Apple Orchard 1980 Fall v1n2 reduced

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specify the file your are looking for,
you "prefix" it with the names of
the directories it is contained in.
Prefixes can get pretty long if you
choose to build up a hierarchy of
directories. For convenience, SOS
will keep track of the prefix for the
directory you are using at a particular
time. So if you have a file
named "X" contained in directory
" Y" which is contained in directory
" Z" you could specify the file as
"Z/Y I X" or you ca n tell SOS that
the prefix is "/Z/Y!" and simply
refer to file "X". Sound complicated
Well, you'll just have to try it.
You' ll like it. All languages on the
Apple 111 access files through the
SOS file system, so you'll only need
to learn it once. Some languages
have minor differences from the
general file specification described
here. Pascal, for instance, uses ":"
instead of"!".
So much for how to specify files,
now what can you do with them
SOS can be asked to perform the
following functions: CREATE either
a file or a directory, DESTROY a file
or a directory, RENAME a file,
OPEN, CLOSE, READ, or WRITE to a
file, plus calls to set the currrent
position in a file or end of file . . .
You can SET and READ PREFIX, ask
for file INFO, and FLUSH ... a file
buffer, that is.
In short, the SOS file management
interface provides a set of
high level capabilities accessable
directly via assembly language, or
at a very high level through the
languages on the Apple Ill. Each
language will have its own convention
for using these capabilities,
but SOS allows a very high level of
consistency.
Device Management Interface
SOS also provides a lower level
access to devices. Although this
level of control is more primitive,
it is useful where more performance
is required. At this level, the
file concepts are stripped away, and
the caller has immediate access to
the device drivers for any devices
configured into the operating
system. As a matter of fact, the high
level ca lls just described are implemented
using these calls!
SOS is a configurable operating
system. Each device on the system is
supported by a set of routines that
can be added to or removed from
the operating system using the
THE APPLE ORCHARD
System Configuration Program
supplied with SOS. Every SOS
"boot" disk contains a file named
SOS.DRIVERS. The driver contains
the necessary information to control
a particular kind of device.
All devices known to the
operating system are controlled by
a device handler and if necessary,
an interrupt handler. The device
handler for a device contains up to
six distinct modules: OPEN, CLOSE,
READ, WRITE, CONTROL, and
STATUS. The modules associated
with each device are direC:tly callable
by the SOS user through the
Device Management System.
Memory Management Interface
Many have asked the inevitable
question: " How does the Apple Ill
address more than 64K bytes of
memory with a 6502." The answer
encompasses both hardware and
software on the Apple 111.
Apple 111 memory hardware
provides two capabilities toward
this end: bank switching and zero
page switching. Thus, the amount
of RAM memory in the system can
extend beyond 64K bytes, and each
code module or set of modules can
have its own zero page.
Apple 111 has three bank
switches, a main bank switch, an 1/ 0
bank switch, and a ROM bank
switch. The main switch selects the
chunk of RAM that will sit in the 8K-
40K address range. The 1/ 0 switch
ca n select between the hardware
1/ 0 bank or a 4k RAM bank, and
finally, the ROM bank switch
selects between a 4k ROM bank or
another 4k RAM bank.
SOS's role is to manage both the
bank switching and zero page
select in a manner that reduces the
apparent complexity of the
memory as the operating system
user views it. This is provided in the
form of a bookkeeping mechanism
for allocating and deallocating
chunks of memory. The REQUEST
SEG system call asks SOS to request
a specific portion of memory. If the
user doesn't care where the
memory is located, he ca n simply
FIND SEG. SOS will search for a free
chunk of memory of the specified
si ze, and assign it. The user can later
expand this chunk of memory.
When a program has finished using
its assigned memory, it assigns it
back to the free space pool using
the RELEASE cal l.
FALL 1980
Several other calls are provided
to obtain information on t he allocation
of memory. The memory
management system may seem to
provide more capability that most
programmers need. However, by
allocating memory through the
operating system, a programmer
will never have to worry about
another module in the system
interfacing with his memory space,
almost as if the Apple 111 had
"virtual" memory.
Interrupt Management
One of the most complex aspects
of writing 1/ 0 intensive programs is
the handling of interrupts. SOS
relieves the program of this difficult
task by providing two types of
interrupt management. The first is a
· peripheral interrupt. These
interrupts are handled entirely by
SOS and its comtituent device
drivers. These device drivers must
be designed with great ca re since
they are literally part of the operating
system itself and because of
danger of data overrun due to critica
l timing constraints. Fortunately,
device drivers with thoroughly
tested interrupt handlers are
provided with SOS for all the builtin
devices on the Apple 111.
A new kind of user software
interrupt mechanism has been
provided to allow user programs
some asynchronous capabilities
without interfering with the ability
of SOS to deal with true peripheral
interrupts. These are, of course, at a
lower priority level than the peripheral
interrupts.
A user software interrupt usually
corresponds to a similar peripheral
interrupt. For example, a user
software interrupt on the
CONSOLE device would probably
correspond to an interrupt on the
Apple 111 keyboard.
Associated with each distinct
user software interrupt is a portion
of the user code called an interrupt
handler which processes the interrupt.
Each interrupt is also assigned
a priority level from 0 to 255 used to
dete rmine the order that
simultaneous inte rrupts are
handled .
A number of SOS calls are provided
to take advantage of the user
interrupt facility. These calls al low
the user to install and remove
various interrupt handlers, set
interrupt priority level, and exit