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@ -1,153 +1,75 @@
.bp
.np
.ce
CHAPTER 5 - THE STRUCTURE OF DOS
.sp2
## CHAPTER 5 - THE STRUCTURE OF DOS
DOS MEMORY USE
DOS is an assembly language program
which is loaded into RAM memory when
the user boots his disk. If the
diskette booted is a master diskette,
the DOS image is loaded into the last
possible part of RAM memory,
dependent upon the size of the actual
machine on which it is run. By doing
this, DOS fools the active BASIC
into believing that there is
actually less RAM memory on the
machine than there is. On a 48K APPLE
II with DOS active, for instance, BASIC believes that
there is only about 38K of RAM. DOS does
this by adjusting HIMEM after it is
loaded to prevent BASIC from using
the memory DOS is occupying.
If a slave diskette is booted, DOS is
loaded into whatever RAM it occupied
when the slave diskette was
INITialized. If the slave was created
on a 16K APPLE, DOS will be loaded in
the 6 to 16K range of RAM, even if
the machine now has 48K.
In this case, the APPLE will appear,
for all intents an purposes, to have
only 6K of RAM.
If the slave was created
on a 48K system, it will not boot on
less than 48K since the RAM DOS
occupied does not exist on a smaller
machine.
.sp1
DOS is an assembly language program which is loaded into RAM memory when the
user boots his disk. If the diskette booted is a master diskette, the DOS image
is loaded into the last possible part of RAM memory, dependent upon the size of
the actual machine on which it is run. By doing this, DOS fools the active BASIC
into believing that there is actually less RAM memory on the machine than there
is. On a 48K APPLE II with DOS active, for instance, BASIC believes that there
is only about 38K of RAM. DOS does this by adjusting HIMEM after it is loaded to
prevent BASIC from using the memory DOS is occupying. If a slave diskette is
booted, DOS is loaded into whatever RAM it occupied when the slave diskette was
INITialized. If the slave was created on a 16K APPLE, DOS will be loaded in the
6 to 16K range of RAM, even if the machine now has 48K. In this case, the APPLE
will appear, for all intents an purposes, to have only 6K of RAM. If the slave
was created on a 48K system, it will not boot on less than 48K since the RAM DOS
occupied does not exist on a smaller machine.
*** INSERT FIGURE 5.1 ***
A diagram of DOS's memory for a 48K
APPLE II is given in
Figure 5.1. As can be seen, there are
four major divisions to the memory
occupied by DOS. The first 1.75K is
used for file buffers. With the
default of MAXFILES 3, there are
three file buffers set aside here.
Each buffer occupies 595 bytes and
corresponds to one potentially
open file. File
buffers are also used by DOS to LOAD
and SAVE files, etc. If MAXFILES is
changed from 3, the space occupied by
the file buffers also changes. This
affects the placement of HIMEM,
moving it up or down with fewer or
more buffers respectively.
A diagram of DOS's memory for a 48K APPLE II is given in Figure 5.1. As can be
seen, there are four major divisions to the memory occupied by DOS. The first
1.75K is used for file buffers. With the default of MAXFILES 3, there are three
file buffers set aside here. Each buffer occupies 595 bytes and corresponds to
one potentially open file. File buffers are also used by DOS to LOAD and SAVE
files, etc. If MAXFILES is changed from 3, the space occupied by the file
buffers also changes. This affects the placement of HIMEM, moving it up or down
with fewer or more buffers respectively.
The 3.5K
above the file buffers is occupied by
the main DOS routines. It is here
that DOS's executable machine language
code begins. The main routines are
responsible for initializing DOS,
interfacing to BASIC, interpreting
commands, and managing the file
buffers. All disk functions are
passed on via subroutine calls to the
file manager.
.bp
The file manager,
occupying about 4.3K, is a collection
of subroutines which perform almost
any function needed to access a disk
file. Functions include: OPEN, CLOSE,
READ, WRITE, POSITION, DELETE,
CATALOG, LOCK, UNLOCK, RENAME, INIT,
and VERIFY. Although the file manager
is a subroutine of DOS it may also be
called by a user written assembly
lanaguage program which is not part
of DOS. This interface is generalized
through a group of vectors in page 3
of RAM and is documented in the next
chapter.
The 3.5K above the file buffers is occupied by the main DOS routines. It is here
that DOS's executable machine language code begins. The main routines are
responsible for initializing DOS, interfacing to BASIC, interpreting commands,
and managing the file buffers. All disk functions are passed on via subroutine
calls to the file manager.
The file manager, occupying about 4.3K, is a collection of subroutines which
perform almost any function needed to access a disk file. Functions include:
OPEN, CLOSE, READ, WRITE, POSITION, DELETE, CATALOG, LOCK, UNLOCK, RENAME, INIT,
and VERIFY. Although the file manager is a subroutine of DOS it may also be
called by a user written assembly lanaguage program which is not part of DOS.
This interface is generalized through a group of vectors in page 3 of RAM and is
documented in the next chapter.
The last 2.5K of DOS is the Read/Write Track/Sector (RWTS) package. RWTS is the
next step lower in protocol from the file manager - in fact it is called as a
subroutine by the file manager. Where the file manager deals with files, RWTS
deals with tracks and sectors on the diskette. A typical call to RWTS would be
to read track 17 sector 0 or to write 256 bytes of data in memory onto track 5
sector E. An external interface is also provided for access to RWTS from a user
written assembly language program and is described in the next chapter.
The last 2.5K of DOS is the
Read/Write Track/Sector (RWTS)
package. RWTS is the next step lower
in protocol from the file manager -
in fact it is called as a subroutine
by the file manager. Where the file
manager deals with files, RWTS deals
with tracks and sectors on the
diskette. A typical call to RWTS
would be to read track 17 sector 0 or
to write 256 bytes of data in memory
onto track 5 sector E. An external
interface is also provided for access
to RWTS from a user written assembly
language program and is described in
the next chapter.
.sp1
.ne5
THE DOS VECTORS IN PAGE 3
.ll30
In addition to the approximately 10K
of RAM occupied by DOS in high
memory, DOS maintains a group of what
are called "vectors" in page 3
of low memory ($300
through $3FF). These
vectors allow access to certain
places within the DOS collection of
routines via a fixed location ($3D0
for instance). Because DOS may be
loaded in various locations,
depending upon the size of the
machine and whether a slave or master
diskette is booted, the addresses of
the externally callable subroutines
within DOS will change. By putting
the addresses of these routines in a
vector at a fixed location,
dependencies on DOS's location in
memory are eliminated. The page 3
vector table is also useful in
locating subroutines within DOS which
may not be in the same memory
location for different versions of
DOS. Locations $300 through $3CF were
used by earlier versions of DOS
during the boot process to load the
Boot 1 program but are used by DOS
3.3 as a data buffer and disk code
translate table.
Presumably, this change was made to
provide more memory for the first
bootstrap loader (more on this
later). The vector
table itself starts at $3D0.
.br
.ll60
.bp
In addition to the approximately 10K of RAM occupied by DOS in high memory, DOS
maintains a group of what are called "vectors" in page 3 of low memory ($300
through $3FF). These vectors allow access to certain places within the DOS
collection of routines via a fixed location ($3D0 for instance). Because DOS may
be loaded in various locations, depending upon the size of the machine and
whether a slave or master diskette is booted, the addresses of the externally
callable subroutines within DOS will change. By putting the addresses of these
routines in a vector at a fixed location, dependencies on DOS's location in
memory are eliminated. The page 3 vector table is also useful in locating
subroutines within DOS which may not be in the same memory location for
different versions of DOS. Locations $300 through $3CF were used by earlier
versions of DOS during the boot process to load the Boot 1 program but are used
by DOS 3.3 as a data buffer and disk code translate table. Presumably, this
change was made to provide more memory for the first bootstrap loader (more on
this later). The vector table itself starts at $3D0.
DOS VECTOR TABLE ($3D0-$3FF)
.np
ADDR USAGE
3D0 A JMP (jump or GOTO) instruction to the DOS warmstart
routine. This routine reenters DOS but does not
@ -197,178 +119,89 @@ ADDR USAGE
called when a non-maskable interrupt occurs.
3FE LO/HI address of a routine which is to be called when
a maskable interrupt occurs.
.bp
WHAT HAPPENS DURING BOOTING
When an APPLE is powered on its
memory is essentially devoid of any
programs. In order to get DOS
running, a diskette is "booted". The
term "boot" refers to the process of
bootstrap loading DOS into RAM.
Bootstrap loading involves a series
of steps which load successively
bigger pieces of a program until all
of the program is in memory and is
running. In the case of DOS,
bootstrapping occurs in four stages.
The location of these stages on the
diskette and a memory map are given
in Figure 5.2 and a description of
the bootstrap process follows.
.sp1
When an APPLE is powered on its memory is essentially devoid of any programs. In
order to get DOS running, a diskette is "booted". The term "boot" refers to the
process of bootstrap loading DOS into RAM. Bootstrap loading involves a series
of steps which load successively bigger pieces of a program until all of the
program is in memory and is running. In the case of DOS, bootstrapping occurs in
four stages. The location of these stages on the diskette and a memory map are
given in Figure 5.2 and a description of the bootstrap process follows.
*** INSERT FIGURE 5.2 ***
The first boot stage (let's call it
Boot 0) is the execution of the ROM
on the disk controller card. When the
user types PR#6 or C600G or 6(ctrl)P, for
instance, control is
.br
.ll30
.br
transfered to
the disk controller ROM on the card
in slot 6. This ROM is a machine
language program of about 256 bytes
in length. When executed, it
"recalibrates" the disk arm by
pulling it back to track 0 (the
"clacketty-clack" noise that is
heard) and then reads sector 0 from
track 0 into RAM memory at location
$800 (DOS 3.3. Earlier versions used
$300). Once this sector is read, the
first stage boot jumps (GOTO's) $800
which is the second stage boot (Boot
1).
The first boot stage (let's call it Boot 0) is the execution of the ROM on the
disk controller card. When the user types PR#6 or C600G or 6(ctrl)P, for
instance, control is transfered to the disk controller ROM on the card in slot
6. This ROM is a machine language program of about 256 bytes in length. When
executed, it "recalibrates" the disk arm by pulling it back to track 0 (the
"clacketty-clack" noise that is heard) and then reads sector 0 from track 0 into
RAM memory at location $800 (DOS 3.3. Earlier versions used $300). Once this
sector is read, the first stage boot jumps (GOTO's) $800 which is the second
stage boot (Boot 1).
Boot 1, also about 256 bytes long,
uses part of the Boot 0 ROM as a
subroutine and, in a loop, reads the
next nine sectors on track 0 (sectors
1 through 9) into RAM. Taken
together, these sectors contain the
next stage of the bootstrap process,
Boot 2. Boot 2 is loaded in one of
two positions in memory, depending
upon whether a slave or a master
diskette is being booted. If the
diskette is a slave diskette, Boot 2
will be loaded 9 pages (256 bytes per
page) below the end of the DOS under
which the slave was INITed. Thus, if
the slave was created on a 32K DOS,
Boot 2 will be loaded in the RAM from
$7700 to $8000. If a master diskette
is being booted, Boot 2 will be
loaded in the same place as for a 16K
slave ($3700 to $4000). In the
process of loading Boot 2, Boot 1 is
loaded a second time in the page
in memory
right below Boot 2 ($3600
for a master diskette). This is so
that, should a new diskette be INITed,
a copy of Boot 1 will be available in
memory to
be written to its track 0 sector 0.
When Boot 1 is finished loading Boot
2, it jumps there to begin execution
of the next stage of the bootstrap.
.br
.ll60
.bp
Boot 2 consists of two parts: a
loader "main program"; and the RWTS
subroutine package. Up to this point
there has been no need to move the
disk arm since all of the necessary
sectors have been on track 0. Now,
however, more sectors must be loaded,
requiring arm movement to access
additional tracks. Since this
complicates the disk access, RWTS is
called by the Boot 2 loader to move
the arm and read the sectors it needs
to load the last part of the
bootstrap, DOS itself. Boot 2 now
locates track 2 sector 4 and reads
its contents into RAM just below the
image of Boot 1 (this would be at
$3500 for a master diskette). In a
loop, Boot 2 reads 26 more sectors
into memory, each one 256 bytes
before the last. The last sector
(track 0 sector A) is read into $1B00
for a master diskette. The 27 sectors
which were read are the image of the
DOS main routines and the file
manager. With the loading of these
routines, all of DOS has been loaded
into memory. At this point, the
bootstrap process for a slave
diskette is complete and a jump is
taken to the DOS coldstart address.
If the diskette is a master, the
image of DOS is only valid if the
machine is a 16K APPLE II. If more
memory is present, the DOS image must
be relocated into the highest
possible RAM present in the machine.
To do this, the master version of
Boot 2 jumps to a special relocation
program at $1B03. This relocator is
512 bytes in length and was
automatically loaded as the two
lowest pages of the DOS image. (In the
case of a slave diskette, these pages
contain binary zeros.) The relocator
determines the size of the machine by
systematically storing and loading on
high RAM memory pages until it finds
the last valid page. It then moves
the DOS image from $1D00 to its final
location ($9D00 for 48K) and, using
tables built into the program, it
modifies the machine language code so
that it will execute properly at its
new home. The relocator then jumps to
the high memory copy of DOS and the
old image is forgotten.
Boot 1, also about 256 bytes long, uses part of the Boot 0 ROM as a subroutine
and, in a loop, reads the next nine sectors on track 0 (sectors 1 through 9)
into RAM. Taken together, these sectors contain the next stage of the bootstrap
process, Boot 2. Boot 2 is loaded in one of two positions in memory, depending
upon whether a slave or a master diskette is being booted. If the diskette is a
slave diskette, Boot 2 will be loaded 9 pages (256 bytes per page) below the end
of the DOS under which the slave was INITed. Thus, if the slave was created on a
32K DOS, Boot 2 will be loaded in the RAM from $7700 to $8000. If a master
diskette is being booted, Boot 2 will be loaded in the same place as for a 16K
slave ($3700 to $4000). In the process of loading Boot 2, Boot 1 is loaded a
second time in the page in memory right below Boot 2 ($3600 for a master
diskette). This is so that, should a new diskette be INITed, a copy of Boot 1
will be available in memory to be written to its track 0 sector 0. When Boot 1
is finished loading Boot 2, it jumps there to begin execution of the next stage
of the bootstrap.
The DOS boot is completed by the DOS
coldstart routine. This code
initializes DOS, making space for the
file buffers, setting HIMEM, building
the page 3 vector table, and running
the HELLO program.
.bp
Previous versions of DOS were
somewhat more complicated in the
implementation of the bootstrap. In
these versions, Boot 1 was loaded at
$300 and it, in turn, loaded Boot 2
at $3600, as does version 3.3. Unlike
3.3, however, 27 sectors of DOS were
not always loaded. If the diskette
was a slave diskette, only 25 sectors
were loaded, and, on 13 sector
diskettes, this meant the DOS image
ended either with sector 8 or sector
A of track 2 depending upon whether
the diskette was a slave or master.
In addition, Boot 1 had a different
form of nibbilization (see chapter 3)
than any other sector on the
diskette, making its raw appearance
in memory at $3600 non-executable.
Boot 2 consists of two parts: a loader "main program"; and the RWTS subroutine
package. Up to this point there has been no need to move the disk arm since all
of the necessary sectors have been on track 0. Now, however, more sectors must
be loaded, requiring arm movement to access additional tracks. Since this
complicates the disk access, RWTS is called by the Boot 2 loader to move the arm
and read the sectors it needs to load the last part of the bootstrap, DOS
itself. Boot 2 now locates track 2 sector 4 and reads its contents into RAM just
below the image of Boot 1 (this would be at $3500 for a master diskette). In a
loop, Boot 2 reads 26 more sectors into memory, each one 256 bytes before the
last. The last sector (track 0 sector A) is read into $1B00 for a master
diskette. The 27 sectors which were read are the image of the DOS main routines
and the file manager. With the loading of these routines, all of DOS has been
loaded into memory. At this point, the bootstrap process for a slave diskette is
complete and a jump is taken to the DOS coldstart address. If the diskette is a
master, the image of DOS is only valid if the machine is a 16K APPLE II. If more
memory is present, the DOS image must be relocated into the highest possible RAM
present in the machine. To do this, the master version of Boot 2 jumps to a
special relocation program at $1B03. This relocator is 512 bytes in length and
was automatically loaded as the two lowest pages of the DOS image. (In the case
of a slave diskette, these pages contain binary zeros.) The relocator determines
the size of the machine by systematically storing and loading on high RAM memory
pages until it finds the last valid page. It then moves the DOS image from $1D00
to its final location ($9D00 for 48K) and, using tables built into the program,
it modifies the machine language code so that it will execute properly at its
new home. The relocator then jumps to the high memory copy of DOS and the old
image is forgotten.
The DOS boot is completed by the DOS coldstart routine. This code initializes
DOS, making space for the file buffers, setting HIMEM, building the page 3
vector table, and running the HELLO program.
Previous versions of DOS were somewhat more complicated in the implementation of
the bootstrap. In these versions, Boot 1 was loaded at $300 and it, in turn,
loaded Boot 2 at $3600, as does version 3.3. Unlike 3.3, however, 27 sectors of
DOS were not always loaded. If the diskette was a slave diskette, only 25
sectors were loaded, and, on 13 sector diskettes, this meant the DOS image ended
either with sector 8 or sector A of track 2 depending upon whether the diskette
was a slave or master. In addition, Boot 1 had a different form of
nibbilization (see chapter 3) than any other sector on the diskette, making its
raw appearance in memory at $3600 non-executable.
The various stages of the bootstrap process will be covered again in greater
detail in Chapter 8, DOS PROGRAM LOGIC.
The various stages of the bootstrap
process will be covered again in greater
detail in Chapter 8, DOS PROGRAM
LOGIC.
.sp1
*** INSERT FIGURE 5.3 HERE ***
.br
.nx CH6.1