</p>
<p>
- Although the Universal Turing Machine conceptually introduced programs stored as data on a tape, Turing could not provide a formal connection to modern architectures, simply because such architectures did not yet exist. Here, by <em>modern</em>, I refer architectures utilizing random-access system memory, dedicated instruction fetch streams with dynamic branching, and discrete processing units. Though Charles Babbage's 1842 Analytical Engine touched on these concepts, they would wait until the 1940s to re-emerge. The practical engineering context of 1936 was limited to calculating machines programmed via patch panels. Hence, for example, there is no explanation in his paper as to why a von Neumann architecture machine (1945) running a program would exhibit the computation theoretic results derived from a computation theory based on the Turing Machine (1936). To establish this missing connection, the volumes of the TTCA, starting in the following chapters, transform the Turing Machine into a modern architecture in a stepwise fashion, while ensuring that at each step the modifications are <RT-term>inconsequential</RT-term> to computational theoretic existence proofs and complexity class results.
+ Although the Universal Turing Machine conceptually introduced programs stored as data on a tape, Turing could not provide a formal connection to modern architectures, simply because such architectures did not yet exist. Here, by <em>modern</em>, I refer architectures utilizing random-access system memory, dedicated instruction fetch streams with dynamic branching, and discrete processing units. Though Charles Babbage's 1842 Analytical Engine touched on these concepts, they would wait until the 1940s to re-emerge. The practical engineering context of 1936 was limited to calculating machines programmed via patch panels. Hence, for example, there is no explanation in his paper as to why a von Neumann architecture machine (1945) running a program would exhibit the computation theoretic results derived from a computation theory based on the Turing Machine (1936). To establish this missing connection, the volumes of the TTCA, starting in the following chapters, transform the Turing Machine into a modern architecture in a stepwise fashion, while ensuring that at each step the modifications are inconsequential to computational theoretic existence proofs and complexity class results.
</p>
<p>
Mathematical logic underpins the computation theory layer. Computation theory speaks of the time and space complexity of algorithms and the existence of solutions to decider problems, which in turn guides the goals of the architecture and organization layers.
</p>
+ <p>
+ The Turing Machine is a computation theory object that is suggestive of a simple architecture, and a computer organization. Any student who has had to do homework problems centered on Turing Machines, will have tracked the flow of data through the machine, i.e. worked at the register transfer level. However, a little work is needed to complete the architecture analog. The fundamentals are present, the read/write head, the tape, the procedure for using the tape, but other components are missing. The manipulation of symbols remains ungrounded. The tape is not well defined. The use of emptiness is non-architectural like. The tape transport is not articulated, though it is implied. The read buffer that holds a value read, so the programmed controller can do a write without clobbering the read data needed for the next transition is not identified as a component. As we proceed, we will likely discover other missing components.
+ </p>
+
<p>
An <RT-term-em>architecture</RT-term-em> provides the programmer with information that is valuable when designing the logic of programs. This includes all ilks of programmers, including OS programmers, driver programmers, and application programmers. This is sometimes called the instruction set architecture. In addition to specifying the instructions, it includes specifying the behavior of the interrupt subsystem, the method of doing I/O (typically memory-mapped or dedicated I/O instructions), DMA, the special registers and their effects, and the standards to be followed for each if any. More recently, this also includes specifying how programs can make use of secure areas. The architecture is specified by an <RT-term>architect</RT-term>.
</p>
+ <p>
+ It is not a requirement of a computer architecture that it can be realized. The Turing Machine architecture developed in a later section serves as an example. Instead, an architecture can serve other purposes, in this case as a stepping stone to another architecture that could be realized.
+ </p>
+
+
<p>
<RT-term-em>Organization</RT-term-em> is the register-transfer level description of the machine, which includes internal buses, external buses and the state machines that implement the protocols used, control units, interrupt structures, methods of doing I/O, and ALU layout. It dictates the logical arrangement of hardware and the procedures that force the data to flow to satisfy the architectural constraints. The organization is made by a <RT-term>design architect</RT-term>. This level, and the next level, are defined in detail in the classic text by Hamacher, Vranesic, and Zaky <RT-cite ref="V. Carl Hamacher, Zvonko G. Vranesic, and Safwat G. Zaky, Computer Organization, 2nd Edition, McGraw Hill, 1984"></RT-cite>.
</p>
If a computer manufacturer keeps the architecture as a constant, all other levels can change, and a customer will be able to run the same software. The same organization can be used with different implementations. Minor changes in manufacturing process can sometimes be used with an older implementation, for example a simple transistor shrink.
</p>
- <p>
- The ENIAC, sometimes credited as the first electronic computer, is an extreme example of using an older implementation in a new process. Rings of ten tubes were wires in circular shift registers so as to act like gears. Binary switch implementations were found in relay computers, and then in the Anastov-Berry machine and ESAC.
- </p>
<h2>The levels are not independent</h2>
</p>
<p>
- The common understanding of the word 'architecture' is that of a what Hamacher and Zaky calls an organization. For example, even the most experienced of architects will say things like a microprocessor has a "superscalar architecture", though whether a processor is a scalar, superscalar, or VLIW machine is clearly a question of computer organization.
+ The common understanding of the word 'architecture' is what Hamacher and Zaky call an <RT-term>organization</RT-term>. For example, even the most experienced of architects will say things like a microprocessor has a "superscalar architecture", though whether a processor is a scalar, superscalar, or VLIW machine is clearly a question of computer organization.
</p>
<p>
</p>
<p>
- This cascades down the stack, as organization instructions implementation, etc. For example, if the architecture has an instruction that names one of N registers as an operand, then the organization has a register file that data flows to and from, and busses to carry that data, the design will specify a register file and layout the busses, and the manufacturing people will build them.
+ This cascades down the stack, as organization instructs implementation, etc. For example, if the architecture has an instruction that names one of N registers as an operand, then the organization has a register file that data flows to and from, and busses to carry that data, the design will specify a register file and layout the busses, and the manufacturing people will build them.
</p>
- <h2><RT-neologism-em>Consequentiality</RT-neologism-em></h2>
+ <h2>Computation theoretic consequentiality</h2>
<p>
- The Turing Machine is an abstraction, as are architectures, organizations, and implementations. Only a computer realization is concrete, but even then we can make observations that are analogous to properties of an abstraction.
+ The Turing Machine is an abstraction, as are architectures, organizations, and implementations. Only a computer realization is concrete, but even then we can make observations that are analogous to properties of an abstraction. Hence, we can use the language of mathematics to talk about machines at all of the levels.
</p>
<p>
- When transforming machine <RT-math>M_0</RT-math> into machine <RT-math>M_1</RT-math>, we say the transformation is <RT-neologism-em>inconsequential</RT-neologism-em> if any computation theory analysis applied to <RT-math>M_1</RT-math> will yield the same answer as it would if applied to <RT-math>M_0</RT-math>; otherwise, the transformation is <RT-neologism-em>consequential</RT-neologism-em>. The remainder of this section says the same in formal language.
+ When a transform applied to machine <RT-math>m_i</RT-math> produces machine <RT-math>m_{i.1}</RT-math>, and this latter machine gets the same results for the same computational inputs, and furthermore, if any computation theory analysis applied to <RT-math>m_{i.1}</RT-math> yields the same answer as it would when applied to <RT-math>m_i</RT-math> — we say that the transform is <RT-neologism-em>computation theoretic inconsequential</RT-neologism-em>. Otherwise, the transformation is said to be <RT-neologism-em>computation theoretic consequential</RT-neologism-em>. The remainder of this section defines these terms more precisely.
</p>
+ <h3>Definition of the same results transform property</h3>
+
<p>
- Suppose that machine abstraction <RT-math>M_1</RT-math> is the result of a transformation, <RT-math>T</RT-math>, applied to machine abstraction <RT-math>M_0</RT-math>.
+ Suppose we are interested in a given Turing Machine <RT-math>m_i</RT-math> where the machine will potentially be run after being given any one of a number of input tapes <RT-math>x_{i,j}</RT-math>, and for each of those inputs the same tape with the results written will be <RT-math>r_{i,j}</RT-math>, then we notate this as:
</p>
-
+
<RT-math>
- M_0 \xrightarrow{T} M_1
+ m_i(x_{i,j}) = r_{i,j}
</RT-math>
<p>
- We can then assign a property to <RT-math>T</RT-math> called its <RT-neologism-em>consequentiality</RT-neologism-em>, as follows.
- </p>
-
- <p>
- We begin with a set of questions that are legal to ask under computation theory <RT-math>C</RT-math> for a machine <RT-math>M_0</RT-math>:
+ Here the subscripts of the same name set up a correspondence. <RT-math>x_{i,j}</RT-math> is the <RT-math>j</RT-math>th input to the machine <RT-math>m_i</RT-math>, etc. The free variable <RT-math>j</RT-math> runs over all the interesting distinct input tapes to be given to machine <RT-math>m_i</RT-math>. So for example, if we had a machine, say <RT-math>m_8</RT-math>, and we had a set of three inputs to be given to <RT-math>m_8</RT-math>, then:
</p>
<RT-math>
- Q_0 = C, M_0 \colon \{ \langle q, d \rangle \}
+ \begin{aligned}
+ m_8(x_{8,0}) &= r_{8,0} \\
+ m_8(x_{8,1}) &= r_{8,1} \\
+ m_8(x_{8,2}) &= r_{8,2}
+ \end{aligned}
</RT-math>
-
+
<p>
- Here <RT-math>q</RT-math> is a question, and <RT-math>d</RT-math> is the domain of computational inputs for machine <RT-math>M_0</RT-math> under <RT-math>C</RT-math>. This is given information.
+ Another machine, perhaps machine <RT-math>m_7</RT-math>, would have its own distinct inputs <RT-math>x_{7,j}</RT-math>, etc.
</p>
<p>
- Then we pose these questions, and receive the answers.
+ Now suppose that a machine <RT-math>m_{i.1}</RT-math> is the result of a transformation, <RT-math>T</RT-math>, applied to machine <RT-math>m_i</RT-math>.
</p>
<RT-math>
- A_0 = C(M_0, Q_0)
+ m_i \xrightarrow{T} m_{i.1}
</RT-math>
<p>
- Here <RT-math>A_0</RT-math> is a set of vectors, with each vector having the form <RT-math>\langle a, q, d \rangle</RT-math>, where <RT-math>a</RT-math> is the answer, and the remaining components distinguish the correspondence with the question.
- </p>
-
- <p>
- If any question in <RT-math>Q_0</RT-math>, for the given machine over the given domain, is not legal to ask of <RT-math>M_1</RT-math> under <RT-math>C</RT-math>, then we say that <RT-math>T</RT-math> is <RT-neologism>consequential</RT-neologism> over <RT-math>Q_0</RT-math>. Otherwise, we continue with asking the same questions about <RT-math>M_1</RT-math>:
+ We can then assign a property to transform <RT-math>T</RT-math> called its <RT-neologism-em>doesn't change results</RT-neologism-em> property, as follows. If and only if:
</p>
<RT-math>
- A_1 = C(M_1, Q_0)
+ \forall j \colon r_{i,j} = r_{i.1,j}
</RT-math>
<p>
- If and only if <RT-math>A_0 = A_1</RT-math>, we say that transform <RT-math>T</RT-math> is <RT-neologism-em>inconsequential over <RT-math>Q_0</RT-math></RT-neologism-em>; otherwise, we say <RT-math>T</RT-math> is <RT-neologism-em>consequential over <RT-math>Q_0</RT-math></RT-neologism-em>.
+ then <RT-math>T</RT-math> doesn't change <RT-math>m_i</RT-math> results. Here we note that we are evaluating a specific machine <RT-math>m_i</RT-math>, so we must add the qualifier '<RT-math>m_i</RT-math> results'. It might be that for another machine with another corresponding set of interesting inputs, the transform would lead to a new machine that produces different results.
</p>
<p>
- Now let us generalize. First, let us define the set of all input domains that are legal for machine <RT-math>M_0</RT-math>, and can be analyzed using <RT-math>C</RT-math>.
+ If, and only if, it is the case that
</p>
<RT-math>
- D(C, M_0) \rightarrow D
+ \forall i, \forall j \colon r_{i,j} = r_{i.1,j}
</RT-math>
<p>
- Then let us define the set of all questions of decidability, along with the two questions of time and space complexity, that can legally be asked under <RT-math>C</RT-math> of machine <RT-math>M_0</RT-math> over the domains in <RT-math>D</RT-math>.
+ then we can say without qualification that <RT-math>T</RT-math> is a <RT-neologism>same results transform</RT-neologism>. Though still implied are the sets of machines and tapes.
</p>
-
- <RT-math>
- Q(C, M_0, D) \rightarrow Q
- </RT-math>
+
+ <h3>Definition of the computation theoretic consequential/inconsequential transform property</h3>
<p>
- We must establish the theoretical baseline by posing these generalized questions to our initial machine:
+ Suppose we still have the given machines, and their corresponding inputs, that were used when determining transform <RT-math>T</RT-math> is a same results transform.
</p>
- <RT-math>
- A_0 = C(M_0, Q)
- </RT-math>
-
<p>
- If any question in <RT-math>Q</RT-math> is not legal to ask of <RT-math>M_1</RT-math> under <RT-math>C</RT-math>, then we say that <RT-math>T</RT-math> is a <RT-neologism>consequential</RT-neologism> transform. Otherwise, we continue on to ask the same questions concerning <RT-math>M_1</RT-math>:
+ Suppose we also have a computation theory <RT-math>C</RT-math> that allows us to analyze some machines so as to answer some questions we find interesting. Suppose furthermore that among these questions are questions of time and space complexity, along with zero or more questions about decidability. Furthermore, we are given a machine, say <RT-math>m_i</RT-math>, for which these questions have answers. We represent this as:
</p>
<RT-math>
- A_1 = C(M_1, Q)
+ a_{i,k} = q_{i,k}(m_i, \{x_{i,j}\})
</RT-math>
<p>
- If and only if <RT-math>A_0 = A_1</RT-math>, we say that transform <RT-math>T</RT-math> is <RT-neologism-em>inconsequential</RT-neologism-em> under <RT-math>C</RT-math>, and otherwise it is <RT-neologism>consequential</RT-neologism> under <RT-math>C</RT-math>. As we are only discussing C in this paper, the "under <RT-math>C</RT-math>" qualification can be dropped as it is clear by context.
+ Here, <RT-math>\{x_{i,j}\}</RT-math> represents the entire domain of <RT-math>j</RT-math> tapes being passed as arguments to the question <RT-math>q_{i,k}</RT-math>. From this, we can observe that if there are <RT-math>n_k</RT-math> questions, then we will have <RT-math>n_k</RT-math> answers. Also, for a specific machine <RT-math>m_i</RT-math>, where there are <RT-math>n_j</RT-math> <RT-math>j</RT-math> values, the domain over which <RT-math>m_i</RT-math> will be analyzed will have <RT-math>n_j</RT-math> tapes in it.
</p>
-
-
- <h2>Is realization consequential?</h2>
-
<p>
-
-
-
- <p>
- The very title of this book has built in the same pitfall, as mostly what we discuss is the flow of data between named components, i.e. register transfer level description, which is organization. Computer architecture is often expressed by describing a <RT-term>reference organization</RT-term>.
+ As we had already discovered when determining <RT-math>T</RT-math> is a same results transform, <RT-math>T</RT-math> transforms machine <RT-math>m_i</RT-math> into machine <RT-math>m_{i.1}</RT-math>.
</p>
-
-
+ <RT-math>
+ m_i \xrightarrow{T} m_{i.1}
+ </RT-math>
<p>
- A key question then, do different computer organizations have consequential effect. By <RT-term-em>consequential<RT-term-em> effect we mean to ask if they exhibit different behavior than that predicted by computer theory. It is certainly possible to force this to happen. Suppose for example, we make a computer that has a clock such that clock pulses are exponentially further apart on every clock tick. Then a linear time program running on the computer would have logarithmic time asymptotic behavior with ever more input data.
+ For our specific machine <RT-math>m_i</RT-math>, if and only if:
</p>
- <p>
- A slowing clock could happen due to some peculiar extenuating circumstances, perhaps because a battery is going dead, but is not a normal situation, and there is a reason for that. If a computer works with a clock period of p0, then any longer clock period would be unnecessarily wasting time.
- </p>
+ <RT-math>
+ \forall k \colon a_{i,k} = a_{i.1,k}
+ </RT-math>
<p>
- Nor is it practical to go the other direction, and make an exponential time program run in linear time by having an every faster clock. The simple reason being that the designers will have already set the clock to the fastest that it can safely go.
+ then <RT-math>T</RT-math> is <RT-neologism-em>computation theoretic inconsequential for <RT-math>m_i</RT-math></RT-neologism-em>.
</p>
<p>
- Adding two Arabic representation numbers of ever lengthening operands is asymptotically a linear time problem. For numbers of fixed length, where that fixed length is not very long, a lookup table can be used. As such each addition would require the same amount of time, but this is merely making the worse case time for bit widths up to the lookup table with the time for all addition. It hides the trend curve, but does not replace it. As th
-
- In theory any program could be run in constant time by using a lookup table.
-
- What about going the other direction? If we have an ever faster
-
-
- To answer this question we would have to be specific about the organization, but on the face of it the answer is "no". Take a superscalar architecture. Suppose that it were possible to run two instructions on every clocks cycle, that is merely a linear term improvement in the step count for completing the program. It doesn't change the asymptotic 'complexity' of programs. A linear time program is still a linear time program, etc.
+ If, and only if, it is the case that:
</p>
- <p>
- The Turing Machine is a computation theory object that is suggestive of a simple architecture, and a computer organization. Any student who has had to do homework problems centered on Turing Machines, will have tracked the flow of data through the machine, i.e. worked at the register transfer level. This makes the Turing Machine a computer organization. All descriptions of Turing Machines
-
-
- however, a little work is needed to complete architecture analog. The fundamentals are present, the read/write head, the tape, the procedure for using the tape, but other components are missing. The manipulation of symbols remains ungrounded. The tape is not well defined. The use of emptiness is non-architecture like. The tape transport is not articulated, though it is implied. The read buffer that holds a value read, so the programmed controller can do write without clobbering the read data needed for the next transition is not identified as a component. As we proceed, we will likely discover other missing components.
- </p>
+ <RT-math>
+ \forall i, \forall k \colon a_{i,k} = a_{i.1,k}
+ </RT-math>
<p>
- It is not a requirement of a computer architecture that it can be realized. The Turing Machine architecture developed in a later section serves as an example. Instead, an architecture can serve other purposes, in this case as a stepping stone to another architecture that could be realized.
+ then we can say without qualification that <RT-math>T</RT-math> is <RT-neologism-em>computation theoretic inconsequential</RT-neologism-em>. Though still implied are the sets of machines and tapes.
</p>
- <p>
- We can now further articulate the goal of these volumes: to first capture the Turing Machine as an architecture, and then to transform that architecture step by step, without making any changes of consequence relative to computation theoretic results, into a modern computer architecture that could be implemented. A secondary goal is to learn something along the way.
- </p>
+
<h1>Symbol</h1>
<p>The original Turing Machine had an infinite tape. In contrast the TTCA machine has a surprising property: for computational problems all of its components remain finite. This follows from the fact that during computation a machine makes a finite number of steps, so the tape can only be expanded to be a finite size.</p>
+ <h1>Consequentiality across the design abstraction stack</h1>
+
+
+ <h2>Dhoice of realization</h2>
+
+ <p>
+ Let us take the example of adding two Arabic representation numbers. Logically this is considered to be a logarithmic time problem. We break the operands into fixed length pieces, and adding them in pairs results in a carry per block. By recursively pairing the blocks and applying the carries, we generate wider carries. Thus we can show that in terms of the logic gates that must be traversed, the sum is a log time operation.
+ </p>
+
+ <p>
+ Physics comes to a different conclusion. In the worst case, a carry into the least significant bit can affect the sum bit some physical distance away. As the operands get longer, this distance grows in proportion. So given the propagation of information at a fixed speed, the bounding evaluation time against growing operand width is linear time. Even if it is log time in gate count, at some point the interconnect delay will dominate.
+ </p>
+
+ <p>
+ The logical analysis of the adder given above allowed for unbounded resources, because as the adder operand increases in size, the number of block adders increases without bounds. In any realization there will be a limit on the number of blocks that can be added in parallel. These groups are then processed one by one, and the carry is propagated between them. Consequently, as the operands grow in length without bounds, the adder evaluation time becomes proportional to the number of groups processed. Processing groups in series is a linear time algorithm.
+ </p>
+
+ <p>
+ It is notable that the time-multiplexed use of computer resources produces the same linear time result as the physics of information propagation analysis for the adder.
+ </p>
+
+ <p>
+ A Turing Machine program faces a situation analogous to physical constraints. Given the operands are found on the tape, and the carry-in can affect the msb of the sum, the head will have to move ever more cells rightward to convey that lsb information up to the msb. Based solely on the propagation time of that information, addition is found to be a linear time algorithm. This propagation remains computation class limiting even if the Turing Machine is given an unbounded number of independent heads.
+ </p>
+
+ <p>
+ There appears to be alignment among physical limitations, resource limited computing, and steps spent by Turing Machines while they carry information across a linear tape. This alignment indicates that a reasonable realization will be computation theoretic inconsequential.
+ </p>
+
+ <p>
+ At this point we have arrived at questions of the <RT-term>physics of computation</RT-term>. The above analysis made use of classical physics. Perhaps realizations based directly on principles from Quantum physics models will be computation theoretic consequential.
+ </p>
+
+
+ <h2>Choice of implementation</h2>
+
+ <p>
+ From Babbage's Analytical Engine of 1842 up to the transition to mechanical relays and vacuum tubes in the 1940s, calculating machines were implemented with gears. The basic principle is apparent to anyone who has seen a mechanical odometer. Consider adding numbers for example: given two odometers, step one back at the same time as stepping a second one forward; when the first one reaches zero, the second will hold the sum. This process can be optimized, but the general idea remains the same. For such machines, a step is a rotation of the main shaft.
+ </p>
+
+ <p>
+ The Harvard Mark I machine had a main axle speed that maxed out at 3000 RPMs, say 2700 RPMs to keep our math simple. Then this is 2700 steps per minute. The ENIAC was a similar implementation, but one that called out the use of circular shift registers of vacuum tubes instead of mechanical gears. Because there were 10 tubes in a ring register, it took 10 clock ticks to complete one 'rotation'. The clock rate maxed out at 450 kilohertz. That would be one rotation every <RT-math>1/45,000</RT-math> of a second, or <RT-math>2.7</RT-math> million RPMs, an improvement of three orders of magnitude!
+ </p>
+
+ <p>
+ Yet, the same program when run on the Mark I took the same number of steps as on the ENIAC. But more importantly, a linear time algorithm on the Mark I was still a linear time algorithm on the ENIAC, etc. Thus, these implementation differences were computation theoretic inconsequential.
+ </p>
+
+ <p>
+ It feels unsatisfactory to leave out the tremendous difference in clock rates. So let us address this feeling by naming an ENIAC main shaft 'rotation' as a standard 'step'. If we do this, then a Mark I shaft rotation would be 1000 ENIAC steps. Yet, this would merely affect the linear constant in the step count formulas. The same programs can be run, with the same inputs, and asymptotic behavior is the same for both machines, because computation classes do not include the constants on the step count equation. Constant time remains constant; linear, polynomial, and exponential time classes are the same as before. Programs that decide questions would get the same answers when they completed.
+ </p>
+
+ <p>
+ Because we made an ENIAC shift register turn completion a 'standard step', we have a relative measure, so there is something we can do to create a computation theoretic consequential difference. Suppose we have two ENIAC machines, and we send one speeding away from Earth at an exponentially increasing rate, i.e. increasing red shift, and we observe it from Earth. We will observe that the clock on the traveling ENIAC is growing ever slower, and that a linear time program running on it will be observed to have exponential time behavior. Unfortunately, relativity does not smile upon us, as the people on the spaceship would not see the inverse, a speeding computation on Earth, but rather they would also observe a slowing one.
+ </p>
+
+ <p>
+ So then, instead we send a spaceship towards Earth, with increasing blue shift, and we would observe that spaceship's ENIAC getting faster and faster. This is still not a computation theoretic speedup, because it is not asymptotic. In finite time, said spaceship would run into Earth, or pass it by and then be red shifting.
+ </p>
+
+ <p>
+ A designer could purposely slow the clock on a second ENIAC so as to emulate red shift. For this to be more than mere theater there would have to be physical reason to run a slower clock than necessary, for example perhaps for conserving an ever dwindling battery. But slowing computation down, or even stopping it, is typically not useful. However, going the other direction, an ever faster clock does not work, as there is a finite maximum physical clock speed.
+ </p>
+
+ <p>
+ We get an increasing blue shift situation with Moore's law. If every generation transistors become exponentially smaller, and thus faster, and we consider step times in years, hopping from new realization to new realization, then indeed linear time algorithms on a single realization would be log time algorithms on the generational computer. But chances are this is not an asymptotic, i.e. limit to infinity, phenomenon either.
+ </p>
+
+ <h2>Choice of organization</h2>
+
+ <p>
+ Superscalar and VLIW computers execute multiple instructions in parallel. Real data dependencies put limitations on how many instructions are available to be executed in parallel, but even discounting this, if a program were executed N instructions at a time, its time to execute would divide by N. This merely affects the linear component of the equation mapping step count to input length, and thus does not change the computation class. Superscalar and VLIW architectures do not affect decisions; indeed they are transparent to programs, so decider problem results cannot change. Hence these techniques are not computation theoretic consequential.
+ </p>
+
+ <p>
+ In general, by definition, organizations do not change a program's view of the machine, as that is part of the architecture. So organizations will not affect decider results. Also, the memory operations will be the same, as that is viewable state, so space complexity does not change unless time complexity changes.
+ </p>
+
+ <p>
+ The realization sets fixed resources, so any attempt at parallelization will be bounded, as in the superscalar and VLIW discussion above. Thus at best it can divide the execution time by N.
+ </p>
+
+ <p>
+ Some organizations can arrange computation in a manner that the base clock can run faster than for other organizations. However, clocks run at a fixed maximum speed. On modern systems they can slow down to reduce heat dissipation or battery consumption, but that does not make programs faster. So if one organization has a faster clock than another, the ratio is merely a linear term contributor. Apart from stopping, there is nothing a clock can do to participate in the decision making of the program.
+ </p>
+
+ <p>
+ Caching of values sent to the system memory again does not participate in the decision making of a program. We are at best looking at improvements in the linear term.
+ </p>
+
+ <p>
+ Branch prediction saves the time required to do a full fetch, but fundamentally it does not change the data flow graph of the program. The same decisions are made.
+ </p>
+
+ <p>
+ Suppose that an organization keeps the operands for a function in a content-addressable memory. When the operands are recognized, it then immediately returns the looked-up value. This approach, called <RT-term>memoization</RT-term>, bypasses the internal looping of the function. Hence, this does participate in the decision making of a program, and could potentially change the computation theoretic complexity of programs with certain properties where the same operands occur in patterns and lead to an expensive computation. <RT-term>Memoization</RT-term> is typically designed into computer languages, rather than being built into the organization of a computer. Chances are, at the computer organization level or lower, it is more efficient to simply execute the presented instructions.
+ </p>
+
+ <h2>Choice of architecture</h2>
+
+ <p>
+ Common decisions made at the architecture level are those for supporting RISC or CISC, the bit layout and handling of operands, the size of the internal register file, how DMA is to be handled, whether to use memory-mapped I/O or have explicit instructions for it, how interrupts are to be implemented and the number of entries in the interrupt table, what special registers are present and what features are available through them, how virtual memory and its user and process IDs are to be implemented with the possible use of a translation lookaside buffer, what onboard execution units will have direct instructions, the built-in data types, questions of unaligned accesses, bus standards to be supported, if sleep modes are to be present, how the machine will get booted, the security rings that will be supported, details of the hardware virtualization layer, special support for the OS, how the system stack will be handled, potential partitioning of address space, support for large buffers, and memory sharing features: none of these are computation theoretic consequential.
+ </p>
+
+ <p>
+ As architecture enters the gray area with organization, cache architecture, bus layouts, bus buffers, direct inclusion of write buffers, perhaps a stack cache, prefetch buffers and split-transaction buses: none of these are computation theoretic consequential either.
+ </p>
+
+ <p>
+ Said features certainly affect performance, but none participate in the decisions the program makes, change the number of execution steps by more than a linear ratio, or alter the memory complexity of the program.
+ </p>
+
+
+
+
<hr>
<h1>TTCA Turing Machine in Lisp</h1>
projects / TM-2026 / commitdiff