Of File.  This mark signals the end of a data file.  $00 for APPLE DOS text
files.

EPILOGUE
The last three bytes of a field on a track.  These unique bytes are used to
insure the integrity of the data which preceeds them.

EXCLUSIVE OR
A logical operation which compares two bits to determine if they are different.
1 EOR 0 results in 1.  1 EOR 1 results in 0.

FIELD
A group of contiguous bytes forming a single piece of data, such as a person's
name, his age, or his social security number.  In disk formatting, a group of
data bytes surrounded by gaps.

FILE
A named collection of data on a diskette or other mass storage medium.  Files
can contain data or programs.

FILE BUFFERS
In APPLE DOS, a collection of buffers used to manage one open file.  Included
are a data sector buffer, a Track/Sector List sector buffer, a file manager
workarea buffer, the name of the file, and pointers.  The DOS command, MAXFILES
3, causes 3 of these file buffers to be allocated.

FILE DESCRIPTOR
A single entry in a diskette directory which describes one file.  Included are
the name of the file, its data type, its length, and its location on the
diskette.

FILE MANAGER
That portion of DOS which manages files.  The file manager handles such general
operations as OPEN, CLOSE, READ, WRITE, POSITION, RENAME, DELETE, etc.

FILE TYPE
The type of data held by a file.  Valid DOS file types are Binary, Applesoft,
Integer-BASIC, Text, Relocatable, S, A, and B.

FIRMWARE
A middle ground between hardware and software.  Usually used to describe
micro-code or programs which have been stored in read-only memory.

GAPS
The spaces between fields of data on a diskette.  Gaps on an APPLE diskette
contain self-sync bytes.

HARD ERROR
An unrecoverable Input/Output error.  The data stored in the disk sector can
never be successfully read again.

HARDWARE
Physical computer equipment, as opposed to programs which run on the equipment.
A disk drive is an example of a hardware component.

HEAD
The read/write head on a diskette drive.  A magnetic pickup, similar in nature
to the head on a stereo tapedeck, which rests on the spinning surface of the
diskette.

HEXADECIMAL/HEX
A numeric system based on powers of 16.  Valid hex digits range from 0 to 9 and
A to F, where A is 10, B is 11, ... , and F is 15.  B30 is 11 256's, 3 16's, and
0 1's, or 2864 in decimal.  Two hexadecimal digits can be used to represent the
contents of one byte.  Hexadecimal is used with computers because it easily
converts with binary.

HIGH MEMORY
Those memory locations which have high address values.  $FFFF is the highest
memory location.  Also called the "top" of memory.

HIMEM
APPLE's zero-page address which identifies the first byte past the available
memory which can be used to store BASIC programs and their variables.

IMMEDIATE COMMAND
A DOS command which may be entered at any time, especially when DOS is waiting
for a command from the keyboard.  Deferred commands are the opposite of
immediate commands.

INDEX
A displacement into a table or block of storage.

INSTRUCTION
A single step to be performed in an assembly language or machine language
program.  Instructions perform such operations as addition, subtraction, store,
or load.

INTEGER
As opposed to floating point.  A "whole" number with no fraction associated with
it.

INTERCEPT
A program which logically places itself in the execution path of another
program, or pair of programs.  A video intercept is used to re-direct program
output from the screen to a printer, for example.

INTERLEAVE
The practice of selecting the order of sectors on a diskette track to minimize
access time due to rotational delay.  Also called "skewing" or interlacing.

INTERRUPT
A hardware signal which causes the computer to halt execution of a program and
enter a special handler routine.  Interrupts are used to service real-time clock
time-outs, BRK instructions, and RESET.

IOB
Input/Output Block.  A collection of parameter data, passed to Read/Write
Track/Sector, describing the operation to be performed.

I/O ERROR
Input/Output Error.  An error which occurs during transmission of data to or
from a peripheral device, such as a disk or cassette tape.

JMP
A 6502 assembly langauge instruction which causes the computer to begin
executing instructions at a different location in memory.  Similar to a GOTO
statement in BASIC.

JSR
A 6502 assembly langauge instruction which causes the computer to "call" a
subroutine.  Similar to a CALL statement in BASIC.

K
A unit of measurement, usually applied to bytes.  1 K bytes is equivalent to
1024 bytes.

KSWL
A vector in zero-page through which input data is passed for from the keyboard
or a remote terminal.

LABEL
A name associated with a location in a program or in memory.  Labels are used in
assembly langauge much like statement numbers are used in BASIC.

LATCH
A component into which the Input/Output hardware can store a byte value, which
will hold that value until the central processor has time to read it (or vice
versa).

LINK
An address pointer in an element of a linked chain of data or buffers.

LIST
A one dimensional sequential array of data items.

LOAD POINT (LP)
The lowest address of a loaded assembly language program -- the first byte
loaded.  Not necessarily the same as the entry point address (EPA).

LOGICAL
A form of arithmetic which operates with binary "truth" or "false", 1 or 0.
AND, OR, NAND, NOR, and EXCLUSIVE OR are all logical operations.

LOOP
A programming construction in which a group of instructions or statements are
repeatedly executed.

LOW MEMORY
The memory locations with the lowest addresses.  $0000 is the lowest memory
location.  Also called the "bottom" of memory.

LOMEM
APPLE's zero-page address which identifies the first byte of the available
memory which can be used to store BASIC programs and their variables.

LSB/LO ORDER
Least Significant Bit or Least Significant Byte.  The 1's bit in a byte or the
second pair of hexadecimal digits forming an address.  In the address $8030, $30
is the LO order part of the address.

MASTER DISK
A DOS diskette which will boot in an APPLE II of any size memory and take full
advantage of it.

MICROSECOND
A millionth of a second.  Equivalent to one cycle of the APPLE II central
processor.  Also written as "Usec".

MONITOR
A machine language program which always resides in the computer and which is the
first to receive control when the machine is powered up.  The APPLE monitor
resides in ROM and allows examination and modification of memory at a byte
level.

MSB/HI ORDER
Most Significant Bit or Most Significant Byte.  The 128's bit of a byte (the
left-most) or the first pair of hexadecimal digits in an address.  In the byte
value $83, the MSB is on (is a 1).

NULL
Empty, having no length or value.  A null string is one which contains no
characters.  The null control character ($00) produces no effect on a printer
(also called an idle).

NIBBLE/NYBBLE
A portion of a byte, usually 4 bits and represented by a single hexadecimal
digit.  $FE contains two nibbles, $F and $E.

OBJECT CODE
A machine language program in binary form, ready to execute.  Object code is the
output of an assembler.

OBJECT MODULE
A complete machine language program in object code form, stored as a file on a
diskette.

OFFSET
The distance from the beginning of a block of data to a particular byte or
field.  Offsets are usually given beginning with 0, for the first byte, 1 for
the second, etc.  Also known as a displacement.

OPCODE
Operation Code.  The three letter mnemonic representing a single assembly
langauge instruction.  JMP is the opcode for the jump instruction.

OPERATING SYSTEM
A machine language program which manages the memory and peripherals
automatically, simplifying the job of the applications programmer.

OR
The logical operation comparing two bits to determine if either of them are 1.
1 OR 1 results in 1 (true), 1 OR 0 results in 1, 0 OR 0 results in 0 (false).

OVERHEAD
The space required by the system, either in memory or on the disk, to manage
either.  The disk directory and VTOC are part of a diskette's overhead.

.nx appendix c.2