Commodore 1541-II - The Buffer Pointer

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 (always 0 on the 1541), and
44track #" and "sector #" are respectively 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, 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,<4BUFFER-POINTER";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 "byte" is the character number within the buffer at which to point.
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 1541 diskette. It works by
reading the start of the directory (Track 18, Sector 1) into disk memory, setting the buffer
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