Memory Read; Memory Write - Commodore 1541 User Manual

Other Resources:
More detailed information about Commodore disk drives can be found in these
books:
Inside Commodore DOS, by Imrners & Neufeld
(Datamost, cl984)
The Anatomy of the 1541 Disk Drive, by Englisch & Szczepanowski
(Abacus, cl984)
Programming the PET/CBM, by West (Level Limited, c 1982)
The PET Personal Computer Guide, by Osborne & Strasmas
(Osborne/McGraw-Hill, c 1982)
MEMORY-READ
The disk contains l 6K of ROM (Read-Only Memory), as well as 2K of RAM (Read-
Write Memory) . You can get direct access to any location within these, or to the buffers
that the DOS has set up in RAM, by using memory commands. Memory-Read allows you
to select which byte or bytes to read from disk memory into the computer. The Memory-
Read command is the equivalent of the Ba ic Peek() function, but reads the disk's
memory instead of the computer's memory.
Note: Unlike other disk commands , those in this chapter cannot be spelled out in
full. Thus, M-R is correct, but MEMORY-READ is not a permitted alternate
wording .
FORMAT FOR THE MEMORY-READ COMMAND:
PRINT# 15, "M-R "CHR$( <address)CHR$(>address)CHR$( # of bytes)
where "<address" is the low order par\, and ">address" is the high order part of the
address in disk memory to be read . If the optional "#of bytes" is specified, it selects
'how many memory locations will be read in, from 1-255 . Otherwise, I character will be
read . If desired, a colon (:) may follow M-R inside the quotation marks .
ALTERNATE FORMAT:
PRINT# I 5, "M-R: "CHR$( <address)CHR$(>address)CHR$(
#
of bytes)
'The next byte read using the Get#
statement through channel #15 (the error
channel), will be from that address in the disk controller's memory, and
success~ve
bytes
will be from successive memory locatjons .
Any
lnput#
from the error channel will give peculiar results when you're uiing this
command.
This can be cleared up by sending any other command to the disk,
except
another memory command.
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AMPLES :
To see how many tries the disk will make to read a particular sector, and whether
k
" one-half track to each side will be attempted if a read fails , and whether
.. see
s .
s" to track one and back will be attempted before declanng the sector unreadable ,
"bUmP . · · · f d' k
can-use the following lines. They will read a special. variable m the zer? page o is
:mory, called REYCNT . It is located at $6A hexadecimal ($6A hexadecimal = 6 x 16
+JO=
106) .
110 OPEN 15,8, 15
ilO PRINT#l5 , "M-R"CHR$(106)CHR$(0)
Open command channel
Same as G = PEEK(
I 06)
130 GET#l5 ,G$:LF G$= ''''THEN G$=CHR$(0)
140 G = ASC(G$)
!So
B =G AND 128:B$= "ON" :IF B THEN B$= "OFF" Check bit 7
!60
S = G AND 64:S$ = "ON":IF S THEN S$ = "OFF" Check bit 6
t70 T = G AND 31 :PRINT "#OF TRIES IS";T Check bits 0-5
!80
PRINT "BUMPS ARE" ;B$ and give results
t90 PRINT "SEEKS ARE" ;S$
ZQOCLOSE 15
Tidy up after
210END
Here's a more general purpose program that reads one or more locations anywhere in disk
memory:
110OPENI5 ,8,15
120 INPUT"# OF BYTES TO READ (O= END)";NL
130 IF NL<I THEN CLOSE 15:END
140 IF NL>255 THEN 120
150 INPUT"STARTING AT ADDRESS";AD
160 AH = INT(AD/256) :AL = AD-AH*256
170 PRINT#15 ,"M-R"CHR$(AL)CHR$(AH)
CHR$(NL)
180 FOR I= I TO NL
190: GET# 15,A$:1F A$="" THEN A$= CHR$(0)
200 :
PRINT ASC(A$);
210
NEXT
I
220 PRINT
230GOTO 120
MEMORY-WRITE
Open command channel
Enter number of bytes wanted
unless done
or way out of line
Enter starting address
Convert it into disk form
Actual Memory-Read
Loop til have all the data
printing it as we go
Forever
The Memory-Write command is the equivalent of the Basic Poke command'. but has
its
effect
in disk memory instead of within the computer. M-W allows you to wnte up
to
l4
bytes at a time into disk memory. The Memory-Execute and some User commands can
be
Used to run any programs written this way.
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