Using Random Files (Advanced); Chapter 8: Internal Disk Commands - Commodore 1541 User Manual

EXAMPLE:
To free the sector in which we wrote our name in the Block Write example, and
allocated in the first Block-Allocate example, we could use the following command:
PRINT#l5, "B-F";O;l ;l
USING RANDOM FILES (ADVANCED
U~ERS
ONLY)
By combining the commands in this chapter, it is possible to develop a file.
handing program that uses random files. What you need to know now is how to
keep track of which blocks on the disk such a file has used. (Even though you know
a sector has not been allocated by your random file, you must also be sure it wasn't
allocated by another unrelated file on the diskette.)
The most common way of recording which sectors have been used by a random file is
in a sequential file. The sequential file stores a list of record numbers, with the track,
sector, and byte location of each record . This means three channels are needed by a
random file: one for the command channel, one for the random data, and the last for the
sequential data.
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CHAPTERS
INTERNAL DISK COMMANDS
Expert programmers can give commands that directly alter the workings of
the 15~ I,
h as skilled programmers can alter the workings of Basic inside the computer with
mucks Pokes and Sys calls. It is also possible to write machine language prograivs that
~
a'nd run entirely within the _ 1541 , either
~y writ~ng
them
in~o dis~
memory from the
Pu ter or by londing them directly from diskette into the desired disk memory buffer.
com • . . ·
Se this is similar to loading and running machine language programs in your
In u •
computer. . . .
As when learning to use Peek() , Poke and Sys in your computer, extreme caution is
advised in using the commands in this chapter. They
~e
essentially machine language
commands, and lack all of Basic's safeguards . If
anyth1~g
goes wrong .. you may have to
the disk drive off and on again (after removing the diskette) to regain control. Do not
WC:tice these commands on any important diskette . Rather, make a spare copy and work
~th
that. Knowing how to program a 6502 in machine language will help greatly, and
you will also need a good memory map of the 1541 . A brief 1541 map appears below .
Location
0000-00FF
0100-0IFF
0200-02FF
0300-07FF
1800-ISOF
lCOO-ICOF
Cl00-F258
F259-FE66
FE67-FE84
FE85-FEE6
FEE7-FFOF
FFE6-FFFF
1541 MEMORY MAP
Purpose
2K of RAM memory
Zero page work area: job queue , important
variables & pointers
Stack work area
Command buffers & tables: channels, parser, output, variables
Data buffers 0-4, I per page of memory .
lnput/Output chips
6522 VIA: I/O to computer
65f2 VIA: I/O to disk controller
Two SK Disk Operating System ROMs
Interface Processor: receive
&
interpret commands from computer
Floppy Disk Controller: executes IP's commands, controls mecha-
nism
IRQ handler: switches from IP to FDC & back every 10 ms .
ROM tables & constants
Patch area
JMP table: User command vectors
73