Commodore Plus 4 - Page 351

The Disk Drive 339
Line-by-Line Explanation
10 Open the command channel and initialize the disk.
20 Check for error. If one is found, exit.
30 Open the direct access channel.
40 Check for error. If one is found, exit.
50 Ask for a string to write.
60 Put the input string followed by a carriage return in the buffer of the 1541.
70 Ask which track and sector to use.
80 Write the buffer to the disk.
90 Check for error. If one is found, exit.
100 Read the sector back into the buffer.
110 Check for error. If one is found, exit.
120 I will count the bytes.
130 Get 1 byte from the buffer.
140 Output the byte to the screen.
150 If the byte is a carriage return, the end of the string has been reached.
Otherwise, go on to the next byte.
160 Close the files.
Block Allocate (B-A)
The BAM is a record of which sectors on the disk are in use. When a sector is
allocated, it is safe from being written on by the DOS in the course of normal
writing (such as a SAVE). It is not safe from being written on by the direct-access
commands, nor is it safe from being deallocated by a validate (COLLECT)
operation. The block allocate command updates the BAM to show the designated
sector as used. The BAM is actually written out to the disk when a direct-access
channel is closed, so it is a good idea to open and close a direct-access channel
when allocating a block, even if it is not needed for any other purpose. The syntax
for a block allocate is
PRINT* fUenumber;'B-A:";drive;track;sector
where Jilenumber is the logical file number of the command channel and drive is
the drive number. The track can be any track on the diskette (1 to 35) and the
sector any sector of that track.