Commodore 1581 - Block-Read Command

As the loop progresses, the contents of the specified track and sector
are copied into computer memory, beginning at the address set by
variable MB in line 160, and may be examined and altered there.
The DOS always checks that the track and sector parameters of the
BLOCK-READ command are within the proper range. If they're not, a
"66 ILLEGAL TRACK AND SECTOR" error occurs. In certain instances it
might be necessary to access a track and sector that are not within what
the DOS considers the proper bounds. This is a special case and, un
less absolutely necessary, should be avoided. Nonetheless, there is a
command identical in function to "Ul" that doesn't check to see if the
track and sector parameters are. within bounds before attempting to
read it. Its format is:
PRINT#15,"B-_";channel #;track #;sector #
(The character following the B- is a shifted R.)
or
PRINT#15,"B-";CHR$(210);channel #;track #;sector #
BLOCK-WRITE
The purpose of a BLOCK-WRITE is to save the contents of a file
buffer into a specified sector. It is thus the reverse of the BLOCK-READ
command. Although the BLOCK-WRITE command (B-W) is still part of
the DOS command set, it is nearly always replaced by the U2 com
mand.
FORMAT FOR THE BLOCK-WRITE COMMAND:
PRINT#15,"U2";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 respectively the track and
sector numbers that should receive the block of data being saved from
the file buffer.
ALTERNATE FORMATS:
PRINT#15,"U2:"channel #;drive #;track #;sector #
PRINT#15,"UB:"channel #;drive #;track #;sector #
PRINT#15,"U2:channel #,drive #,track #,sector #"
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