Commodore 1581 - Original Block-Read and Block-Write Commands

mines how many bytes of that sector are read into a disk memory
buffer. A program may check to be sure it doesn't attempt to read past
the end of data actually loaded into the buffer, by watching for the
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. 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
COMMANDS:
PRINT#15,"BLOCK-READ";channel #;drive #;track #;sector #
abbreviated as: PRINT#15,UB-R";channel #;drive #;track #;sector #
and
PRINT#15,uBLOCK-WRITE";channel #;drive #;track #;sector #
abbreviated as: PWNT#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, and "track #" and "sector #" are the track and sector
numbers containing the desired block of data to be partially read into
or written from the file buffer.
-NOTE
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.
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. 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.
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