Block Execute - Commodore 1541 User Manual

FORMAT FOR THE MEMORY-WRITE COMMAND:
PRINT# 15,' ' M-W' 'CHR$( <address)CHR$(>address)CHR$
(#of bytes)CHR$(data byte(s))
where "<address" is the low order part, and ">address" is the high order part f
address in disk memory to begin writing, "# of bytes" is the number of m o the
I . h ·11 . emory
oc.allon.s t at .w1 be wntten (from 1-34), and "data byte" is I or more byte values to
be
wntt~n
.mto disk memory, each as a CHR$() value.
If desired,
a colon(:) may follow
M
W wl!hm the quotation marks . ·
ALTERNATE FORMAT:
PRINT# 15 ,' 'M-W :' ' CHR$( <address )CHR$( >address )CHR$
(#of bytes)CHR$(data byte(s))
EXAMPLES :
We can use this line to tum off the
"bum~s" w~en
loading DOS-protected
program~
(i.e.,
prog:ams . that have been protected agamst bemg copied by creating and checking for
specific disk errors) .
PRINT# 15 , " M-W"CHR$( 106)CHR$(0)CHR$( I )CHR$( 133)
The following line can be used to recover bad sectors, such as when an important file has
been damaged and cannot be read normally .
PRINT# 15,' 1M-W " CHR$( I 06)CHR$(0)CHR$( I )CHR$(3 I)
. The above two examples may be very useful under some circumstances . They are the
~q~1valent
of POKE I 06 ,
13~
and '.0KE I 06,31 respectively, but in disk memory , not
ms1de the
c?mput~r.
As mentioned m the previous section's first example, location 106 in
the 1541 d1.sk dnve signifies three separate activities to the drive, all related to error
recov.ery ·.BI! 7 (the high bit), if set means no bumps (don't thump the drive back to track
I). Bit 6, if set, means no seeks . In that case, the drive won ' t attempt to read the half-track
a?<>ve and below the assigned track to see if it can read the data that way . The bottom 6
bi!~
are the count of how many times the disk will try to read each sector before and after
trymg
see~s an~
bumps before giving up. Since 31 is the largest number that can
be
expressed m 6 bits, that is the maximum number of tries allowed .
Fro~
this example, you can see the value of knowing something about Peeks, Pokes,
~nd
machme-language before using direct-access disk commands, as well as their poten-
llal power.
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~MORY-EXECUTE
Any routine in disk memory, either
in
RAM or ROM, can be executed with the
\fclllory-Execute command. It is the equivalent of the Basic Sys call to a machine
IJll8uage program or subroutine, but works in disk memory instead of within the com-
p0ter.
f()RMAT FOR THE MEMORY-EXECUTE COMMAND:
PRINT#15 , ''M-E''CHR$(<address)CHR$(>address)
*'1Cre "<address" is the low order part, and ">address" is the high order part of the
address in disk memory at which execution is to begin .
ALTERNATE FORMAT:
PRINT# 15 , "M-E:' 'CHR$( <address)CHRS(>address)
EXAMPLE:
Here is a Memory-Execute command that does absolutely nothing. The first instruc-
tion it executes is an RTS, which ends the command:
PRINT# 15, "M-E"CHR$(88)CHR$(242)
A more plausible use for this command would be to artificially trigger an error message .
Don't forget to check the error channel, or you'll miss the message:
PRINT#15 ,''M-E''CHR$(201)CHR$(239)
However, most uses require intimate knowledge of the inner workings of the DOS, and
preliminary setup with other commands, such as Memory-Write.
BLOCK-EXECUTE
This rarely-used command will load a sector containing a machine language routine
into
a memory buffer from diskette, and execute it from the first location within the
buffer, until a ReTum from Subroutine (RTS) instruction ends
the
command.
FORMAT FOR THE BLOCK-EXECUTE COMMAND:
PRINT#l5 , "B-E";channel # ;drive #;track #;sector#
~re
"channel # " is the channel number specified when the file into which the block
\\ill be loaded was opened, " drive#" is
the drive number (always 0 on~ 1541), and
"track #" and "sector# " are respectively the track and sector numbers containing the
desired block of data to be loaded into the file buffer and executed there .
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