Commodore 1541-II - Page 84

FORMAT FOR THE MEMORY-WRITE COMMAND:
PRINT# 15, * 'M- W 'CHR$(<address)CHR$(>address)CHR$
(# of bytes)CHR$(data byte(s))
where "<address" is the low order part, and t4>address" is the high order part of the
address in disk memory to begin writing, *'# of bytes" is the number of memory
locations that will be written (from 1-34), and "data byte" is 1 or more byte values to be
written into disk memory, each as a CHR$() value. If desired, a colon (:) may follow M-
W within the quotation marks.
ALTERNATE FORMAT:
PRINT# 15/ 'M-W:' 'CHR$(<address)CHR$(>address)CHR$
(# of bytes)CHR$(data byte(s))
EXAMPLES:
We can use this line to turn off the "bumps" when loading DOS-protected programs (i.e.,
programs that have been protected against being copied by creating and checking for
specific disk errors).
PRINT#15,ttM-W"CHR$(106)CHR$(0)CHR$(l)CHR$(133)
The following line can be.used to recover bad sectors, such as when an important file has
been damaged and cannot be read normally.
PRINT#15,ttM-W"CHR$(106)CHR$(0)CHR$(l)CHR$(31)
The above two examples may be very useful under some circumstances. They are the
equivalent of POKE 106,133 and POKE 106,31 respectively, but in disk memory, not
inside the computer. As mentioned in the previous section's first example, location 106 in
the 1541 disk drive signifies three separate activities to the drive, all related to error
recovery. Bit 7 (the high bit), if set means no bumps (don't thump the drive back to track
1). Bit 6, if set, means no seeks. In that case, the drive won't attempt to read the half-track
above and below the assigned track to see if it can read the data that way. The bottom 6
bits are the count of how many times the disk will try to read each sector before and after
trying seeks and bumps before giving up. Since 31 is the largest number that can be
expressed in 6 bits, that is the maximum number of tries allowed.
From this example, you can see the value of knowing something about Peeks, Pokes,
and machine-language before using direct-access disk commands, as well as their poten
tial power.
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