Note: Descriptions are shown in the official language in which they were submitted.
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CA9-77-004
1 Field of the Invention
The present invention relates to data processing systems. More
particularly, the invention relates to an instruction modifying method
and apparatus. The method and apparatus of the present invention is
capable of modifying any portion of an instruction in the data pro-
cessing system prior to its execution.
Prior Art
In modern computers, especially mini and micro-computers, instruc-
tion storage is usually provided by read only storage devices. This
10 results in a loss of flexibility compared to a stored program computer
in which the instructions may be modified in sequence and in operation
code.
In the past, complex indexing methods and apparatus including ad-
ditional index registers have been enlisted in an effort to alleviate
these shortcomings. Usually such methods and circuitry have applied
only to a portion of the total instruction set and/or have a limited
number of index registers with attendant loading and modifying com-
plexities and/or cannot modify the operation portion of the instruction
itself.
U.S. Patent 3,503,046 which outlines an instruction modification
circuit suffers from obvious shortcomings in that the primary means for
indexing is the arithmetic registers. This is unduly restrictive and
requires additional machine cycles to load the register. Dummy machine
cycles are wasted to modify the command and additional cycles are
required for setting/resetting a flip-flop to modify the instructions
desired. Only instruction address modification is provided by the
aforesaid patent.
Objects of the Present Invention
The principal objects of the present invention are to provide
30 improved method and apparatus for the modification of instructions
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CA9-77-004 ~ P7 ~3
l without requiring additional registers or complex circuitry while still
being capable of modifying substantially any or all instructions.
Statement of the Invention
As is well known in the art, modern computers, particularly mini
and micro-computers, provide instruction storage by read only storage
devices while data storage is accomplished by the use of volatile or
modifiable memories such as a random access stores (RAS).
According to the present invention, instructions stored in a read
only store (ROS) are modifiable by data received from a random access
lO store in the computer. An instruction is retrieved from the read only
store and data is retrieved from the random access store, and, if an
indexing gate is active, the instruction and the data are combined in an
adder. The output of the adder constitutes the modified instruction.
With this simple arrangement the read only store instruction set can be
modified manyfold, limited only by the capacity of the random access
store. Thus, with the addition of a small amount of structure, immense
versatility is achieved.
In accordance with the present invention, method and apparatus is
provided for modifying instructions in a data processing system having
20 at least two memories.
A first of these memories, typically a read only store, is ad-
dressed to selectively access a stored instruction. The addressed and
accessed instruction obtained from the first memory is then directed to
instruction modifier means which may include an adder. Data is ad-
dressed in another memory, typically a random access store, in accor-
dance with an instruction received from the first memory during a pre-
vious processing cycle, possibly the immediately preceding processing
cycle. The addressed data is directed to instruction modification
gating means which may be selectively energized to thereby selectively
30 furnish the addressed data to the instruction modifier means. The
CAg-77-004 ~ 7~
1 data received by the instruction modifier means and the accessed in-
struction received by it are combined by means of the instruction
modifier means to prpvide a modified instruction to the data processing
system typically to be put to use by the system in much the same fashion
as unmodified instructions.
As may be readily inferred by experts in the art a greatly expanded
instruction set has been achieved.
In order that the present invention may be readily carried into
effect, it will now be described with reference to the accompanying
drawings, wherein;
FIG. 1 is a block diagram of a data processing system of the prior
art to which the instruction modification method of the present inven-
tion may be applied.
FIG. 2 is a block diagram of a data processing system employing the
method and apparatus of the present invention.
FIG. 3 is a simplified circuit diagram of an embodiment of the
present invention as used in a data processing system.
To ease understanding of the present invention so that it may be
readily carried into effect typical current practice in the processing
20 of instructions and data will be discussed followed by a description of
the present invention all with reference to the abovementioned drawings.
The data processing system of the prior art, as depicted in Fig. 1,
comprises two memories, an instruction store 1 (I-store) for the storage
of instructions, and a data store 2 (D-store) for the storage of data.
An instruction comprises an operational portion or operand and an
address portion or address label as is well known in the art although
variants have been developed with additional portions for specialized
uses. It will be realized by those skilled in the art that the method
and apparatus of the present invention are readily adaptable to those
30 variants as well.
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CA9-77-004
1 Each memory has associated with it its own address register operat-
ing by branching or incrementing in known fashion for selectively ad-
dressing and accessing its contents. The instruction store 1 is ad-
dressed by an instru~tion address register 3 (IAR) connected to it, the
Data store 2 is addressed by its data address register 4 (DAR). Thus,
particular instructions and data can be selectively accessed from their
respective memories.
Interconnection of the registers by branch line 5 permits branching
of instructions within a program while line 6 provides output instruc-
10 tions to initiate data accessing from data store 2 and other instructionsequencing.
The output of instruction store 1, an instruction, is fed to
operation register 7 which, when triggered by a clock pulse from the
processing system in a known manner, supplies the instruction to an
input 10 of an arithmetic logic unit 8 for operation upon the data
received at input 11 of the arithmetic logic unit 8 (ALU) from the data
store 2 connected to it and information supplied by the accumulator 9
connected to ALU input 12 and ALU output 13.
The accumulator output 14 and ALU input 12 are connected to input/
20 output devices in a known manner.
Generally, in digital data processing systems similar to those
illustrated in Figs. 1, 2 and 3, instructions which are stored in the
system must be fetched (accessed) from an instruction storage memory,
such as the instruction store 1 in the illustration, and applied to the
data stored and fetched from a data storage memory, such as the data
store 2 of the system.
In modern computers, more particularly mini and micro-computers,
the instruction storage memory is made from read only storage (ROS)
devices commonly known as read only memories (ROM). Typically, the read
30 only memories may be obtained with built-in instructions predetermined
in manufacture or as Programmable read only memories (PROM) which are
programmed with a series of instructions before insertion in a data
processing system.
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CA9-77-004
1 Once in place in a system, it is not usually economically feasible
to change the programming of the PROM or to replace it as the down time
would be unacceptable.
In systems like that of Fig. 1, the data memory is modifiable.
Typically, a random access memory or a serial memory which allows inser-
tion, modification or deletion of memory stored data by the system
either internally or by external inputs as is well known is provided.
As is well known in the art in system data processing, two main
phases of operation are carried out cyclically, usually serially, in
parallel or both. These operations consist of instruction accessing or
fetching and instruction application or the execution of instructions
on the data to obtain the desired result.
It will be most logical to examine the operation of a data pro-
cessing system by considering its operation midstream.
First, we will consider instruction accessing, and second, in-
struction execution. While a next sequential instruction is being
selectively accessed from the instruction store 1 read only memory in
accordance with the corresponding address label stored in the IAR 3, the
execution cycle of the system is selectively fetching or accessing data
from D-store 2 according to the corresponding addresses in the DAR 4 and
combining it with data fed back from the output 14 of the accumulator 9
by means of the ALU 8 in a known manner.
Once the cycle of these operations has been completed, the output
13 of ALU ~ is a product of the function specified ln the operation
register 7, the data output of data D-store 2 and the output 14 of
accumulator 9. The output 13 is then set in the accumulator 9 in a
known manner by the processing system.
Subsequently the IAR 3 is incremented to address the next sequen-
tial instruction or is set directly via branching line 5 in a known
manner if a branching instruction is to occur. DAR 4 is set to address
the next data word according to the address label of the instruction
fetched from the I-store 1.
CA9-77-004
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1 In applying the method and apparatus of the present invention to
the abovementioned system of the prior art, instruction modifier means,
gating means and special instructions hereinafter called modification
instructions are provided as described in more detail below.
Similarly performing structures in each of the following described
diagrams will be labelled similarly to those of Fig. 1.
As depicted in Fig. 2, the improved data processing system of the
present invention comprises at least two addressable memories (al-
though the use of additional memories for increased versatility is also
contemplated) one of which is I-store 1', and another, D-store 2'.
The information contents of each is selectively accessible by their
respective index registers, namely IAR 3' and DAR 4', respectively.
In this example, the instruction modifier means comprises instruc-
tion modifier adder (IMA) 15 (although modifier means other than adders,
for instance, AND/OR or Exclusive OR devices, etc could be used de-
pending on the results desired) fed by (a) the I-store 1' at its input
16 for accepting instructions therefrom and (b) by IMA gate 17 at its
input 18.
IMA gate 17 derives its data input from D-store 2'.
Operation register 7' (connected to IMA 15), ALU 8' and accumulator
9' have the same role as their counterparts in Fig. 1. Branch line 5'
connecting IAR 3' and DAR 4' provides branching capability as before.
Line 19 connects IMA 15 output 20 to DAR 4' for selective accessing of
data from associated D-store 2'.
As stated above, the application of the method of the invention to
data processing requires the provision of a special instruction called a
modification instruction which is stored in the instruction store 1'
along with other conventional instructions.
In the processing systems of most concern, the instruction store 1'
is unmodifiable so that its contents, the instructions, are not alter-
CA9-77-004 ~L~6~37~
1 able within the instruction store 1'. The modification instruction when
fetched from the instruction store 1' by means of its IAR 3'~ acts like
any other conventio~al instruction obtained from the store in the in-
struction or execution phase of operations. The operand and address
portion of the instruction code are fetched or accessed from the in-
struction store 1' at more or less the same time as data is being
fetched from the data store 2' according to the address (i.e. the
address of the instruction received previously along line 19) in the
data address register 4'.
Provided no previous modification instruction has called for the
modification of the presently considered modification instruction this
instruction is passed unmodified through IMA 15 (IMA gate 17 not having
been activated by a pulse via trigger line 21 and hence remaining in a
blocking state).
On the activation of operation register 7' by a clock pulse on
trigger line 22 in a known manner the modification instruction is pro-
vided to ALU input 10' setting ALU 8' to the desired function. Data
supplied from D-store 2' at ALU input 11' and previously processed data
from accumulator 9' at ALU input 12' are then combined arithmetically by
ALU 8' in accordance with the setting operand received at input 10' in
a known manner.
At the end of the phase the accumulator 9' is set to ALU output 13'
to thereby feed output devices (not shown).
At the end of this phase of operation, the IAR 3' is incremented,
typically by a clock pulse, and the address portion or label of the
aforementioned modification instruction enters and sets DAR 4'.
During the next phase of the operation the next sequential instruc-
tion is accessed or fetched from instruction store 1' according to the
address in IAR 3' in a known manner and at substantially the same time
CA9-77-004 ~L~Lq~6~7~3
`~ 1 as data is fetched from D-store 2' via its address obtained from the
address label of the previous instruction in DAR 4' which, in this
example was a modification instruction.
According to the method of the invention, the data from D-store 2'
is gated by IMA gate 17 only if a modification instruction ls being
executed to achieve modification and not if the instruction executed
(i.e. a previous one) is a normal non-modification instruction from
instruction store 1'.
The execution of the previous modification instruction causes the
subsequent activation of IMA gate 17 to feed data from the data store
2' to the input 18 of IMA 15 which combines the instructions received at
input 16 and the data received at input 18 in an arithmetic manner, such
as by adding, to output a modified instruction address to the DAR 4' and
a modified instruction operation code or operand to the operation re-
gister 7'.
Thus the improvement of the invention results in the modification
instruction modifying any following instruction, the modified instruc-
tion can be applied in the arithmetic logic unit 8' and elsewhere in the
system in a known manner.
It will be realized by those skilled in the art that, when the
output of operation register 7' decodes a modification instruction
operand, known means provides a substantially instantaneous trigger
signal to IMA gate 17 via trigger 21 so that only the next following
instruction can be modified. However, if known decoding and storage
devices are employed any following instruction may be modified.
Fig. 3 is a simplified circuit diagram of an embodiment of the
present invention used in a typical data processing system such as that
shown in Fig. 2 illustrating suitable components capable of carrying out
the method of the invention.
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CA9-77-004
1 It will be understood by those familiar with the art that the
specific embodiment to be described is applicable to microcomputers
using 8 bit words, and 8 bit instructions (4 bits thereof being reserved
for the address portion and the other 4 bits for the operand).
With suitable modification, which will be immediately apparent, the
present invention can be used with other data word and instruction sizes.
Of course, any increase in size would expand the processing capability
and increase the complexity of the system.
Typically mini and micro-computers are assembled from integrated
circuit building blocks which are readily available commercially.
Preferably, off the shelf items, such as registers and gates are used
where possible but other items, such as read only memories (ROM's),
may be ordered to satisfy individual preferences. With this in mind,
this invention, as described in relation to Fig. 2, will be presented
with illustrations of suitable components.
As illustrated in Fig. 3, two addressable memories are provided
(although additional memories may be useful in some instances). The
instruction store 1", which is capable of storing 8 bit instructions, is
derived from two masked programmed integrated circuits labelled la and lb,
respectively. Each of the two integrated circuits are available as
standard Part Nos. 74187 from Texas Instruments Incorporated and are
available with preselected programming. As each circuit is capable of
storing only 4 bit units of information two circuits are required. Unit
la is used for storage of the address portion of instructions and unit
lb for storage of the corresponding operands.
The ROM's are programmed with two types of instructions; regular
instructions and modifier or modification instructions. The latter type
have operands with selected coding having certain common characteristics
capable of being decoded to provide a trigger signal for instruction
modifier gating means to allow subsequent instruction modifications.
CA9-77-004 i~ 6~
The other memory, data store 2" is also formed in two parts. Each
part consists of an integrated circuit forming a 16 by 4 bits RAM. The
RAM is available as standard Part No. 7489 from Texas Instruments
Incorporated. The RAM is labelled as 2a and 2b. Concurrent operation
of each RAM permits operation with 8 bit data words.
Instruction store 1" is addressed by associated IAR 3" via in-
struction access cables A and A' to permit selective access to stored
instructions. The accessed stored instructions are outputted onto
output cables B and B', respectively. The outputs on cables B and B'
provide address and corresponding operand portions of instructions to
IMA 15.
Similarly, the D-store 2" is addressed by its associated DAR 4" via
data register or output cable D to selectively access stored data. Data
output cables E and E' feed ALU 8".
In this example, IAR 3" comprises two cooperatively connected
registers 3a and 3b. The registers 3a and 3b may be activated by in-
crementing or by branch loading via branch cable c (comprised of lines
Cl, C2, C3, and C4). Input lines Kl and K2 are used for load low and
load high, respectively in a known manner. Registers 3a and 3b may be
integrated circuits which are commercially available as Part Numbers 74163
from Texas Instruments Incorporated.
DAR 4" is a 4 bit, pulse operated latching register deriving its
input from the IMA address output to cable C.
Operation register 7" derives its input from IMA operand output
cable C' and, when loaded, outputs to the ALU along ALU operand input
cable D'. Registers DAR 4" and 7" may be of the type commercially
available as Part No. 7495 from Texas Instruments Incorporated.
Modification adder 15 comprises two adders labelled 15a, and 15b and
provides the modification means in this example. Each of adders 15a or
CA9-77-004 ~ ?6~7~3
1 15b can provide a 4 bit sum for the modification operations. Each ad-
der 15a or 15b may be of the type commercially available as Part No. 7483
from Texas Instruments Incorporated.
Other devices can be used for the modification means, such as
AND/OR circuits, Exclusive OR circuits or many other function circuits
depending on the type of modification of instruction desired and it is
understood that the invention is not limited to adder type modifiers
alone.
Input information to the modification adder 15' is obtained via in-
struction output cables B and B'. Cable B is connected to modification
adder component 15a, and cable B' to adder component 15b to deliver the
corresponding address and operand portions of the instruction. Gate
output lines J and J' from modification adder gate components 17a and
17b, respectively of modification adder gate 17' deliver modification
inputs to adder components 15a and 15b, respectively. Gate Components
17a, 17b, in this example, are integrated circuit gates which are com-
mercially available as Part No. 74157 from Texas Instruments Incorporated.
Inputs E, and E' for gates 17a and 17b, respectively, are derived from
the identically labelled data output cables E and E' from D-store 2".
Outputs J and J' appear only when trigger input K3 is activated by
the corresponding output of decoder 23 fed by operand input cable D'.
Decoder 23 may comprise a PROM integrated circuit decoder of type
82S123 available commercially from Signetics Corporation with suitable
bit patterns stored. Decoder 23 provides loading inputs Kl and K2 to
load the instruction address register 3" high and low, respectively, and
to activate the IMA gate 17' when appropriate signals or information are
present on cable D'. In the case of the present invention, this is when
the operand portion of a modification instruction is present on the cable
D'. Other outputs Kn, etc., may be used at other points in the pro-
CA9-77-004
7B
cessing system, for example, to output the accumulator 9" or permit
inputs to the ALU 8".
The components of arithmetic logic unit 8" comprising paired type
integrated circuits, available as Part No. 74181 from Texas Instruments
Incorporated and labelled 8a and 8b. Units 8a and 8b derive input via
data output cables E and E' and input/out cables F and F' respectively.
Operand input cable D' sets the function desired to compare the information
fed by the aforesaid cables.
ALU output cables G and G' feed components 9a and 9b of the ac-
10 cumulator 9". Accumulator 9" outputs to input/output devices when loadline 24 is activated in a known manner by the system.
Accumulator 9" comprises a pair of 4 bit registers and has an
output line disconnect feature for disconnecting from the ALU without
affecting the accumulator contents. Each register of accumulator 9" may
be an integrated circuit of the type available commercially as Part
No. 74295 from Texas Instruments Incorporated.
Although the operation of the method and apparatus of the present
invention should now be apparent the operation of the embodiment of the
invention illustrated in Fig. 3 in accordance with the method of the
20 invention will now be described.
As stated above, two types of instructions are stored in the instruc-
tion store 1", normal instructions and modification instructions. The
latter type when detected by line decoder 23 or like device, procure the
modification of the next following instruction.
Consider the system in mid operation, an instruction is to be
fetched from instruction store 1". To obtain the instruction, the
instruction address register may be operated by pulling increment line
to cause sequential loading or by loading the contents of modification
- l3
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CA9-77-004
1 adder output cable C by pulsing selectively high load line K2 or low
load line Kl in a known manner. This will enable selection of a block
of instructions or a separate instruction within a block of instruc-
tions.
Each activation of the IAR 3" selectively addresses an instruction
in the I store via access cables A and A'. The selected instruction
then appears on output cables B and B' and impresses the instruction on
the inputs of modification adder input components 15a and 15b", re-
spectively.
If the immediately preceding instruction was not a modification in-
struction, decoder 23 will not provide a trigger input K3 to gate 17'
(17a and 17b) and, consequently, gate output lines J and J' will be
inactive. Hence, whatever instruction appeared on lines B and, B' will
appear unmodified on modification adder output lines C and C' and the
instruction will be implemented normally by the system.
If, on the other hand, the previous instruction was a modification
instruction, gate 17' will be active to thereby provide information on
cables E and E' from the data store 2" to lines J and J'. The modifica-
tion adder will combine these arithmetically to obtain a modified instruction
(the instruction accessed from I-store 1" as modified by data accessed
by the previous modification instruction address).
Either the operand, address label or both parts of an instruction
may be modified in this manner.
It is contemplated by the invention that all or any part of the
data, eg. 2 of 8 bits may be applied for modification purposes to any
part of either operand address or both parts of an instruction.
It will also be realized that, when and if the instruction to be
modified is a null instruction, data accessed from D-store 2" will be
inserted as the modified instruction, in effect permitting storage of
instructions in the Data store 2", if desired.
CA9-77-004 ~L
1 At the end of an instruction access cycle, a timing pulse is im-
pressed on load line 25 of the DAR 4" and operation register 7" and
causes the loading of output cables D and D' respectively while IAR 1"
is usually incremented once again to access new instructions.
The address portion of the modified instruction with which we are
presently concerned enters data store 2" via cable D while the cor-
responding modified operand appearing on cable D' sets a function in ALU
8".
ALU 8" combines data impressed on it via cables E and E' and input/
output cables F and F'.
At the next cycle, accumulator 9" (9a, 9b) is set to the output of
ALU 8". This output appears on input/output lines F and F' and supplies
peripheral devices such as printers, teletype units or magnetic tape
units for instance.
In this manner an instruction may be modified, the data address
register may be loaded with a modified instruction address, and the
operation register may be loaded with a modified operand.
It should be apparent that the decoder 23 may be readily designed
or selected to permit modification of any subsequent instruction rather
than just the immediately subsequent one as disclosed above. Various
apparent modifications to the method and the apparatus of the invention
are apparent and could be made without departing from its spirit or
scope.
