Block-Write - Commodore 1551 User Manual

EXAMPLE:
Here is a complete program to read a sector into disk memory using U I, and from
there into computer memory via Get#. (If a carriage return will appear at least once in
every 88 characters of data, Input# may
be
used in place of Get#).
110 MB
=
7936:REM $1 FOO
120 INPUT"TRACK TO READ";T
130 INPUT"SECTOR TO READ";S
140 OPEN 15,8,15
150 OPEN 5,8,5, "#"
160 PRINT#15,"UI";5;0;T;S
170 FOR I
=
MB TO MB
+
255
180 GET#5,A$:IF A$
=" "
THEN A$
=
CHR$(O)
190: POKE I,ASC(A$)
200 NEXT
210 CLOSE 5:CLOSE 15
220 END
Define a memory buffer
Select a track
and sector
Open command channel
Open direct access channel
Read sector into disk buffer
Use a loop to
copy disk buffer
into computer memory
Tidy up after
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. This is the basis for programs like "DlSPLA Y T & S" on the
Test/Demo diskette.
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 command.
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 (always 0 on the 1551); 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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