Commodore 1541-II - Chapter 8: Internal Disk Commands

CHAPTER 8
INTERNAL DISK COMMANDS
Expert programmers can give commands that directly alter the workings.pf the 1541,
much as skilled programmers can alter the workings of Basic inside the computer with
Peeks, Pokes and Sys calls. It is also possible to write machine language programs that
load and run entirely within the 1541, either by writing them into disk memory from the
computer, or by loading them directly from diskette into the desired disk memory buffer.
In use, this is similar to loading and running machine language programs in your
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 are essentially machine language
commands, and lack all of Basic's safeguards. If anything goes wrong, you may have to
turn the disk drive off and on again (after removing the diskette) to regain control. Do not
practice these commands on any important diskette. Rather, make a spare copy and work
with 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
0100-01FF
0200-02FF
0300-07FF
1800-180F
1C00-1C0F
C100-F258
F259-FE66
FE67-FE84
FE85-FEE6
FEE7-FF0F
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, 1 per page of memory.
Input/Output chips
6522 VIA: I/O to computer
65?2 VIA: I/O to disk controller
Disk Operating System ROM
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
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