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BACKGROUND
A computer includes both a physical machine, namely the hardware, and the
instructions which cause the physical machine to operate, namely the
software. Software includes both application and system programs. If the
program is simply to do tasks for a user, such as solving specific
problems, it is referred to as application software. If a program controls
the hardware of the computer and the execution of the application
programs, it is called system software. System software further includes
the operating system, the program which controls the actual computer or
central processing unit (CPU), and device drivers which control the input
and output devices (I/O) such as printers and terminals.
A general purpose computer is fairly complicated. Usually there will be a
queue of application programs waiting to use the central processing unit.
The operating system will need to determine which program will run next,
how much of the CPU time it will be allowed to use and what other computer
resources the application will be allowed to use. Further, each
application program will require a special input or output device and the
application program must transfer its data to the operating system which
controls the device drivers.
The operating system is generally too large and complex to be stored in
non-volatile Read Only Memory (ROM). Additionally, there are generally
periodic changes made to the operating system which makes it impractical
to store it in ROM. The operating system is usually stored on a magnetic
disk and read into Random Access Memory (RAM) to be executed. The problem
is that the access to the disk requires the disk hardware driver and the
rest of the system software. It would therefore appear that there is no
mechanism to start the computer, since to read the operating system off
the disk requires that the operating system be present.
To get around this problem a technique called boot-strapping or booting is
used. In booting, a small program is provided in a special ROM, called the
boot-ROM. When the computer is first started, the contents in the boot-ROM
are read and that program executed. The boot-ROM program is a small
program which instructs the CPU where on a disk to find a larger boot
program, which is termed the "boot-block" program. It also instructs the
CPU how to load the program into memory and execute it. The boot-block
program is executed to copy the operating system off the disk. Once the
operating system is in memory, it is executed and the computer is
completely functional.
A network of general purpose computers may be constructed by having a
number of general purpose computers, termed nodes, communicate with one
another over a communications link. If the computers are distant from each
other, the network is termed a wide area network (WAN), while, if they are
close together, for example, in the same building, it is called a local
area network (LAN). In one type of LAN, each computer or node is connected
to a common communication link by way of an interface, called a network
device.
The computers in a network communicate by sending messages to each other.
Each message consists of a header field, which contains an identifier
which comprises the address of the network device of the computer to which
the message is directed and an identifier which comprises the address of
the network device of the computer sending the message, and a data field,
which contains the information being sent between the computers.
Each network device monitors the messages on the link and copies the
message from the link into the computer's memory and notifies the computer
if the message header contains the network device's address or if a
message header includes a broadcast address, indicating that the message
is being sent to all devices on the network or a multicast address,
indicating that the message is being sent to all devices within a certain
address range. When a computer wishes to transmit information to another
computer, it attaches the address of the intended recipient to its own
address and attaches both to the information to form a message, which is
transmitted over the communication link.
The availability of local area networks has added to the versatility of
computers. It did not take users long to realize that the network could
make files on disks belonging to one computer system on the network
available to all computers on the network. Programs were developed which
simplified access to the files on the disks of another computer system
Eventually the concept evolved to assign special functions to certain
computers on the network. For example, one computer would assign logical
name to each physical device accessible to the network. In that way a user
instead of requesting a file on a specified disk on a specified system can
simply request the file using some logical name, and a computer on the
network which was designated to do the correlation then requests the file
on the specified disk and system, treating the disk on that system as if
it were the user's local disk. This translation of logical names to
physical devices is transparent to the user. The computer doing the
translating in this case is termed a disk or file server. Other server
functions have been defined, such as a print server, which allows a file
to be printed without specifying to which computer the printer is
attached.
It is also possible to assign a user to a disk and then use the disk server
to connect that user to that disk regardless of what computer on the LAN
the user is on. A disk server program, termed a Local Area Disk (LAD),
allows a user on one computer to treat a file on a disk on another
computer as a virtual disk. This virtual disk acts as if it were a disk on
the user s computer.
One of the files which can be accessed across the network are the operating
system files. In fact, even before the LAD concept made the access to a
file easy, many systems were developed to "downline" load an operating
system to a computer on the network. It became unnecessary, therefore,
that the computer, which is receiving the operating system, have an
operating system on a disk or have a disk at all. For such a system to
function, there are only two requirements. The first is that the network
device be capable of generating a message and transmitting it over the
communications link to request the operating system from another computer
on the network. The second is that the network device be capable of
loading the operating system it received into memory and causing the CPU
to execute it.
In one prior arrangement, upon powering on the computer, the network device
transmitted a request to be booted. This request, broadcast to all the
other computers on the network, includes a simple message which contains
the network device s hardware address. A computer on the network, upon
receiving this request, checks its database to determine if it contains a
listing identifying an operating system for the requesting computer. If
the receiving computer finds the list entry of the requesting computer in
its database, it becomes the host computer and retrieves the requested
operating system from its disk, attaches the requesting computer's address
to the file to form a message, and transmits the message on the network.
The requesting computer receives the message copies it into its memory, and
initiates the operating system execution. Although in principle the
loading of an operating system into a computer on the network is simple to
understand, its actual implemention is fairly complex.
To go into more detail, upon powering on, the requesting computer performs
a self-test/power-up sequence. As part of the sequence, the processor
looks for a boot device. If it fails to find a boot device, the processor
will allow the network device to request a boot over the network.
Typically, the network device will not use the protocols usually used to
communicate between computers. The reason for this is that such protocols
are more complex than is necessary for such a simple task, and the network
device would have to support a great deal of functionality it would not
need. So instead, a simple protocol is used which consists of a small set
of specialty messages for performing, testing, making boot requests, etc.
(see for example DECnet.TM. Digital Network Architecture Maintenance
Operations Functional Specification, order number (AA-x436A-TK DIGITAL
EQUIPMENT CORPORATION, MAYNARD, MASS). The boot request message, for
example, might simply contain the code number for a boot request, the code
number for the type of device making the request, what type of computer
the request is for, and whether the program requested is a boot strap
program or the operating system program. To these codes the network device
of the requesting computer attaches its address, and an address indicating
this is a broadcast to all other computers on the network.
When another computer on the network receives this message, it takes the
hardware address of the message and looks through its list of computers
for which it has operating system programs. If it fails to find the
hardware address of the requesting computer listed in its database, the
receiving computer simply ignores the message. If no computer has
responded to the requesting computer within a certain amount of time, the
requesting computer will again transmit a boot request. If the receiving
computer finds the address of the requesting computer listed in its
database, it then determines whether the requesting systems operating
system is to be loaded immediately or in a series of steps.
The problem with this method of loading an operating system using the
network is that too much information must be contained within the network
prior to the boot request. That is, a computer node on the network needs
to know, a priori, what programs, including boot-programs and operating
system, each bootable network computer on the network would require.
Further, for the operating system to work with the boot-programs, it is
necessary that the operating system be modified to contain "hooks" or
entry points that the boot-programs can call.
SUMMARY OF INVENTION
The invention provides a new and improved system and method of downloading
operating system or other executable programs to a computer on a network
without a boot device and without requiring a modification of the loadable
image.
Upon power up, the network device requests a boot and a minimum-boot
program is transferred over the network into the requesting computer s
memory. This minimum-boot program contains the functionality necessary to
establish a network connection to a disk server on the network. When
chained into the power-up/self-test program of the requesting computer,
the minimum-boot program causes the computer to function as though it has
a local disk with an operating system image, while actually transmitting
all the disk accesses made by the computer across the network. Because the
network is treated like a disk, there is no need to modify the operating
system for downline loading.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention is pointed out with particularity in the appended claims.
The above and further advantages of this invention may be better
understood by referring to the following description taken in conjunction
with the accompanying drawings, in which:
FIG. 1 is block diagram of a system constructed in accordance with the
invention including a host system and a node to be booted;
FIG. 2 is a block diagram of the minimum boot program structure.
FIG. 3a-3e together is a step diagram of the boot procedure.
DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
FIG. 1 depicts a network consisting of a computer node 10, which does not
include a local boot device, and a computer host node 14, which has a disk
20 containing the operating system of node 10, connected by a
communications link 12. The node 10 has a communications interface, called
the network device 26 by which it transmits and receives messages on the
communications link 12. Similarly, node 14 has a communications interface
28.
The node 10 has many of the elements of an ordinary computer, such as a
memory 27 and a central processing unit (CPU) 24. In addition it may have
peripheral devices such as a video display terminal 16 and a local disk
storage device 29.
The host 14 also has the elements of a computer including a CPU 30 and a
memory 32 as well as peripherals such as a video display terminal 18 and a
disk 20. The disk 20 on the host 14 not only contains the host's operating
system 23 but also a minimum-boot program 34 needed to load the node 10
and a virtual disk file 33 containing the operating system 22 for node 10.
Upon powering up, the processor 24 of computer 10 executes a
power-on/self-test sequence during which it attempts to find a boot device
which contains a file with its operating system in it. If it finds no boot
device, the network device 26 broadcasts a request for an operating system
over the network communications line 12. The network device 28 on the host
14 determines that the message is in broadcast mode and notifies the CPU
30. The CPU 30 of host 14 determines that the message is a boot request
and searches its memory 32 to attempt to identify an operating system for
the node 10. If it finds such an identification, it constructs a message
by attaching the hardware address of the network device 26 of node
computer 10 onto a minimum-boot program file 34 and places the message on
the communications line 12.
The communications device 26 of the node 10 receives the message and copies
the minimum boot program into the node's low memory addresses 36 of its
memory 27. The CPU 24 then begins to execute the minimum boot program. The
minimum-boot program is then copied into the high addresses 46 of memory
27 and the power-on/self-test sequence completes.
At this point the software on the node 10 is essentially treating the file
33 on disk 20 on host 14 as its virtual local disk. The node 10 reads a
boot-block 56 from the virtual disk file 33 on disk 20 and uses it to load
its operating system 22. It is important to note that all this is taking
place over the network link 12 and not through access of the node's 10
local disk storage 29.
To understand the sequence of events in detail, it is first necessary to
consider the nature of the minimum-boot program. The minimum-boot program,
shown in FIG. 2, consists of a header 41 which identifies it as a network
program to the host s operating system. The next portion is a boot-control
program 42 which will prepare the node 10 for the downline load. A Local
Area Disk (LAD) portion 48 is a program which allows a file on a disk on
the network to be treated as a virtual local disk by node 10. A Local Area
Systems Transport (LAST) 50 provides the actual local area network
communications functions as described in co-pending application entitled
"LOCAL AREA SYSTEM TRANSPORT" By Bruce E. Mann Et al. Attorney docket No.
PD 88-0421. A Data Link Layer (DLL) 52 is the protocol providing the
mechanism for communications between nodes through the network device 26
on node 10 and network device 28 on node 14. Finally, a Scheduler program
54 provides the real-time scheduling and timers required by network
communications.
FIG. 3 shows in detail the steps performed by node 10 and host 14 during a
downline load of the operating system for node 10. To the left of the
figure are the steps performed by the node 10 and to the right are the
steps performed by the host 14. Transfers between the two over the
communications link 12 are shown in the center of the figure. The
direction of the arrows indicates the direction of message flow.
With reference to FIGS. 1, 2, and 3, when the computer 10 is powered on, it
initiates a power-on/self-test (Step 100). During the power-on/self-test,
the CPU 24 checks addresses on the bus to determine if an option board is
present on the system. If it finds an option board, it processes any
initialization routines located on the board. When the option board is
network device 26, the initialization routines (Step 102) on the device
requests a boot from the network by broadcasting a boot request (Step
104). Another computer 14 on the network receives (Step 106) the boot
request and searches (Step 108) its database for information concerning
the requesting node 10. If the requesting node 10 is in the database, the
appropriate minimum boot routine is recovered (Step 110) from disk 20 and
transmitted (Step 112) to the requesting node 10.
Upon receipt (Step 114) of the minimum boot routine by node 10, it copies
it into low memory 36 and executes it (Step 116) in particular the boot
control program 42. The minimum boot-control program 42 of the
minimum-boot routine first determines (Step 118) the amount of memory 27
on node 10. In one embodiment, in which node 10 includes an IBM compatible
personal computer, this is accomplished by initiating an interrupt 12
interrupt service routine. The boot-control 42 modifies (Step 120) the
memory size reported by subtracting the size cf the minimum boot program
(FIG. 2) from the memory size returned by interrupt-12 in a location in
memory. This is done to make this portion of memory unavailable to the
node 10. The minimum boot routines are copied (Step 122) into high memory
46, and the boot control is chained (Step 124) to a routine which enables
a boot block to be read from a local disk. This is done to allow the power
up/self test sequence to complete and to regain control upon its
completion.
The power-up/self-test sequence regains (Step 126) control upon completion
of the network device 26 initialization routines and completes (Step 128).
Upon completion, the power-up/self-test sequence attempts (Step 130) to
read the boot block on the local disk. In this embodiment in which node 10
includes an IBM compatible personal computer, this is accomplished by
initiating an interrupt 19 interrupt service routine. This causes the
boot-control routine 42 to regain control (Step 132). The boot-control
routine 42 calls (Step 134) the LAD program 48, the LAST program 50, the
DLL program 52 and the Scheduler 54 at their entry points to initialize
them. In this embodiment in which node 10 includes an IBM compatible
personal computer, any disk access request generates an interrupt 13. LAD
is chained to interrupt 13 so that any disk access request will be
intercepted by LAD (Step 136).
The boot-control program 42 regains (Step 138) control, and enables the
registers of the network device 26 to be read (Step 140) to determine the
network address of node 10. The boot-control program 42 calls (Step 142)
the LAD program 48 to establish a local area disk connection. Once the
local area disk connection is established with server (Step 144), the
virtual disk is available to the node. Once the local area disk connection
has been established, the communications parameters may in fact, be
changed. For example, in one embodiment, shown in FIGS. 3C and 3D, it is
at this time that the DECnet network parameters for the node 10, such as a
network address, are read (Step 146) by the server so that the LAD routine
48 on the node 10 can be used to read (Step 148) the parameter file. Once
the parameters are read, the LAD connection is broken (Step 150) and
re-establshed (Step 152) using the new LAD parameters. In the general case
where the communication parameters need not be changed, steps 146, 148,
150, and 152 need not be not taken and the node 10 continues with the boot
procedure by requesting a read of boot-sector 56 to be retrieved.
The boot control program (42) issues a read request for the boot-sector 56
(Step 154). In one embodiment, in which node 10 includes an IBM compatible
personal computer, this is accomplished by asserting interrupt 13. Since
the LAD program 48 is chained in the interrupt service routine (Step 136)
it intercepts this request and directs (Step 156) it, to the host 14. This
host 14 reads (Step 158) the boot-sector 56 on the disk 20 and allows the
node 10 to read it (Step 160). LAD 48 on the node 10 then exchanges
messages (Step 162) with host 14 that facilitates the transfer of
boot-sector 56 to node 10, and its execution (Step 164).
The boot-sector 56 enables the node 10 to request the operating system by
asserting Step 166 interrupt-13 which is again intercepted by LAD 48 and
directed (Step 168) to the server. The server reads (Step 170) the
operating system from file 33 on disk 20 and makes it available (Step 172)
to DAD 48 on node 10. LAD 48 allows node 10 to read (Step 174) the
operating system 22 and load it into memory 27. When all the operating
system is loaded, LAD is unchained from interrupt-13 (Step 176) from LAD
and returned to the local disk 29. Control is transferred to the
initialization code of the executable program or operating system (Step
178).
It should be stated that in this embodiment the host and the server were
indicated to be the same node 14. This is not a requirement, and it is
possible to have another implementation in which they are separate.
It must be noted that none of the steps of the downline load of the
operating system required that the operating system be modified in any
way. By treating the host disk as if it were the local disk, any
executable image may be loaded without modification.
Having shown the preferred embodiment, those skilled in the art will
realize many variations are possible which will still be within the scope
and spirit of the claimed invention Therefore, it the intention to limit
the invention only as indicated by the scope of the claims.
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