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6.001 SICPRegister Machines

• what and why• datapaths• instructions• abstract operations

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Plan

• Design a central processing unit (CPU) from:• wires• logic (networks of AND gates, OR gates, etc)• registers• control sequencer

• Our CPU will interpret scheme as its machine language

• Today: Iterative algorithms in hardware• Next: Recursive algorithms in hardware• Then: Scheme in hardware (EC-EVAL)

• EC-EVAL exposes more details of scheme than M-EVAL

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The ultimate goal

GCD24

44

48

306

Ultimate machine

48

306

Circuit

diagram

Universal machine

(define (gcd a b)

….)

48

306

Procedure description

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A universal machine

• Existence of a universal machine has major implications for what “computation” means

• Insight due to Alan Turing (1912-1954)• “On computable numbers with an application to the

Entscheidungsproblem, A.M. Turing, Proc. London Math. Society, 2:42, 1937

• Hilbert’s Entscheidungsproblem (decision problem) 1900: Is mathematics decidable? That is, is there a definite method guaranteed to prodcue a correct decision about all assertions in mathematics?

• Church-Turing thesis: Any procedure that could reasonably be considered to be an effective procedure can be carried out by a universal machine (and thus by any universal machine)

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Euclid's algorithm to compute GCD

(define (gcd a b)

(if (= b 0)

a

(gcd b (remainder a b))))

• Given some numbers a and b• If b is 0, done (the answer is a)• If b is not 0:

• the new value of a is the old value of b• the new value of b is the remainder of a b• start again

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Example register machine: datapath

a b =

0rem

t

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Example register machine: instructions

(controller

test-b

(test (op =) (reg b) (const 0))

(branch (label gcd-done))

(assign t (op rem) (reg a) (reg b))

(assign a (reg b))

(assign b (reg t))

(goto (label test-b))

gcd-done)

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Complete register machine

a b =

0rem

tinstructions

sequencer

program counter

condition

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Datapath terminology

a b =

0rem

t

register

operation

button wire

constant

test

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Datapath components

• Button• when pressed, value on input wire flows to output

• Register• output the stored value continuously• change value when button on input wire is pressed

• Operation• output wire value = some function of input wire values

• Test• an operation• output is one bit (true or false)• output wire goes to condition register

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Incrementing a register

sum0

+

1

X

Y? 0 1 2

? 1 2 3

an op thatadds its inputs

press sum ?

X 0Y 1Y 2

• What sequence of button presses will result in the register sum containing the value 2?

X Y Y

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Euclid's algorithm to compute GCD

(define (gcd a b)

(if (= b 0)

a

(gcd b (remainder a b))))

• Given some numbers a and b• If b is 0, done (the answer is a)• If b is not 0:

• the new value of a is the old value of b• the new value of b is the remainder of a b• start again

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Datapath for GCD (partial)

• What sequence of button presses will result in: the register a containing GCD(a,b) the register b containing 0

• The operation rem computes the remainder of a b

Z 9 6 3X 6 6 3Y 6 3 3 Z 6 3 0X 3 3 0Y 3 0 0

press a b t9 6 ?

a b

rem

X

Y

t

Z

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Example register machine: instructions

(controller

test-b

(test (op =) (reg b) (const 0))

(branch (label gcd-done))

(assign t (op rem) (reg a) (reg b))

(assign a (reg b))

(assign b (reg t))

(goto (label test-b))

gcd-done)

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Instructions

• Controller: generates a sequence of button presses• sequencer• instructions

• Sequencer: activates instructions sequentially• program counter remembers which one is next

• Each instruction:• commands a button press, OR• changes the program counter

– called a branch instruction

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Button-press instructions: the sum example

(controller

(assign sum (const 0)) <X>

(assign sum (op +) (reg sum) (const 1)) <Y>

(assign sum (op +) (reg sum) (const 1)))

sum0

+

1

X

Y

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Unconditional branch

(controller

0 (assign sum (const 0))

increment

1 (assign sum (op +) (reg sum) (const 1))

2 (goto (label increment)))

sum0

+

1

X

Y

PC nextPC press0 1 X1 2 Y2 3 1 --1 2 Y2 3 1 --

sequencer: nextPC <- PC + 1 activate instruction at PC PC <- nextPC start again

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Conditional branch

a b

rem

t(controllertest-b

(assign t (op rem) (reg a) (reg b)) (assign a (reg b)) (assign b (reg t)) (goto (label test-b)) )

=

0

insts

sequencer

program counter

condition

(test (op =) (reg b) (const 0)) (branch (label gcd-done))

gcd-done

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Conditional branch details

(test (op =) (reg b) (const 0))• push the button which loads the condition register

from this operation's output

(branch (label gcd-done))• Overwrite nextPC register with value if condition register

is TRUE• No effect if condition register is FALSE

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Check your understanding

• Draw the datapath for the following instruction sequence• Describe what function it computes

(controller

test-n

(test (op =) (reg n) (const 0))

(branch (label done))

(test (op =) (reg n) (const 1))

(branch (label done))

(assign n (op -) (reg n) (const 2))

(goto (label test-n))

done)

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Answer: computes the function odd?

n

-

2

0 = 1=

• input in register n• output in register n: 1 if input was odd, 0 if input was even(define (odd? n) (cond ((= n 0) n) ((= n 1) n) (else (odd? (- n 2)))))

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Datapaths are redundant

• We can always draw the data path required for aninstruction sequence

• Therefore, we can leave out the data path when describing a register machine

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Abstract operations

• Every operation shown so far is abstract:• abstract = consists of multiple lower-level operations

• Lower-level operations might be:• AND gates, OR gates, etc (hardware building-blocks)• sequences of register machine instructions

• Example: GCD machine uses

(assign t (op rem) (reg a) (reg b))

• Rewrite this using lower-level operations

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Less-abstract GCD machine(controllertest-b (test (op =) (reg b) (const 0)) (branch (label gcd-done)) ; (assign t (op rem) (reg a) (reg b)) (assign t (reg a))rem-loop (test (op <) (reg t) (reg b)) (branch (label rem-done)) (assign t (op -) (reg t) (reg b)) (goto (label rem-loop))rem-done (assign a (reg b)) (assign b (reg t)) (goto (label test-b))gcd-done)

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Importance of register machine abstraction

• A CPU is a very complicated device• We will study only the core of the CPU

• eval, apply, etc.• We will use abstract register-machine operations for all the

other instruction sequences and circuits:

(test (op self-evaluating?) (reg exp))

• remember,(op +) is abstract, (op <) is abstract, etc.• no magic in (op self-evaluating?)

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Summary

• Introduced register machines• registers, buttons, tests, constants

• Register machine instructions• button press• unconditional branch (goto)• conditional branch (branch)

• Abstraction in register machines

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Recitation problem

• Draw the datapath and write the instruction sequence for the register machine that implements

(define (max a b)

(if (> a b) a b))

• The two input values are in registers labeled a and b• The output should be left in a register labeled out