L3.4. Programming language structures

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来自 Universitat Autònoma de Barcelona 的课程

Digital Systems: From Logic Gates to Processors

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Universitat Autònoma de Barcelona

Digital Systems: From Logic Gates to Processors

164 个评分

This course gives you a complete insight into the modern design of digital systems fundamentals from an eminently practical point of view. Unlike other more "classic" digital circuits courses, our interest focuses more on the system than on the electronics that support it. This approach will allow us to lay the foundation for the design of complex digital systems. You will learn a set of design methodologies and will use a set of (educational-oriented) computer-aided-design tools (CAD) that will allow you not only to design small and medium size circuits, but also to access to higher level courses covering so exciting topics as application specific integrated circuits (ASICs) design or computer architecture, to give just two examples. Course topics are complemented with the design of a simple processor, introduced as a transversal example of a complex digital system. This example will let you understand and feel comfortable with some fundamental computer architecture terms as the instruction set, microprograms and microinstructions. After completing this course you will be able to: * Design medium complexity digital systems. * Understand the description of digital systems using high-level languages such as VHDL. * Understand how computers operate at their most basic level (machine language).

从本节课中

Combinational circuits (II)

<b><font size=4 color=#B22222><b>Click on "v More" to read the purpose of this module</b></font> </b> <br/><br/>We continue the study of combinational circuits. While in the previous module we were working on the classical design techniques of combinational circuits, this one is focused on other issues such as a brief introduction to computer aided design tools (CAD tools), or the direct synthesis of combinational circuits from its algorithmic description.<br/> Read the "Lesson index" for more information. <br><br/><b>To solve the exercises in this module VerilUOC_Desktop is needed. </b>Remember that the "VerilUOC_Desktop" section in module 2 contains all the information you need about this tool.

L3.4. Programming language structures 14:28

与讲师见面

Elena Valderrama
Professor
Departamento de Microelectrónica y Sistemas Electrónicos. UAB.
Jean-Pierre Deschamps
Professor
Lluis Terés
Professor
Centro Nacional de Microelectrónica del CSIC
Merce Rullan
ProfesoraTitular
Departamento de Microelectrónica y Sistemas Electrónicos. UAB.
Joaquín Saiz Alcaine
Ingeniero Informático
Departamento de Microelectrónica y Sistemas Electrónicos. UAB.
David Bañeres
Profesor Agregado
Estudios de Informática, Multimedia y Telecomunicación. UOC.
Juan Antonio Martínez
Collaborator
APSI - UAB

[MUSIC].

This lesson deals with the relation

between programming language instructions and digital circuits.

This relation justifies the use of hardware description languages,

very similar to programming languages, and the availability

of digital circuit synthesis tools.

Actually, several programming language instructions

have a straightforward translation in terms of

digital components. Four examples will be given

in this lesson. The "if then else"

construct, the "case" statement,

the loops "for loop", and the procedure

calls. Let us start with the "if-then-else"

instructions. Here is an example:

it is a binary decision algorithm

that computes the value

of a switching function f of six

variables, x0, x1, y0,

y1, y2, and y3, and the

algorithm is the following:

the first condition "if x1 is equal to 0" then

there is another internal "if"

instruction that say that "if x0 is equal

to 0" then f is equal to y0; else

f is equal to y1. So we see

that if x1 is equal to 0 and if x0 is

equal to 0 then function f is equal

to y0. If x1 is equal to 0 but x0 equal

to the other value (the else part of the

instruction) then the function is equal to y1.

In the other case, if x1 is equal to 1, if

x0 is equal to 0, then f is equal to y2, and

in the other case, f is equal to y3.

To this algorithm corresponds this circuit.

The external if (if then else

with end if here), this first

if instruction correspond to the output multiplexer of this circuit.

According to the value of x1, we choose between two branches

within this circuit, and to every if-then-else,

internal if-then-else instruction, correspond those multiplexers

that, according to the value of x0 in both cases,

chooses the value of the function f.

So, there is an obvious relation between if "condition" then

"some operation" else "other operation", between this

type of construct and 2-to-1 multiplexers.

A second example is the "case" statement.

The same binary decision algorithm can be defined by

a "case instruction". A second example is

the "case" statement. The same binary decision algorithm

can be defined by a "case" instruction:

CCase x is when 00, f is equal

to y0, when x is equal to 01,

f is equal to y1 and so on,

and to this instruction correspond

a 4-to-1 multiplexer with a

2-bit input x that allows to choose for

f one of those values, one of those input variables.

"For loops" are associated with iterative structures.

Consider the algorithmic definition of a 4-digit

decimal adder. An example before: when

we add, for example,

9,273 and

4,389

we use a step

by step method. We add 3 and 9;

this is 12; actually 1 and 2;

we generate a carry equal to 1;

Then we add 1, 7, and 8,

equal to 16. Once again, there is a carry;

1 plus 2 plus 3 equal to 6.

We see there is no carry, and finally

9 plus 4 equal to 13 and the obtained

result is this. So, we

start with an initial carry equal to 0 and

then we repeat four times the same

sequence of operations. We add one digit of x, one

digit of y, and

an incoming carry. If the result is greater

than 9, we must subtract 10 and generate an

outgoing carry, and that's all.

The corresponding circuit has an iterative

architecture. To the loop body, corresponds

this component, and the complete

circuit is made up of four components

connected in series. The outgoing carry

of one step, of one component, is connected to the

incoming carry of the next component and

this corresponds to the fact that, in the program, you can see that carry

number i+1 is updated at step i.

Now two comments about loop implementations.

The first comment is that other sequential implementations will

be seen later on. Another comment is that not

any loop can be implemented in this way. An

example is the case of while "some condition"

loop, and then "some sequence of operations".

If the maximum number of times that this condition will hold true

is either unknown or is known, but is a very large number, a

sequential implementation must be considered.

The last example is the call to a function

or to a procedure. Once again, let us

see an example. Assume that some procedure MAC

(multiply and accumulate) has

been defined. It is a procedure whose

parameters w, x and y are input parameters

and z an output parameter, and

it executes this operation: it adds to

some number w the product of 2 numbers x and y.

It allows, for example, to compute this type of expression:

z is a sum of 8 products:

x1·y1 + x2·y2 and

so on, up to x8·y8,

and the corresponding algorithm

is this one. Initially, the

first value of w is 0 and then we

call 8 times the MAC procedure

with parameters: the value of w,

the corresponding values of x and

y and the result of the operation;

that means that at each

step we compute a new value

of w equal to the current

value plus the product

of two numbers, xi and yi.

The corresponding circuit is

an iterative one (this circuit);

to the "for" loop

corresponds this iterative

structure and to every MAC

procedure call corresponds

a component that must be

defined. This is another example of top-down

hierarchical description. We say top-down

because we start with a complete system, the top level in this case,

and after that, we will have to develop the MAC component.

in this example the "down" level of this hierarchical description.

Now some comments. We have seen how some classical

programming language instructions have a straightforward translation

into digital components. First comment:

that is the reason why hardware description

languages, very similar to programming languages, have been defined.

For example

VHDL, Verilog,

or even C/C++ with some additional time functions,

objects and so on, and another comment:

this relation between programming language instructions

and digital components also explains how

it has been possible to generate software packages able

to translate programs to circuits. Summary of

this lesson: we have seen that binary decision

algorithms can be implemented with multiplexers,

that case constructs can also be implemented with multiplexers.

We have seen that loops correspond to iterative structures,

and that procedure calls correspond to

hierarchical descriptions. Finally, a conclusion

about hardware description languages and synthesis tools.

[BLANK_AUDIO]

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