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