Apple Orchard 1980 Fall v1n2 reduced

FALL 1980
THE APPLE ORCHARD
PAGE 49
There are many sections of the
Apple I l's disk operating system
(DOS) which seem especially designed
to keep you waiting for
eternity. My pet bugaboo for a long
time has been the file handling
routines. Since I couldn't do anything
about them, I began looking
for other places in the DOS to
improve disk speed. The most
obvious start was the "INIT" command.
Fortunately it yielded to
improvements, which I will try to
describe.
This article applies to DOS 3.2.1
in a 48K machine. Addresses mentioned
may be converted to 3.2 or
3.1 by subtracting four bytes from
the given address. Users with less
memory must subtract the appropriate
amount of memory (1K +
1024 bytes).
Before describing the modifications
I made, you should know how
the "INIT" command works, then
you will understand what the
changes do. It is convenient to
think of commands in the DOS as
operating at two levels. The first is
the command level with deals with
interpretation of the "INIT" command
you type. Most command
level operations aren't worth modifying
unless you plan to rewrite
about 10K of machine language
DOS. That is why file handling is
difficult to improve. The second
level is machine level. This mainly
involves a routine called Read/
Write Track and Sector (or RWTS)
which does all of the disk access.
RWTS, fortunately, is well documented
in the original Wozpak.
Most time during INIT is spent at
the machine level in RWTS's disk
formatter. For the purpose of this
explanation we will assume that
the INIT command has been properly
entered and decoded by
INSIDE INITIALIZATION
by Joseph H. Budge
Carolina Apple Corps
co~mand level DOS. We shall
enter at the point where RWTS is
being called with the command to
format a diskette:
After turning on the disk drive,
RWTS calibrates the drive's read/
write head to track zero. Calibration
is accomplished by stepping
the read head through 127 tracks.
Since there are only 35 tracks in the
first place, the head hits a stop at
track 0 and halts there, generating
the jackhammer-like noise you
frequently hear. Once calibrated,
RWTS steps through each track,
erasing and formatting as it goes. To
erase, RWTS writes over everything
with one byte, $FF, in a loop that
repeats 7,000 times per track. This
will write over each track several
times before the track is formatted.
The erasure cleans off any previous
data that may be on the disk, at least
most of the time. Sometimes the
manufacturer may put a test signal
on the disk that's too strong to
erase. lnthatcasealargenumberof
1/0 ERRORs on each disk in the
batch tell you to go find a bulk
eraser. Audio recording tape erasers
do a good job.
After erasure, formatting a track
proceeds sector by sector, with
each sector essentially identical. To
format a sector, RWTS first writes a
number of timing bytes onto the
disk. These bytes are $FF's with a
special spacing on the disk, and
there must be at least 16 of them
before each sector. Next RWTS
writes an address block for the
sector. The block contains four
elements. Three starting marks
($DS, $AA and $BS) begin the
block; they tell the DOS that data is
to follow. Then comes the address
data itself. Volume number, track
number, and sector number are
written in encoded form. Next
comes the checksum. Finally three
end of data marks bring up the rear
($AA, $EB, and $FF) .
It is helpful to know thatdata isn 't
put onto the disk in the same form
as the DOS receives it from your
programs or the keyboard. All data
sent to the DOS arrives there in the
standard Apple format of eight-bit
words (bytes). But the data can't be
stored on the disk that way for a
variety of reasons. Instead, the
RWTS routines go through a complicated
procedure to recode the
data. In brief: the data is split into
five bit nybbles*, data from each
nybble is combined with data from
other nybbles, creating a pointer to
a table. RWTS gets a byte from the
table and sends that value to the
disk. Believe it or not, the value sent
to the disk is once again an eight-bit
word. But it now only represents
five bits of your information which
are all scrambled. This encoded
byte is sent to the disk hardware,
which stores each bit on the
diskette in serial form, one bit at a
time. Naturally the read section is
smart enough to figure out all this
encoding to extract the information
when you want it.
The upshot of all this encoding is
that no data is stored on the disk as
one byte. The sector number 0, for
example, is written as "$AA, $AA."
The volume number, track number,
and sector number each take
two bytes. One sector of data,
normally 256 bytes, becomes 410
bytes when encoded. Timing
marks, beginning marks, and ending
marks are the only single bytes
ever put on the disk.
The last two paragraphs of diversion
help explain what INIT does
next. You will recall that we left
RWTS just as it finished formatting a
sector's address block. Now, ordinarily
the address block would be
followed by a few timing bytes and
then the sector's data block. Like an
address block, the data block contains
beginning marks, encoded
data, a checksum, and ending
marks. The beginning marks for a
data block are: ($DS,$AAand $AD),
while its end marks are: ($DE, $AA
and $EB). These are different from
the beginning and ending marks in
an address block. But the formatter
in RWTS is lazy. Since no data needs
to be written during the formatting
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