280
The
VIC
20
User
Guide
with
VI.
The B-R command reads only the data in a particular block
(sector). The VI command reads all the information in the block, including
the two bytes that precede the 254 data bytes. These bytes contain the link to
the next block. The link
is
the track and sector to go to next.
U2
The
V2
command
is
similar to BLOCK-WRITE. The format
of
the
V2
command
is
identical to that
of
B-
W.
The difference between these two
commands
is
that invoking
B-
W terminates the file.
That
is,
the track and
sector link (the two bytes
that
precede the 254 bytes
of
data
in the block) are
set to indicate
that
this block
is
the end
of
a file. The next track
and
sector
are not pointed to.
Obviously, this can cause some problems.
If, for instance, you wanted
to write some
data
into the middle
of
some existing data, using the
B-
W
instruction would end the file in the middle.
V2, however, would not.
It
allows you to write in the next track and sector
or
to leave it the same.
U3·U9
The next
few
user commands are similar to the MEMORY -EXECVTE
command. They
jump
to a specific location in memory
and
begin executing
at that location. The locations they jump to are listed in Table 8-3.
The syntax for the
V3 command, for example,
is
OPEN
15,8,15
PRINTfU5,
"U3:"
CLOSE
15
The command
V:
(or
VJ)
will
jump
the DOS to its power-up routine.
The locations
that
the user commands
jump
to are only three bytes long.
This
is
because they are intended to contain a
JMP
machine code instruc-
tion to go to a program that you define.
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