The Buft'er Pointer - Commodore 1541 User Manual

EXAMPLES:
To restore track 18, sector 1 of the directory from the disk buffer filled by the Black
Read example on page 82, we can use
PRINT#l5,"U2";5;0;18;1
We'll return to this example on the next page, after we learn to alter the directory in a
useful way.
We can also use a Block Write to write a name in Track l, Sector 1, a rarely-used
sector. This can be used as a way of marking a diskette as belonging to you . Here is a
program to do it, using the alternate form of the Block Write command:
110 INPUT" YOUR NAME" ;NA$
120 OPEN 15,8, 15
130 OPEN 4,8,4, "#"
140 PRINT#4,NA$
150 PRINT#l5,"U2" ;4;0;1;1
160 CLOSE 4
170 CLOSE 15
!RO END
Enter a name
Open command channel
Open direct access channel
Write name to buffer
Write buffer to Track 1,
Sector 1 of diskette
Tidy up after
THE ORIGINAL BLOCK-READ AND BLOCK-WRITE COMMANDS (EXPERT
USERS ONLY)
Although the Block Read and Block Write commands are nearly
~I
ways replaced by
the U 1 and U2 commands respectively. the original commands can still be used , as long
as you fully understand their effects . Unlike U 1 and U2, B-R and B-W allow you to rea. d
or write less than a full sector. In the case of B-R, the first byte of the selected sector
15
used to set the buffer pointer (see next section), and determines how many bytes of
th~t
sector are read into a disk memory buffer. A program may check to be sure it doesn
1
h
. f
the
attempt to read past the end of data actually loaded into the buffer, by watc mg or
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. and
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·
MANOS:
PRINT#l5,"BLOCK-READ";channel #;drive #;track #;sector#
abbreviated as: PRINT#l5,"B-R";channel #;drive #;track #;sector#
and
PRlNT#15,"BLOCK-WRITE";channel #;drive #;track#; ector #
68
llJl>reviated as: PRINT#l5,"B-W";channel #;drive #;track #;sector#
wtiere "channel #" is the channel number specified when the file into which the block
\\'ill be read was opened, "drive#" is the drive number (always 0 on the 1541), and
··uack #" and "sector #" are respectively the track and sector numbers containing the
11e5ired block of data to be partially read into or written from the file buffer.
IMPORT ANT NOTES:
I . 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, and 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.
THE BUFFER POINTER
The buffer pointer points to where the next Read or Write will begin within a disk
memory buffer. By moving the buffer pointer, you can access individual bytes within a
block in any order. This allows you to edit any portion of a sector, or organize it into
fields, like a relative record .
FORMAT FOR THE BUFFER-POINTER COMMAND:
PRINT# 15, "BUFFER-POINTER" ;channel #;byte
USually abbreviated as: PRINT# 15, "B-P" ;channel #;byte
""here
"channel #" is the channel number specified when the file reserving the buffer
'N&s
opened, and "byte" is the character number within the buffer at which to point.
ALTERNATE FORMATS:
PRINT#l5 ,"B-P:"channel #;byte
PRINT#l5, "B-P:channel #;byte"
EXAMPLE:
Here is a program that locks the first program or file on a 1541 diskette. It works by
!tading the start of the directory (Track 18, Sector I) into disk memory, setting the buffer
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