Commodore 1581 - 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,uBUFFER-POINTERn;channel#;byte
usually abbreviated as: PRINT#15,"B-P";channel #;byte
where "channel #" is the channel number specified when the file
reserving the buffer was opened, and ubyte" is the character number
within the buffer at which to point (from 0 through 255).
ALTERNATE FORMATS:
PRINT#15,"B-P:"channel #;byte
PRINT#15,"B-P:channel #;byte"
EXAMPLE:
Here is a program that locks the first program or file on a diskette.
It works by reading the start of the directory (Track 40, Sector 3) into
disk memory, setting the buffer pointer to the first file type byte (see
Appendix C for details of directory organization), locking it by setting
bit 6 and rewriting it.
110 OPEN 15,8,15 Open command channel.
120 OPEN 5,8,5,"#" Open direct access channel.
130 PRINT#15/'Ul";5;0;40;3 Read Track 40, Sector 3.
140 PRINT#15,"B-P";5;2 Point to Byte 2 of the buffer.
15OGET#5A$:IFA$ = ""
THEN A$ = CHR$(0) Read it into memory.
160 A=ASC(A$) OR 64 Turn on bit 6 to lock.
170 PRINT#15,"B-P";5;2 Point to Byte 2 again.
180 PRINT#5,CHR$(A); Overwrite it in buffer.
190 PRINT#15,uU2";5;0;40;3 Rewrite buffer to diskette.
200 CLOSE 5 Tidy up after.
210 CLOSE 15
220 END
74