THE ORIGINAL BLOCK-READ AND BLOCK-WRITE COMMANDS (EXPERT
USERS)
Although the BLOCK-READ and BLOCK-WRITE commands are nearly always
replaced by the U I and
U2
commands respectively, the original commands can still
be
• used,
as
long as you fully understand their effects. Unlike U I and U2, B-R and
B-
Wallow
you to read or write less than a full sector. In the case of B-R, the first byte of the selected
sector
is
used to set the buffer pointer (see next section), and determines how many bytes
of
that sector are read into a disk memory buffer. A program
may
check
to
be
sure
it
doesn't attempt to read past the end
of
data actually loaded into the buffer, by watching for
the value
of
the file status variable ST to change from 0
to
64. When the buffer
is
written
back to diskette by
B-
W, the first byte written
is
the current value
of
the buffer pointer.
Only that many bytes are written into the specified sector. B-R and B-W may thus be
useful
in
working with custom-designed
file
structures.
FORMAT FOR THE ORIGINAL BLOCK-READ AND BLOCK-WRITE COM-
MANDS:
PRINT#15,"BLOCK-READ";channel #;drive #;track #;sector #
abbreviated
as:
PRINT#15,"B-R";channel
#;drive #;track #;sector #
and
PRINT#15,"BLOCK-WRITE";channel #;drive #;track #;sector #
abbreviated
as:
PRINT#15,"B-W";channel
#;drive #;track #;sector #
where "channel
#"
is
the channel number specified when the
file
into which the block
will be read was opened,
"drive
#"
is
the drive number, and "track
#"
and "sector
#"
are the track and sector numbers containing the desired block
of
data
to
be
partially read
into or written from the
file
buffer.
IMPORTANT NOTES:
1.
In a true BLOCK-READ, the first byte
of
the
selected sector
is
used
to
determine how many bytes
of
that sector to read into the disk memory buffer. It
thus cannot be used to read an entire sector into the buffer,
as
the first data byte
is
always interpreted
as
being the number of characters
to
read, rather than part of the
data.
2.
Similarly,
in
a true BLOCK-WRITE, when the buffer
is
written back
to
diskette, the first byte written
is
the current value
of
the buffer pointer. Only that
many bytes are written into the specified sector.
It
cannot
be
used to rewrite an
entire sector onto diskette unchanged, because the first data byte
is
overwritten
by
the buffer pointer.
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