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u Terms and acronyms
by Peter Schepers
Many strange terms have come along
with computers in general, and I will
not attempt to explain them all, but
some of the ones in this document may
not be entirely clear. I will attempt
to make things a little easier by
explaining some of the more common
ones.
<CR> - Short form for a Carriage
Return ($0D) symbol.
<LF> - Short form for a Line Feed
($0A) symbol.
ARCHIVE - A file format which
contains other files, and in which
compression is an integral part of the
design. Some examples are ZIP, SRK,
SDA, ARC, LHA, WRA.
ASCII - This is an acronym for
"American Standard Code for
Information Interchange". The standard
is a 7-bit code covering control
codes, punctuation, alphanumeric (A to
Z, 0 to 9) as well as math and a few
other symbols. Since it is a 7-bit
code, it ranges from $00 to $7F
(0-127). This leaves the top 128-255
definable by the vendor. The PC world
has corrupted this standard making it
8-bit.
BAM - An acronym for "Block
Availability Map". Here is where the
disk operating system keeps track of
what sectors are allocated (or
available) for each track.
BLOCK - This refers to sectors which
on a logical level are grouped
together. On a 1541 disk, it could be
a series of sectors linked together in
a file, or a partition on a 1581 disk.
In the PC world it represents a
"cluster" of sectors. Generally if I'm
referring to a grouping of sectors
thats *not* 256 bytes large, then I
talk in blocks.
BYTE - A group of 8 bits, the
contents of a memory location.
CHAIN - A series of sectors linked
together. One sector will have a
pointer to another, and that sector
will point to another, until the
chain has no more forward pointers. A
file stored on a 1541 disk would be
considered a chain of sectors, but it
also has a directory entry
explaining what the chain is for.
CONTAINER - A file format which
simply contains other files, and
no compression takes place. Some
examples are T64, P00, SPY, ARK,
LNX.
FILETYPE - In the Commodore world,
this would be the kind of file, be it
SEQ, REL, PRG, USR, GEOS etc. In the
DOS world, this would possibly be
the file extension, be it EXE, TXT,
DOC. It tells the user what file it
is, making usage easier.
GCR - An acronym for "Group Code
Recording". This is the encoding
method Commodore uses to physically
store the information on most of the
5.25" disks (i.e. 1541). It encodes an
8 bit sequence (2 4-bit nybbles) into
a 10 bit sequence (2 5-bit nybbles)
so that long repeated sequences of
1's or 0's are avoided. These must be
avoided so that the timing of
reading/writing to the disk won't
become "out of sync". As a user, you
would not normally see the GCR
information since the drive does all
the conversion to normal HEX data
before it gives it to you.
HIGH/LOW - The bytes here are stored
backwards compared to the LOW/HIGH
method. See LOW/HIGH for more
information.
IMAGE - A file format which is a PC
equivalent of a physical Commodore
media. Some examples are D64 (1541),
D71 (1571), D81 (1581), D2M (FD2000),
X64.
LINK - This is the track/sector
values, stored in the first two bytes
of a sector, which point to (or "link"
to) the t/s location of the next
sector. A series of these links
comprise a "chain" of sectors.
LOW/HIGH - This is how values are
stored when they exceed one byte. A
good example of this is the sector
count of a D64 file. To calculate the
actual value, take the second value,
multiply it by 256 and add the low
value. You will now get the real
decimal value. i.e.
(HIGH*256)+LOW=result. If you look at
is as a HEX value, swap the bytes
around and put them together for the
16-bit HEX value. i.e. $FE $03 would
be $03FE as a 16-bit HEX value.
LSB/MSB - See LOW/HIGH.
LSU - This is my own acronym meaning
"last sector useage". It is the
value stored in byte position $01 (the
"sector" value of the t/slink) of
the last sector of a file. This value
is the offset into the sector where
the last byte is stored. It also
represents the byte count + 1, since a
value of 255 would actually mean only
254 bytes of file data exists (full
sector less the 2 bytes for t/schain).
Without reasonable knowledge of the
disk layout, this byte can be
confusing, and hard to explain.
NYBBLE - A grouping of 4 bits (half
a byte), either the first or last
4bits of an 8-bit binary number, or
one half of a two-digit
hexadecimal number. Typically, a byte
will be broken down into two parts,
the top 4 bits and the bottom 4 bits.
These are referred to as the upper and
lower nybble respectively, and are
represented by two hexadecimal digits
in base 16.
PETASCII - (or PETSCII) This is
Commodore's version of ASCII (the
PETpart of the name comes from the
first computer to use the code,
the PET or Personal Electronic
Transactor).Most of the codes from
0-127 are the same as ASCII, but there
are differences, especially noticible
when converting text from a C64 to
a DOS machine. Where ASCII has
uppercase characters, PETASCII has
lower case ones, and vice versa. Also,
the top 128 characters (128 to 255)
are quite different from the PC
"standard".
RLE - An acronym for "Run Length
Encoding". This is a simple
compression method, employed by most
compression programs, and also used by
some file formats (ZipCode, CPK). It
encodes sequences (or "runs", hence
the name "RUN length...") of the same
byte (i.e. 00 00 00 00 00 00) into a
smaller string using a shorter code
sequence, makingthe resultant file
smaller than the original. This is the
simplest form of file compression.
SECTOR - It is best described as the
method that the drive uses to store
the smallest group of bytes physically
on the disk. On the 1541 this refers
to a group of 256 bytes stored
together in a single sector. On a PC
disk, this is typically 512 bytes.
SIGNATURE - A group of bytes,
usually near or at the front of the
file, which are used to identify the
type of file. I.e. a PC64 file will
always have the signature string
"C64file" contained at the beginning
of the file.
TAR - An acronym for "Tape
ARchiver", a UNIX application, and
method of backing up information.
Commodore Free Reprinted with the
permission of Peter Schepers
http://www.unusedino.de/ec64/technical
/formats/intro.html
...end...
www.commoorefree.com