276
The
VIC
20
User
Guide
Here
is
a routine you can use to free any block on the diskette.
10
OPEN
15.8.15
20
INPUT
"TRACK";
A
30
INfI'UT
"SECTOR";
B
40
PRINTt15.
"B-F:"0;A;B
50
CLOSE
15
99
END
DISKEnE
MEMORY
MANIPUlATION
The
1540
controller interprets external commands and causes the disk
drive mechanism to carry them out. The controller contains a 6502 micro-
processor, just like the one inside the VIC
20.
It
has 2K of RAM and the
Disk Operating System (DOS), which
is
contained on two ROM chips.
Some
of
this memory
is
used for the buffers discussed in the last
few
sections; some
is
used for housekeeping purposes such as maintaining the
BAM
data
and special file information; and some
is
available for you to use
for special applications.
The RAM that
is
available for you to write routines on
is
the same
RAM that
is
used by DOS for the buffers.
If
you decide to write a special
machine language program into those areas you will have to keep track of
which areas you are using and which areas you will reserve for buffers.
There are
five
pages of memory, each of which contains
256
bytes.
Buffer Memory Location
(Hexadecimal)
#1
300-3FF
#2
400-4FF
#3
500-5FF
#4
600-6FF
#5
700-7FF
It
is
not advisable to use buffer
#5,
because this buffer
is
often used by
DOS for various housekeeping activities. Although it
is
not always in use,
data placed there may alter DOS procedures or may be written over by
DOS.
The memory space in buffers I through 4
is
used only by the buffers.
If
you haven't requested a buffer, and you know that one has not been opened
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