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beneath-apple-dos/ch08.txt

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CHAPTER 8 - DOS PROGRAM LOGIC
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This chapter will take a detailed
look at the operation of the DOS
program itself to aid the APPLE user
in understanding it and to help him
to make intelligent use of its
facilities. Each subroutine and group
of variables or constants will be
covered separately by storage
address. The enterprising programmer
may wish to create a disassembly of
DOS on his printer and transfer the
annotations given here directly to
such a listing. Addresses used will
be for DOS 3.3 and for a 48K master
diskette version of DOS. Slot 6 is
assumed. Unless specifically
indicated by a $ character, lengths
are given in decimal, addresses in
hexadecimal (base 16).
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DISK II CONTROLLER CARD ROM - BOOT 0
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ADDRESS
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C600-C65B This routine is the first code executed when a disk
is to be booted. It receives control via PR#6 or
C600G or 6 control-P.
Dynamically build a translate table for converting
disk codes to six bit hex at location $356-$3FF.
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Call an RTS instruction in the monitor ROM and
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extract the return address from the stack to find out
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the address of this controller card ROM.
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Use this address to determine the slot number of this
drive by shifting $Csxx.
Save the slot number times 16 ($s0)
Clear disk I/O latches, set read mode, select drive
1, turn disk drive on.
Pull disk arm back over 80 tracks to recalibrate the
arm to track zero.
Set up parms to read sector zero on track zero to
location $800.
Execution falls through into a general sector read
subroutine at C65C.
.np
C65C-C6FA This subroutine reads the sector number stored at
$3D on the track indicated by $41 to the address
stored at $26,$27.
Look for D5/AA/96 sector address header on the disk.
If D5/AA/AD is found and sector data was wanted, go
to C6A6.
C683 Handle a sector address block.
Read three double bytes from the disk and combine
them to obtain the volume, track, and sector number
of the sector being read from the disk at this time.
Store the track at $40.
Compare the sector found to the sector wanted and the
track found to the track wanted.
If no match, go back to C65C.
Otherwise, if sector is correct, go to C65D to find
the sector data itself.
C6A6 Handle sector data block.
Read the 85 bytes of secondary data to $300-$355.
Read 256 bytes of primary data to the address stored
at $26,$27.
Verify that the data checksum is valid.
If not, start over at C65C.
"Nibbilize" the primary and secondary data together
into the primary data buffer ($26,$27).
Increment $27 (address page of read data) and $3D
(sector number to be read) and check against $800
to see if additional sectors need to be read.
If so, reload slot*16 and go back to C65C to read
next sector. (This feature is not used when loading
DOS but is used when loading from a BASICS diskette.)
Otherwise, go to $801 to begin executing the second
stage of the bootstrap.
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FIRST RAM BOOTSTRAP LOADER - BOOT 1
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ADDRESS
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0801-084C This routine loads the second RAM loader, Boot 2,
including RWTS, into memory and jumps to it.
If this is not the first entry to Boot 1, go to $81F.
Get slot*16 and shift down to slot number.
Create the address of the ROM sector read subroutine
(C65C in our case) and store it at $3E,$3F.
Pick up the first memory page in which to read Boot 2
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from location $8FE, add the length of Boot 2 in
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sectors from $8FF, and set that value as the first
address to which to read (read last page first).
081F Get sector to read, if zero, go to $839.
Translate theoretical sector number into physical
sector number by indexing into skewing table at $84D.
Decrement theoretical sector number (8FF) for next
iteration through.
Set up parameters for ROM subroutine (C65C) and
jump to it. It will return to $801 when the sector
has been read.
0839 Adjust page number at 8FE to locate entry point of
Boot 2.
Perform a PR#0 and IN#0 by calling the monitor.
Initialize the monitor (TEXT mode, standard window,
etc.)
Get slot*16 again and go to Boot 2 ($3700 for a
master disk, $B700 in its final relocated location).
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DOS 3.3 MAIN ROUTINES
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ADDRESS
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9D00-9D0F Relocatable address constants
9D00 Address of first DOS buffer at its file name field.
9D02 Address of the DOS keyboard intercept routine.
9D04 Address of the DOS video intercept routine.
9D06 Address of the primary file name buffer.
9D08 Address of the secondary (RENAME) file name buffer.
9D0A Address of the range length parameter used for LOAD.
9D0C Address of the DOS load address ($9D00).
9D0E Address of the file manager parameter list.
.np
9D10-9D1C DOS video (CSWL) intercept's state handler address
table. States are used to drive the handling of DOS
commands as they appear as output of PRINT statements
and this table contains the address of the routine
which handles each state from state 0 to state 6.
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9D1E-9D55 Command handler entry point table. This table
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contains the address of a command handler subroutine
for each DOS command in the following standard order:
INIT A54F
LOAD A413
SAVE A397
RUN A4D1
CHAIN A4F0
DELETE A263
LOCK A271
UNLOCK A275
CLOSE A2EA
READ A51B
EXEC A5C6
WRITE A510
POSITION A5DD
OPEN A2A3
APPEND A298
RENAME A281
CATALOG A56E
MON A233
NOMON A23D
PR# A229
IN# A22E
MAXFILES A251
FP A57A
INT A59E
BSAVE A331
BLOAD A35D
BRUN A38E
VERIFY A27D
.np
9D56-9D61 Active BASIC entry point vector table. The addresses
stored here are maintained by DOS such that they
apply to the current version of BASIC running.
9D56 Address of CHAIN entry point to BASIC.
9D58 Address of RUN.
9D5A Address of error handler.
9D5C Address of BASIC coldstart.
9D5E Address of BASIC warmstart.
9D60 Address of BASIC relocate (APPLESOFT only).
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9D62-9D6B Image of the entry point vector for INTEGER BASIC.
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This image is copied to 9D56 if INTEGER BASIC is made
active.
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9D6C-9D77 Image of the entry point vector for the ROM version
of APPLESOFT.
.np
9D78-9D83 Image of the entry point vector for the RAM version
of APPLESOFT.
.np
9D84-9DBE DOS coldstart entry routine.
Get the slot and drive numbers and store as default
values for command keywords.
Copy APPLESOFT ROM or INTEGER BASIC entry point
vector into current BASIC entry point vector.
Remember which BASIC is active.
Go to 9DD1.
.np
9DBF-9DE9 DOS warmstart entry routine.
Get the remembered BASIC type and set the ROM card
as necessary (calls A5B2).
9DD1 Remember whether entry is coldstart or warmstart
Call A851 to replace DOS keyboard and video
intercepts.
Set NOMON C,I,O.
Set video intercept handler state to 0.
Coldstart or warmstart the current BASIC (exit DOS).
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(DOS will next gain control when BASIC prints its
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input prompt character)
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9DEA-9E50 First entry processing for DOS. This routine is
called by the keyboard intercept handler when the
first keyboard input request is made by BASIC after
a DOS coldstart.
If RAM APPLESOFT is active, copy its entry point
vector to the active BASIC entry point vector and
blank out the primary file name buffer so that no
HELLO file will be run.
Set MAXFILES to 3 by default.
Call A7D4 to build the DOS file buffers.
If an EXEC was active, close the EXEC file
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Set the video intercept state to 0 and indicate
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warmstart status by calling A75B.
If the last command executed was not INIT (this DOS
was not just booted), go to 9E45.
Otherwise, copy an image of the DOS jump vector to
$3D0-$3FF.
Point $3F2,$3F3 to DOS warmstart routine.
Set the AUTOSTART ROM power-up byte since the RESET
handler address was changed.
Set the command index for RUN (to run the HELLO file)
and go to A180 to execute it.
9E45 See if there is a pending command.
If so, go to A180 to execute it. Otherwise, return
to caller.
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9E51-9E7F An image of the DOS 3-page jump vector which the
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above routine copies to $3D0-$3FF. See Chapter 5 for
a description of its contents.
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9E81-9EB9 DOS keyboard intercept routine.
Call 9ED1 to save the registers at entry to DOS.
If not coldstarting or reading a disk file,
go to 9E9E.
Get value in A register at entry and echo it on the
screen (erases flashing cursor).
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If in read state (reading a file) go to A626 to get
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next byte from disk file.
Otherwise, call 9DEA to do first entry processing.
Put cursor on screen in next position.
If EXECing, call A682 to get the next byte from the
EXEC file.
Set the video intercept state to 3 (input echo).
Call 9FBA to restore the registers at entry to DOS.
Call the true keyboard input routine.
Save the input character so that it will be restored
with the registers in the A register.
Do the same with the new X register value.
Exit DOS via 9FB3.
.np
9EBA-9EBC A jump to the true KSWL handler routine.
.np
9EBD-9ED0 DOS video intercept routine.
Call 9ED1 to save the registers at entry to DOS.
Get the video intercept state and, using it as an
index into the state handler table (9D11), go to
the proper handler routine, passing it the character
being printed.
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9ED1-9EEA Common intercept save registers routine.
Save the A, X, Y, and S registers at AA59-AA5C.
While in DOS, restore the true I/O handlers (KSWL and
CSWL) to $36-$39.
Return to caller.
.np
9EEB-9F11 State 0 output handler. --start of line--
If a RUN command was interrupted (by loading RAM
APPLESOFT) go to 9F78 to complete it.
If read flag is on (file being read) and output is a
"?" character (BASIC INPUT), go to state 6 to skip
it.
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If read flag is on and output is prompt character
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($33) go to state 2 to ignore the line.
Set state to 2 (ignore non-DOS command) just in case.
If output character is not a control-D, go to
state 2.
Otherwise, set state to 1 (collect possible DOS
command), set line index to zero, and fall through
to state 1.
.np
9F12-9F22 State 1 output handler. --collect DOS command--
Using line index, store character in input buffer at
$200.
Increment line index.
If character is not a carriage return, exit DOS
via 9F95 (echo character on screen if MON I).
Otherwise, go to command scanner at 9FCD.
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9F23-9F2E State 2 output handler. --non-DOS command ignore--
If the character is not a carriage return, exit DOS
via 9FA4 (echo character on screen).
Otherwise, set state back to 0 and exit DOS via
9FA4.
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9F2F-9F51 State 3 output handler. --INPUT statement handler--
Set state to 0 in case INPUT ends.
If character is not a carriage return, echo it on
screen as long as EXEC is not in effect with NOMON I
but exit DOS in any case. (KSWL will set state=3)
Otherwise, call A65E to see if BASIC is executing a
program or is in immediate mode. If EXEC is running
or if BASIC is in immediate mode, go to state 1 to
collect the possible DOS command.
Otherwise, exit DOS, echoing the character as
appropriate.
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9F52-9F60 State 4 output handler. --WRITE data to a file--
If the character is a carriage return, set state to
5 (start of write data line).
Call A60E to write the byte to the disk file.
Exit DOS with echo on screen if MON O.
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9F61-9F70 State 5 output handler. --Start of WRITE data line--
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If the character is a control-D, go to state 0 to
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immediately exit write mode.
If the character is a line feed, write it and exit,
staying in state 5.
Otherwise, set the state to 4 and go to state 4.
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9F71-9F77 State 6 output handler. --Skip prompt character--
Set state to 0.
Exit DOS via 9F9D (echo if MON I).
.np
9F78-9F82 Finish RUN command, interrupted by APPLESOFT RAM LOAD
Reset the "RUN interrupted" flag.
Call A851 to replace the DOS CSWL/KSWL intercepts.
Go to A4DC to complete the RUN command.
.np
9F83-9F94 DOS command scanner exit to BASIC routine.
If first character of command line is control-D,
go to echo exit (9F95).
Otherwise, set things up so BASIC won't see the DOS
command by passing a zero length line (only a
carriage return). Fall through to echo exit.
.np
9F95-9FB0 Echo character on screen (conditionally) and exit DOS
9F95 Echo only if MON C set, otherwise, go to 9FB3.
9F99 Echo only if MON O set, otherwise, go to 9FB3.
9F9D Echo only if MON I set, otherwise, go to 9FB3.
9FA4 Always echo character.
Call 9FBA to restore registers at entry to DOS.
Call 9FC5 to echo character on screen.
Save contents of the registers after echoing.
Fall through to DOS exit routine.
.np
9FB3-9FC4 DOS exit routine and register restore.
Call A851 to put back DOS KSWL/CSWL intercepts.
Restore S (stack) register from entry to DOS.
9FBA DOS register restore subroutine.
Restore registers from first entry to DOS and return
to caller.
.np
9FC5-9FC7 A jump to the true CSWL routine.
.np
9FC8-9FCC Skip a line on the screen.
Load a carriage return into the A register and
call 9FC5 to print it.
.np
9FCD-A179 DOS command parse routine.
.br
.nx ch8.1
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