chapitre 3 – notions sur les instructions d’un ordinateur

2 LMD Architecture des ordinateurs Chap. 3 Univ. Tiaret Mr A. CHENINE

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Chapitre 3 – Notions sur les instructions d’un ordinateur

1. Langage haut niveau (HLL), assembleur, langage machine

Fig. 1 software levels

Programs that convert a user’s program written in some language to another language are called translators.

Fig. 2 translation process


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Compilation

interpretation

The compiler translates the C text into ‘machine code’ for the processor – Machine code is a set of consecutive memory words, encoding instructions for the processor. – Machine code are binary words. In case of the MIPS processor, each instruction is 32 bits Translation examples


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Assembler Three types of statements in assembly language

Typically, one statement should appear on a line 1. Executable Instructions

Generate machine code for the processor to execute at runtime Instructions tell the processor what to do

2. Pseudo-Instructions and Macros Translated by the assembler into real instructions Simplify the programmer task

3. Assembler Directives Provide information to the assembler while translating a program Used to define segments, allocate memory variables, etc. Non-executable: directives are not part of the instruction set

Instructions Assembly language instructions have the format:

[label:] mnemonic [operands] [#comment] Label: (optional)

Marks the address of a memory location, must have a colon Typically appear in data and text segments

Mnemonic Identifies the operation (e.g. add, sub, etc.)

Operands Specify the data required by the operation Operands can be registers, memory variables, or constants Most instructions have three operands

L1: addiu $t0, $t0, 1 #increment $t0 Comments are very important!

Explain the program's purpose When it was written, revised, and by whom Explain data used in the program, input, and output Explain instruction sequences and algorithms used Comments are also required at the beginning of every procedure

Indicate input parameters and results of a procedure Describe what the procedure does

Single-line comment Begins with a hash symbol # and terminates at end of line


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2. THE ASSEMBLY PROCESS [2]

In the following sections we will briefly describe how an assembler works. Although each machine has a different assembly language, the assembly process is sufficiently similar on different machines that it is possible to describe it in general. 2.1 Two-Pass Assemblers

Because an assembly language program consists of a series of one-line statements, it might at first seem natural to have an assembler that read one statement, then translated it to machine language, and finally output the generated machine language onto a file, along with the corresponding piece of the listing, if any, onto another file. This process would then be repeated until the whole program had been translated. Unfortunately, this strategy does not work.

Consider the situation where the first statement is a branch to L. The assembler cannot assemble this statement until it knows the address of statement L. Statement L may be near the end of the program, making it impossible for the assembler to find the address without first reading almost the entire program. This difficulty is called the forward reference problem, because a symbol, L, has been used before it has been defined; that is, a reference has been made to a symbol whose definition will only occur later. Forward references can be handled in two ways. First, the assembler may in fact read the source program twice. Each reading of the source program is called a pass; any translator that reads the input program twice is called a two-pass translator. On pass one, the definitions of symbols, including statement labels, are collected and stored in a table. By the time the second pass begins, the values of all symbols are known; thus no forward reference remains and each statement can be read, assembled, and output. Although this approach requires an extra pass over the input, it is conceptually simple. The second approach consists of reading the assembly program once, converting ing it to an intermediate form, and storing this intermediate form in a table in memory. Then a second pass is made over the table instead of over the source program. If there is enough memory (or virtual memory), this approach saves I/O time. If a listing is to be produced, then the entire source statement, including all the comments, has to be saved. If no listing is needed, then the intermediate form can be reduced to the bare essentials. Either way, another task of pass one is to save all macro definitions and expand the calls as they are encountered. Thus defining the symbols and expanding the macros are generally combined into one pass. 2.2 Pass One The principal function of pass one is to build up a table called the symbol table, containing the values of all symbols. A symbol is either a label or a value that is assigned a symbolic name by means of a pseudoinstruction such as ret EQU 1

2.3 Pass Two The function of pass two is to generate the object program and possibly print the assembly listing. In addition, pass two must output certain information needed by the linker for linking up procedures assembled at different times into a single executable file.

3. Hardware


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3.1 Arithmetic operations 3.1.1 Addition/Subtraction

Fig.3 Block diagram for addition/subtraction (From [1])

Logisim implementation

Control signals are needed to control whether or not the complementer is used, depending on whether the operation is addition or subtraction.


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Fig.4 Serial addition 3.1.2 Unsigned Multiplication complex operation, the steps are:

Fig.5 Multiplication of Unsigned Binary Integers

1. Multiplication involves the generation of partial products, one for each digit in the multiplier. These partial products are then summed to produce the final product.

2. The partial products are easily defined. When the multiplier bit is 0, the partial product is 0. When the multiplier is 1, the partial product is the multiplicand.

3. The total product is produced by summing the partial products. For this operation, each successive partial product is shifted one position to the left relative to the preceding partial product.

4. The multiplication of two n-bit binary integers results in a product of up to 2n bits in length (e.g., 11 * 11 = 1001).


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Fig.6 a. Multiplication diagram b. Example

Logisim implementation


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Fig.7 Flowchart for unsigned Multiplication Another method for multiplication

Fig. 8 Multiplication of two 2-bit numbers �

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Fig. 9 Logic circuit of 2x2-bit numbers �

Question:

Compare fig. 9 and fig. 6.

4. Pipeline �

Technique utilisée pour optimiser le temps d’exécution d’un processus répétitif. Si le temps d’exécution d’un processus est T, l’exécution séquentielle de m processus prend un

temps m*T.


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Le pipeline

Le temps passé par une instruction dans un étage est appelé (temps de) cycle processeur La longueur d’un cycle est déterminée par l’étage le plus lent. Souvent égal à un cycle d’horloge, parfois 2

L’idéal est d’équilibrer la longueur des étages du pipeline Sinon le étages les plus rapides ‘attendent’ les plus lents Pas optimal

Insertion de registres intermédiaires entre les étages (Registres pipeline)

Exemple avec un pipeline à 5 étages :

1. Lecture de l’instruction (IF) 2. Décodage de l’instruction (ID) 3. Exécution de l’instruction (EX) 4. Accès mémoire (MEM) 5. Ecriture du résultat (WB)


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1. Lecture de l’instruction (IF) La prochaine instruction à exécuter est chargée à partir de la case mémoire pointée par le compteur

de programme (PC) dans le registre d'instruction RI. Le compteur de programme est incrémenté pour pointer sur l'instruction suivante.

2. Décodage de l’instruction (ID)

Cette étape consiste à préparer les arguments de l'instruction pour l'étape suivante (UAL) où ils seront utilisés. Ces arguments sont placés dans deux registres A et B. Si l'instruction utilise le contenu d’un ou deux registres, ceux-ci sont lus et leurs contenus sont rangés

dans A et B. Si l'instruction contient une valeur immédiate, celle-ci est étendue (signée ou non signée) à 16 bits et

placée dans le registre B. Pour les instructions de branchement avec offset, le contenu de PC est rangé en A et l'offset étendu

dans B. Pour les instructions de branchement avec un registre, le contenu de ce registre est rangé en A et B

est rempli avec 0. Les instructions de rangement mettent le contenu du registre qui doit être transféré en mémoire dans

le registre C.

3. Exécution de l’instruction (EX)


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Cette étape utilise l’UAL pour combiner les arguments. L'opération réalisée dépend du type de l'instruction. Instruction arithmétique ou logique (ADD, AND et NOT)

Les deux arguments contenus dans les registres A et B sont fournis à l'UAL pour calculer le résultat. Instruction de chargement et rangement (LW et SW)

Le calcul de l'adresse est effectué à partir de l'adresse provenant du registre A et de l'offset contenu dans le registre B. Instruction de branchement

Pour les instructions contenant un offset, addition avec le contenu du PC. Pour les instructions utilisant un registre, le contenu du registre est transmis.

4. Accès mémoire (MEM) Cette étape est uniquement utile pour les instructions de chargement et de rangement. Pour les instructions arithmétiques et logiques ou les branchements, rien n'est effectué. L'adresse du

mot mémoire est contenue dans le registre R. Dans le cas d'un rangement, la valeur à ranger provient du registre C. Dans le cas d'un chargement, la valeur lue en mémoire est mise dans le registre R pour l'étape

suivante.


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5. Ecriture du résultat (WB)

Le résultat des opérations arithmétiques et logiques est rangé dans le registre destination. La valeur lue en mémoire par les instructions de chargement est aussi rangée dans le registre

destination. Les instructions de branchement rangent la nouvelle adresse dans PC.


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Reference [1] William Stallings, Computer organization and architecture, Designing for Performance, tenth edition, [2] Tanenbaum A.S., Austin T. - Structured computer organization-Pearson (2013)

chapitre 3 – notions sur les instructions d’un ordinateur

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