Computer Architecture Notes

CS6303 - Computer Architecture Notes

Unit 1

Design Principles of Computer Architecture CISC vs. RISC

Instructions directly executed by hardware

Maximize instruction issue rate (ILP)

Simple instructions (easy to decode)

Access to memory only via load/store

Plenty of registers

Pipelining

Basic Computer Organization

Bus-Based Computer Organization

Data path:

Memory Dynamic Random Access Memory (DRAM)

The choice for main memory

Volatile (contents go away when power is lost)

Fast

Relatively small

DRAM capacity: 2x / 2 years (since 96); 64x size improvement in last decade

Static Random Access Memory (SRAM)

The choice for cache

Much faster than DRAM, but less dense and more costly

Magnetic disks

The choice for secondary memory

Non-volatile

Slower

Relatively large

Capacity: 2x / 1 year (since 97)250X size in last decade

Solid state (Flash) memory

The choice for embedded computers

Non-volatile

Optical disks

Removable, therefore very large

Slower than disks

Magnetic tape

Even slower

Sequential (non-random) access

The choice for archival

System Software Operating system supervising program that interfaces the users program with the hardware (e.g., Linux, MacOS, Windows)

- Handles basic input and output operations

- Allocates storage and memory

- Provides for protected sharing among multiple applications

Compiler translate programs written in a high-level language (e.g., C, Java) into instructions that the hardware can executeTECHNOLOGY

A transistor is simply an on/off switch controlled by electricity. The integrated circuit (IC) combined dozens to hundreds of transistors into a single chip. To describe the tremendous increase in the number of transistors from hundreds to millions, the adjective very large scale is added to the term, creating the abbreviation VLSI, for very large-scale integrated circuit.CLASSES OF COMPUTERS

Desktop computers

Designed to deliver good performance to a single user at low cost usually executing 3rd party software, usually incorporating a graphics display, a keyboard, and a mouse

Servers

Used to run larger programs for multiple, simultaneous users typically accessed only via a network and that places a greater emphasis on dependability and (often) security

Modern version of what used to be called mainframes, minicomputers and supercomputers Large workloads Built using the same technology in desktops but higher capacity Gigabytes to Terabytes to Peta bytes of storage

Expandable

Scalable Reliable Large spectrum: from low-end (file storage, small businesses) to supercomputers (high end scientific and engineering applications) Examples: file servers, web servers, database serversSupercomputers A high performance, high cost class of servers with hundreds to thousands of processors, terabytes of memory and petabytes of storage that are used for high-end scientific and engineering applicationsEmbedded computers (processors) A computer inside another device used for running one predetermined application Microprocessors everywhere! (washing machines, cell phones, automobiles, video games) Run one or a few applications Specialized hardware integrated with the application (not your common processor) Usually stringent limitations (battery power) High tolerance for failure (dont want your airplane avionics to fail!) Becoming ubiquitous Engineered using processor cores

The core allows the engineer to integrate other functions into the processor for fabrication on the same chip

Using hardware description languages: Verilog, VHDL

Embedded Processor CharacteristicsThe largest class of computers spanning the widest range of applications and performance

Often have minimum performance requirements. Often have stringent limitations on cost. Often have stringent limitations on power consumption.

Often have low tolerance for failure.PERFORMANCE

Defining Performance

Lets suppose we define performance in terms of speed. This still leaves two possible definitions. You could define the fastest plane as the one with the highest cruising speed, taking a single passenger from one point to another in the least time. If you were interested in transporting 450 passengers from one point to another, however, the 747 would clearly be the fastest, as the last column of the figure shows. Similarly, we can define computer performance in several different ways.

Throughput and Response Time Do the following changes to a computer system increase throughput, decrease response time, or both?

1. Replacing the processor in a computer with a faster version

2. Adding additional processors to a system that uses multiple processors

for separate tasksfor example, searching the World Wide Web

Example

Time taken to run aprogram = 10s on A, 15s on BRelative performance =Execution TimeB / Execution TimeA

=15s/10s

=1.5

So A is 1.5 times faster than B

Measuring Execution TimeElapsed time

Total response time, including all aspects Processing, I/O, OS overhead, idle time Determines system performance

CPU time

Time spent processing a given job Discounts I/O time, other jobs shares

Comprises user CPU time and systemCPUtime Different programs are affected differentlybyCPU and system performance

CPU ClockingOperation of digital hardware governed by a constant-rate clock

Clock period: duration of a clock cycle

e.g., 250ps = 0.25ns = 2501012s

Clock frequency (rate): cycles per second

e.g., 4.0GHz = 4000MHz = 4.0109Hz CPU TIME

Example Computer A: 2GHz clock, 10s CPU time

Designing Computer B

Aim for 6s CPU time

Can do faster clock, but causes 1.2 clock cycles

How fast must Computer B clock be?

Instruction Count and CPI Instruction Count for a program

Determined by program, ISA and compiler

Average cycles per instruction

Determined by CPU hardware

If different instructions have different CPI

Average CPI affected by instruction mix

CPI Example Computer A: Cycle Time = 250ps, CPI = 2.0

Computer B: Cycle Time = 500ps, CPI = 1.2

Same ISA

Which is faster, and by how much?

CPI in More Detail If different instruction classes take different numbers of cycles

Weighted average CPI

CPI Example Alternative compiled code sequences using instructions in classes A, B, C Class A B C

CPI for class 1 2 3

IC in sequence 12 1 2

IC in sequence 2 4 1 1

Sequence 1: IC = 5

Clock Cycles= 21 + 12 + 23= 10

Avg. CPI = 10/5 = 2.0

Sequence 2: IC = 6

Clock Cycles= 41 + 12 + 13= 9

Avg. CPI = 9/6 = 1.5

Performance depends on Algorithm: affects IC, possibly CPI Programming language: affects IC, CPI Compiler: affects IC, CPI Instruction set architecture: affects IC, CPI, Tc

computers are constructed using a clock that determines when events take place in the hardware. These discrete time intervals are called clock cycles (or ticks, clock ticks, clock periods, clocks, cycles). Designers refer to the length of a clock period both as the time for a complete clock cycle (e.g., 250 picoseconds, or 250 ps) and as the clock rate (e.g., 4 gigahertz, or 4 GHz), which is the inverse of the clock period. In the next subsection, we will formalize the relationship between the clock cyclesof the hardware designer and the seconds of the computer user.Computer Performance and its Factors

Instruction Performance

The term clock cycles per instruction, which is the average number of clock cycles each instruction takes to execute, is often abbreviated as CPIClassic cpu performance

POWERWALL

UNIPROCESSOR AND MULTIPROCEASSOR

The power limit has forced a dramatic change in the design of microprocessors. Figure shows the improvement in response time of programs for desktop microprocessors over time. Since 2002, the rate has slowed from a factor of 1.5 per year to less than a factor of 1.2 per year.

As an analogy, suppose the task was to write a newspaper story. Eight reporters working on the same story could potentially write a story eight times faster. To achieve this increased speed, one would need to break up the task so that each reporter had something to do at the same time. Thus, we must schedule the subtasks. If anything went wrong and just one reporter took longer than the seven

others did, then the benefits of having eight writers would be diminished. Thus, we must balance the load evenly to get the desired speedup. Another danger would be if reporters had to spend a lot of time talking to each other to write their sections. You would also fall short if one part of the story, such as the conclusion, couldnt be written until all of the other parts were completed. Thus, care must be taken to reduce communication and synchronization overhead. For both this analogy and parallel programming, the challenges include scheduling, load balancing, time for synchronization, and overhead for communication between the parties. As you might guess, the challenge is stiffer with more reporters for a newspaper story and more processors for parallel programming

OPERATIONS OF COMPUTER HARDWARE

OPERANDS OF COMPUTER HARDWARE

A very large number of registers may increase the clock cycle time simply because it takes electronic signals longer when they must travel farther. Guidelines such as smaller is faster are not absolutes; 31 registers may not be faster than 32. Yet, the truth behind such observations causes computer designers to take them seriously. In this case, the designer must balance the craving of programs

for more registers with the designers desire to keep the clock cycle fast.Another reason for not using more than 32 is the number of bits it would take in the instruction forma

Memory OperandsAs explained above, arithmetic operations occur only on registers in MIPS instructions; thus, MIPS must include instructions that transfer data between memory and registers. Such instructions are called data transfer instructions. To access a word in memory, the instruction must supply the memory address. Memory is just a large, single-dimensional array, with the address acting as the index to that array, starting at 0. For example, in Figure the address of the third data element is 2, and the value of Memory[2] is 10.

Given the importance of registers, what is the rate of increase in the number of

registers in a chip over time?

1. Very fast: They increase as fast as Moores law, which predicts doubling the

number of transistors on a chip every 18 months.

2. Very slow: Since programs are usually distributed in the language of the

computer, there is inertia in instruction set architecture, and so the number

of registers increases only as fast as new instruction sets become viable.

REPRESENTING INSTRUCTION

LOGICAL INSTRUCTION

Case/Switch Statement

Most programming languages have a case or switch statement that allows the programmer to select one of many alternatives depending on a single value. The simplest way to implement switch is via a sequence of conditional tests, turning the switch statement into a chain of if-then-else statements. Sometimes the alternatives may be more efficiently encoded as a table of addresses of alternative instruction sequences, called a jump address table or jump table, and the program needs only to index into the table and then jump to the appropriate sequence. The jump table is then just an array of words containing addresses that correspond to labels in the code. The program loads the appropriate entry from the jump table into a register. It then needs to jump using the address in the register. To support such situations, computers like MIPS include a jump register instruction (jr), meaning an unconditional jump to the address specified in a register. Then it jumps to the proper address using this instructionNested Loop

Procedures that do not call others are called leaf procedures. Life would be simple if all procedures were leaf procedures, but they arent. Just as a spy might employ other spies as part of a mission, who in turn might use even more spies, so do procedures invoke other procedures. Moreover, recursive procedures even invoke clones of themselves. Just as we need to be careful when using registers in procedures, more care must also be taken when invoking nonleaf procedures.

ADDRESSING MODES

1. Immediate addressing, where the operand is a constant within the instruction

itself

2. Register addressing, where the operand is a register

Base or displacement addressing, where the operand is at the memory location

whose address is the sum of a register and a constant in the instruction

4. PC-relative addressing, where the branch address is the sum of the PC and a

constant

in the instruction

5. Pseudodirect addressing, where the jump address is the 26 bits of the instruction

concatenated with the upper bits of the PC

UNIT 2

ADDITION AND SUBTRACTION

Digits are added bit by bit from right to left, with carries passed to the next digit to the left, just as you would do by hand. Subtraction uses addition: the appropriate operand is simply negated before being added.

Binary Addition and Subtraction

Example

1. Add 610 and 710

2. Subtract 610 from 710

Adding 610 and 710 can be done as follows:

0000 0000 0000 0000 0000 0000 0000 0111two = 7ten

+ 0000 0000 0000 0000 0000 0000 0000 0110two = 6ten

= 0000 0000 0000 0000 0000 0000 0000 1101two = 13ten

The 4 bits to the right have all the action;

Subtracting 6ten from 7ten can be done directly:

0000 0000 0000 0000 0000 0000 0000 0111two = 7ten

0000 0000 0000 0000 0000 0000 0000 0110two = 6ten

= 0000 0000 0000 0000 0000 0000 0000 0001two = 1ten

or Subtraction can be done via addition using the twos complement representation of -6:

0000 0000 0000 0000 0000 0000 0000 0111two = 7ten

+ 1111 1111 1111 1111 1111 1111 1111 1010two = 6ten

= 0000 0000 0000 0000 0000 0000 0000 0001two = 1ten

The above figure shows the sums and carries. The carries are shown in parentheses, with the arrows showing how they are passed.

Binary addition, showing carries from right to left. The rightmost bit adds 1 to 0, resulting in the sum of this bit being 1 and the carry out from this bit being 0. Hence, the operation for the second digit to the right is 0 + 1 + 1. This generates a 0 for this sum bit and a carry out of 1. The third digit is the sum of 1 + 1 + 1, resulting in a carry out of 1 and a sum bit of 1. The fourth bit is 1 + 0 + 0, yielding a 1 sum and no carry.

When the result from an operation cannot be represented with the available hardware, in this case a 32-bit word. When can overflow occur in addition? When adding operands with different signs, overflow cannot occur. The reason is the sum must be no larger than one of the operands. For example, -10 + 4 = -6. Since the operands fit in 32 bits and the sum is no larger than an operand, the sum must fit in 32 bits as well. Therefore, no overflow can occur when adding positive and negative operands. There are similar restrictions to the occurrence of overflow during subtract, but its just the opposite principle: when the signs of the operands are the same, overflow cannot occur. To see this, remember that x - y = x + (-y) because we subtract by negating the second operand and then add. Therefore, when we subtract operands of the same sign we end up by adding operands of different signs. From the prior paragraph, we know that overflow cannot occur in this case either.

Adding or subtracting two 32-bit numbers can yield a result that needs 33 bits to be fully expressed. The lack of a 33rd bit means that when overflow occurs, the sign bit is set with the value of the result instead of the proper sign of the result. Since we need just one extra bit, only the sign bit can be wrong. Hence, overflow occurs when adding two positive numbers and the sum is negative, or vice versa. This means a carry out occurred into the sign bit. Overflow occurs in subtraction when we subtract a negative number from a positive number and get a negative result, or when we subtract a positive number from a negative number and get a positive result. This means a borrow occurred from the sign bit.

Two kinds of Overflow Conditions

MIPS detects overflow with an exception, also called an interrupt on many computers. An exception or interrupt is essentially an unscheduled procedure call. The address of the instruction that overflowed is saved in a register, and the computer jumps to a predefined address to invoke the appropriate routine for that exception. The interrupted address is saved so that in some situations the program can continue after corrective code is executed.

MIPS includes a register called the exception program counter (EPC) to contain the address of the instruction that caused the exception. The instruction move from system control (mfc0) is used to copy EPC into a general-purpose register so that MIPS software has the option of returning to the offending instruction via a jump register instruction.

MULTIPLICATION

The multiplication of decimal numbers in longhand to remind ourselves of the steps of multiplication and the names of the operands. For reasons that will become clear shortly, we limit this decimal example to using only the digits 0 and 1. Multiplying 100010 by 100110:

Multiplicand 100010

Multiplier x 100110

---------

1000

0000

0000

1000

-------------------

Product 100100010

The first operand is called the multiplicand and the second the multiplier. The final result is called the product.

The old algorithm shows that taking the digits of the multiplier one at a time from right to left, multiplying the multiplicand by the single digit of the multiplier, and shifting the intermediate product one digit to the left of the earlier intermediate products.

The number of digits in the product is considerably larger than the number in either the multiplicand or the multiplier

If the sign bits were ignored then, the length of the multiplication of an n-bit multiplicand and an m-bit multiplier is a product that is n + m bits long. n + m bits are required to represent all possible products

Multiply must cope with overflow because we frequently want a 32-bit product as the result of multiplying two 32-bit numbers

In the above example1. Just place a copy of the multiplicand (1 multiplicand) in the proper place if the multiplier digit is a 1, or2. Place 0 (0 multiplicand) in the proper place if the digit is 0.

SEQUENTIAL VERSION OF THE MULTIPLICATION ALGORITHM AND HARDWARE

This design mimics the algorithm what we learned in school. The following figure shows the hardware.

We have drawn the hardware so that data flows from top to bottom to resemble more closely the paper-and-pencil method. Lets assume that the multiplier is in the 32-bit Multiplier register and that the 64-bit Product register is initialized to 0. From the paper-and-pencil example above, its clear that we will need to move the multiplicand left one digit each step, as it may be added to the intermediate products. Over 32 steps, a 32-bit multiplicand would move 32 bits to the left. Hence, we need a 64-bit Multiplicand register, initialized with the 32-bit multiplicand in the right half and zero in the left half. This register is then shifted left 1 bit each step to align the multiplicand with the sum being accumulated in the 64-bit Product register.

THREE BASIC STEPS NEEDED FOR EACH BIT

The least significant bit of the multiplier (Multiplier0) determines whether the multiplicand is added to the Product register. The left shift in step 2 has the effect of moving the intermediate operands to the left, just as when multiplying with paper and pencil. The shift right in step 3 gives us the next bit of the multiplier to examine in the following iteration. These three steps are repeated 32 times to obtain the product. If each step took a clock cycle, this algorithm would require almost 100 clock cycles to multiply two 32-bit numbers.

This algorithm and hardware are easily refined to take 1 clock cycle per step. The speed-up comes from performing the operations in parallel: the multiplier and multiplicand are shifted while the multiplicand is added to the product if the multiplier bit is a 1. The hardware just has to ensure that it tests the right bit of the multiplier and gets the pre shifted version of the multiplicand. The hardware is usually further optimized to halve the width of the adder and registers by noticing where there are unused portions of registers and adders.

Replacing arithmetic by shifts can also occur when multiplying by constants. Some compilers replace multiplies by short constants with a series of shifts and adds. Because one bit to the left represents a number twice as large in base 2, shifting the bits left has the same effect as multiplying by a power of 2. Almost every compiler will perform the strength reduction optimization of substituting a left shift for a multiply by a power of 2.

EXAMPLE FOR A MULTIPLY ALGORITHM

Using 4-bit numbers to save space, multiply 210 310, or 00102 00112.

The value of each register for each of the steps is given in the above figure. The final value is 0000 01102 or 610.

SIGNED MULTIPLICATION

In the signed multiplication, convert the multiplier and multiplicand to positive numbers and then remember the original signs.

The algorithms should then be run for 31 iterations, leaving the signs out of the calculation.

The shifting steps would need to extend the sign of the product for signed numbers. When the algorithm completes, the lower word would have the 32-bit product.

FASTER MULTIPLICATION

Faster multiplications are possible by essentially providing one 32-bit adder for each bit of the multiplier: one input is the multiplicand ANDed with a multiplier bit, and the other is the output of a prior adder.

Connect the outputs of adders on the right to the inputs of adders on the left, making a stack of adders 32 high.

The above figure shows an alternative way to organize 32 additions in a parallel tree. Instead of waiting for 32 add times, we wait just the log2 (32) or five 32-bit add times.

Multiply can go even faster than five add times because of the use of carry save adders It is easy to pipeline such a design to be able to support many multiplies simultaneously

MULTIPLY IN MIPS

MIPS provides a separate pair of 32-bit registers to contain the 64-bit product, called Hi and Lo. To produce a properly signed or unsigned product, MIPS has two instructions: multiply (mult) and multiply unsigned (multu). To fetch the integer 32-bit product, the programmer uses move from lo (mflo). The MIPS assembler generates a pseudoinstruction for multiply that specifies three generalpurpose registers, generating mflo and mfhi instructions to place the product into registers.

DIVISION

The reciprocal operation of multiply is divide, an operation that is even less frequent and even more quirky. It even offers the opportunity to perform a mathematically invalid operation: dividing by 0.

Lets start with an example of long division using decimal numbers to recall the names of the operands and the grammar school division algorithm. For reasons similar to those in the previous section, we limit the decimal digits to just 0 or 1.

EXAMPLE

The example is dividing 1,001,01010 by 100010:

Divides two operands, called the dividend and divisor, and the result, called the quotient, are accompanied by a second result, called the remainder. Here is another way to express the relationship between the components:

Dividend = Quotient Divisor + Remainder

where the remainder is smaller than the divisor. Infrequently, programs use the divide instruction just to get the remainder, ignoring the quotient. The basic grammar school division algorithm tries to see how big a number can be subtracted, creating a digit of the quotient on each attempt. Our carefully selected decimal example uses only the numbers 0 and 1, so its easy to figure out how many times the divisor goes into the portion of the dividend: its either 0 times or 1 time. Binary numbers contain only 0 or 1, so binary division is restricted to these two choices, thereby simplifying binary division.

Lets assume that both the dividend and the divisor are positive and hence the quotient and the remainder are nonnegative. The division operands and both results are 32-bit values, and we will ignore the sign for now.

A DIVISION ALGORITHM AND HARDWARE

The above figure shows hardware to mimic our grammar school algorithm. We start with the 32-bit Quotient register set to 0. Each iteration of the algorithm needs to move the divisor to the right one digit, so we start with the divisor placed in the left half of the 64-bit Divisor register and shift it right 1 bit each step to align it with the dividend. The Remainder register is initialized with the dividend.

THREE STEPS OF THE FIRST DIVISION ALGORITHM.

The above figure shows three steps of the first division algorithm. Unlike a human, the computer isnt smart enough to know in advance whether the divisor is smaller than the dividend. It must first subtract the divisor in step 1; remember that this is how we performed the comparison in the set on less than instruction. If the result is positive, the divisor was smaller or equal to the dividend, so we generate a 1 in the quotient (step 2a). If the result is negative, the next step is to restore the original value by adding the divisor back to the remainder and generate a 0 in the quotient (step 2b). The divisor is shifted right and then we iterate again. The remainder and quotient will be found in their namesake registers after the iterations are complete.

EXAMPLE

A DIVIDE ALGORITHM

Using a 4-bit version of the algorithm to save pages, lets try dividing 710 by 210, or 0000 01112 by 00102.

The above figure shows the value of each register for each of the steps, with the quotient being 3ten and the remainder 1ten. Notice that the test in step 2 of whether the remainder is positive or negative simply tests whether the sign bit of the Remainder register is a 0 or 1. The surprising requirement of this algorithm is that it takes n + 1 steps to get the proper quotient and remainder.

This algorithm and hardware can be refined to be faster and cheaper. The speedup comes from shifting the operands and the quotient simultaneously with the subtraction. This refinement halves the width of the adder and registers by noticing where there are unused portions of registers and adders.

The following figure shows the revised hardware.

SIGNED DIVISION

The one complication of signed division is that we must also set the sign of the remainder. Remember that the following equation must always hold:Dividend = Quotient Divisor + Remainder

To understand how to set the sign of the remainder, lets look at the example of dividing all the combinations of 710 by 210. The first case is easy:

+7 +2: Quotient = +3, Remainder = +1

Checking the results:

7 = 3 2 + (+1) = 6 + 1

If we change the sign of the dividend, the quotient must change as well:

7 +2: Quotient = 3

Rewriting our basic formula to calculate the remainder:

Remainder = (Dividend Quotient Divisor) = 7 (3 +2) = 7(6) = 1

So,

7 +2: Quotient = 3, Remainder = 1

Checking the results again:

7 = 3 2 + (1) = 6 1

The reason the answer isnt a quotient of 4 and a remainder of +1, which would also fit this formula, is that the absolute value of the quotient would then change depending on the sign of the dividend and the divisor! Clearly, if

(x y) (x) y

programming would be an even greater challenge. This anomalous behavior is avoided by following the rule that the dividend and remainder must have the same signs, no matter what the signs of the divisor and quotient. We calculate the other combinations by following the same rule:

+7 2: Quotient = 3, Remainder = +1

7 2: Quotient = +3, Remainder = 1

Thus the correctly signed division algorithm negates the quotient if the signs of the operands are opposite and makes the sign of the nonzero remainder match the dividend.

FASTER DIVISION

We used many adders to speed up multiply, but we cannot do the same trick for divide. The reason is that we need to know the sign of the difference before we can perform the next step of the algorithm, whereas with multiply we could calculate the 32 partial products immediately.

There are techniques to produce more than one bit of the quotient per step. The SRT division technique tries to guess several quotient bits per step, using a table lookup based on the upper bits of the dividend and remainder. It relies on subsequent steps to correct wrong guesses. A typical value today is 4 bits. The key is guessing the value to subtract. With binary division, there is only a single choice.These algorithms use 6 bits from the remainder and 4 bits from the divisor to index a table that determines the guess for each step. The accuracy of this fast method depends on having proper values in the lookup table. SUBWORD PARALLELLISM

A subword is a lower precision unit of data contained within a word. In subword parallelism, multiple subwords are packed into a word and then process whole words. With the appropriate subword boundaries this technique results in parallel processing of subwords. Since the same instruction is applied to all subwords within the word, This is a form of SIMD(Single Instruction Multiple Data) processing.

It is possible to apply subword parallelism to noncontiguous subwords of different sizes within a word. In practical implementation is simple if subwords are same size and they are contiguous within a word. The data parallel programs that benefit from subword parallelism tend to process data that are of the same size.

For example if word size is 64bits and subwords sizes are 8,16 and 32 bits. Hence an instruction operates on eight 8bit subwords, four 16bit subwords, two 32bit subwords or one 64bit subword in parallel.

Subword parallelism is an efficient and flexible solution for media processing because algorithm exhibit a great deal of data parallelism on lower precision data.

It is also useful for computations unrelated to multimedia that exhibit data parallelism on lower precision data.

Graphics and audio applications can take advantage of performing simultaneous operations on short vectors

Example: 128-bit adder:

Sixteen 8-bit adds

Eight 16-bit adds

Four 32-bit adds

Also called data-level parallelism, vector parallelism, or Single Instruction, Multiple Data (SIMD)

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