Path: news.uiowa.edu!chi-news.cic.net!io.org!van-bc!pm06.pomo.wis.net!user
From: dmanzer@wimsey.com (Doug Manzer)
Newsgroups: comp.sys.apple2.programmer,comp.sys.apple2
Subject: HELP! Disk device driver won't boot.
Date: Wed, 31 Jan 1996 19:57:52 -0800
Organization: Online at Wimsey
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Message-ID: <dmanzer-3101961957520001@pm06.pomo.wis.net>
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Xref: news.uiowa.edu comp.sys.apple2.programmer:6074 comp.sys.apple2:98367

I'm trying to write device driver firmware for my PC-based file
server for the Apple 2. The firmware resides on a converted parallel
printer card which connects to a parallel port on the PC. So far I've
gotten an initial version of the device driver to communicate with
the server, and can format a volume, read and save files, etc. (A
ProdDos "volume" is implemented as a binary file on the PC).

However, I can't get it to boot reliably. It works occasionally, but 
mostly I get a message:

            *** UNABLE TO LOAD PRODOS ***

The specs I'm using are in Gary Little's book _Apple ProDOS_,
Advanced Features for Programmers, which says:

  "The first two blocks (0 and 1) of every standard ProDOS-formatted
  disk contain an assembly language program, called the _boot record_,
  that is automatically loaded into memory at $800 and executed (by a
  call to $801)."

Ok, I wrote code to do that on entry at $C100 (it's hard-coded for
slot 1 at the moment). This part seems to work, as I can tell by
debug messages from the server on the PC's screen that two blocks
are being read from the ProDOS volume file.

Then I call $801, where the following should happen:

  "When the boot record starts executing, it first loads the volume
  directory blocks (it assumes that these are located at their standard
  positions: blocks 2-5) into the 2K memory area from $C00-$13FF."

However, I can tell from the PC that it doesn't get that far before
hanging up with UNABLE TO LOAD PRODOS.

Here's some of the code I'm using (the $C1 stuff) -- this is the only
version that ever worked, where a ProDOS volume file is fixed at 1 MB
for the time being:


C:\PD\P2A.ASM                       Jan 29, 1996 7:25pm   Page 1
(Jan 29, 1996 6:03pm)

    4                        ;=======
    5                        ; PC file server - ProDos device driver
    6                        ;=======
    7                        
    8 0100                   stack   equ     $0100
    9 C080                   ioport  equ     $C080   ; add $n0 for actual
i/o addr
   10 C060                   cin     equ     $C060   ; cassette input port
   11                        
   12 0042                   command equ     $42     ; command code for disk op
   13 0043                   sldrive equ     $43     ; slot/drive #
   14 0044                   buffer  equ     $44     ; & $45
   15 0046                   block   equ     $46     ; & 47
   16                        
   17 0000                   datsav  equ     0
   18 CFFF                   exp_off equ     $CFFF   ; expansion ROM soft sw
   19 C800                   entry   equ     $C800   ; real work done here
   20                        
   21                        ;=======
   22                        ; START OF PROGRAM CODE
   23                        ;=======
   24                        
   25 C100                           .org    $C100
   26                        
   27                        ; disk controller signature bytes
   28 C100  C9 20                    cmp     #$20
   29 C102  C9 00                    cmp     #00
   30 C104  C9 03                    cmp     #03
   31 C106  C9 00                    cmp     #00
   32                        
   33                        ; boot entry
   34 C108  A9 00                    lda     #0      ; action code for boot
   35 C10A  8D FF CF                 sta     exp_off ; expansion ROM off
   36 C10D  20 00 C8                 jsr     entry
   37                        
   38                        ; ProDos command entry point
   39 C110                   cmd:
   40 C110  A9 01                    lda     #1      ; action code for cmd
   41 C112  8D FF CF                 sta     exp_off ; expansion ROM off
   42 C115  20 00 C8                 jsr     entry
   43 C118  60                       rts
   44                        
   45 C1FC                           .org    $C1FC
   46                        
   47                        ; device size in blocks ($0800 blocks = 1 mb)
   48 C1FC' 00                       .byte   $00     ; block count lo
   49 C1FD' 08                       .byte   $08     ; block count hi
   50                        
   51                        ; device attributes
   52 C1FE' 47                       .byte   $47     ; like ProFile
   53                        
   54                        ; ProDos device driver entry point address lo
   55 C1FF' 10                       .byte   <cmd
   56                        
   57                                .END    ;END OF PROGRAM

NOTES:

1. After a crash, location DEVADR11 ($BF12) still contains 01 C1,
for the $C110 ProDOS cmd entry address.

2. I've also tried $C3 at $C107. Didn't help.

3. Entry point at $C800 loads $10 to x-reg; examines A-reg for action
code. If boot, code reads in boot blocks and jumps to $801. This
relates to something I'm curious about: how does the boot code know
what interface card it's supposed to read the directory blocks from?
Perhaps it reads the interrupt addr MSB in $07F8, but putting $C1 in
there didn't make a difference. Or maybe it examines the stack to see
what card it was called from. I believe I've got this covered, as the
jsr entry at $C10D puts a $C1 on the stack; the boot code then does a
jmp to $801 after reading in the boot blocks.

4. Device size in blocks at $C1FC/C1FD -- I've tried 0000 here and
returning X=00, Y=08, A=0, and C-flag=0 on a status call.

5. Device attributes: tried all sorts of different things here,
including $FF which sometimes worked. Ultimately I want bit 7 set
as the volumes are "removable"; bit 6 set for interruptible (although
I'm not running anything that uses interrupts at the moment), and
two volumes. ProDOS only sometimes sees two volumes when bit 4
is set, but when I try to format either volume it crashes.

My best guess is that there's a register or memory location not being
set, and when it does boot it's because the correct values are there
coincidentally.

Any help appreciated, 

Regards, D.M.