The A ssembly Assembly Language Level Chapter 7 1

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1 The Assembly Language Level Chapter 7 1

2 Contemporary Multilevel Machines A six-level l computer. The support method for each level is indicated below it.2

3 Assembly Language Level a) It is implemented by translation rather than interpretation. b) Programs that convert a user s program written in some language to anther language are called translator. c) The language in which h the original i program is written is called the source language and the language to which it is converted is called the target language. d) Translation is used when a processor is available for the target t language but not for the source language. 3

4 Assembly Language Level a) In translation, the original program is converted to an equivalent program called an object program whose execution is carried out only after the translation has been completed. b) In interpretation, there is only one step: executing the original source program. No equivalent program needs to be generated first. 4

5 Translator a) Translator can be roughly divided into two groups. b) When the source language is essentially a symbolic representation for a numerical machine language, the translator is called an assembler and the source language is called an assembly language. c) When the source language is a high-level language and the target t language is either a numerical machine language or a symbolic one, the translator is called a compiler. 5

6 Assembly Language a) A pure assembly language is a language in which each statement produces exactly one machine instruction. b) It is much easy to program in symbolic forms. c) The assembly programmer has access to all the features and instructions available on the target machine. d) An assembly language program can run only on one family of machines, whereas a program written in a high-level language can potentially run on many machines. 6

7 Why Use Assembly Language? a) Performance b) Access to the machine 7

8 Why Use Assembly Language? Comparison of assembly language and high level language Comparison of assembly language and high-level language programming, with and without tuning. 8

9 Format of an Assembly Language Statement (1) Computation ti of N = I + J. (a) Pentium 4. 9

10 Format of an Assembly Language Statement (2) Computation ti of N = I + J. (b) Motorola 680x

11 Format of an Assembly Language Statement (3) Computation ti of N = I + J. (c) SPARC. 11

12 Pseudoinstructions a) In addition to specifying which machine instructions to execute, an assembly language g program can also contain commands to the assembler itself. b) Commends to the assembler itself are called pseudoinstructions or sometimes assembler directives. 12

13 Pseudoinstructions (1) Some of the pseudoinstructions available in the Some of the pseudoinstructions available in the Pentium 4 assembler (MASM). 13

14 Pseudoinstructions (2) Some of the pseudoinstructions available in the Some of the pseudoinstructions available in the Pentium 4 assembler (MASM). 14

15 Macros a) Assembly language programmers frequently need to repeat sequences of instructions several times within a program. b) A way is to make the sequence into a procedure and call it wherever it is needed. But it requires a procedure call instruction and a return instruction. c) A macro definition is a way to give a name to a piece of text. After a macro has been defined, the programmer can write the macro name instead of the piece of program. d) A macro is an abbreviation for a piece of text. 15

16 Macro Definition, Call, Expansion (1) Assembly language code for interchanging P and Q twice Assembly language code for interchanging P and Q twice. (a) Without a macro. (b) With a macro. 16

17 Macros a) A macro header gives the name of the macro. b) The text comprises the body of the macro. c) A pseudoinstruction marks the end of the definition. d) When the assembler encounters a macro definition, it saves it in a macro definition table for subsequent use. e) The use of a macro name as an opcode is known as a macro call and its replacement by the macro body is called macro expansion. 17

18 Macro Definition, Call, Expansion (2) Comparison of macro calls with procedure calls. 18

19 Macros with Parameters Formal parameters At Actual parameters Nearly identical sequences of statements. (a) Without a macro. (b) With a macro. 19

20 Advanced Features a) Avoid label duplication to allow a label to be declared local, with the assembler automatically generating a different label on each expansion of the macro. 20

21 The Assembly Process a) It might seen natural to have an assembler that reads one statement, then translates it to machine language, and finally outputs the generated machine language onto a file. Unfortunately, this strategy does not work. b) A branch to L, can not be assembled until the address of statement L is known. Forward reference problem a reference has been made to a symbol whose definition will only occur later. 21

22 The Assembly Process a) The assembler may in fact read the source program twice. Two pass. The translator that reads the input program twice is called a two-pass translator. t b) 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. c) Another approach consists of reading the assembly program once, converting it to an intermediate form, and storing this form in a table in memory. Then a second pass is made over the table instead of over the source program. (need enough memory) 22

23 Two Pass Assemblers (1) The instruction location counter (ILC) keeps track of the address where the instructions will be loaded in memory. In this example, the statements prior to MARIA occupy 100 bytes. 23

24 Two Pass Assemblers (2) A symbol table for the program of Fig

25 Two Pass Assemblers (3) A few excerpts from the opcode table for a Pentium 4 assembler. 25

26 Pass One (1)... Pass one of a simple assembler. 26

27 ... Pass One (2)... Pass one of a simple assembler. 27

28 Pass One (3)... Pass one of a simple assembler. 28

29 Pass Two a) The function of pass two is to generate the object program and possibly print the assembly listing. b) 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. 29

30 Pass Two (1)... Pass two of a simple assembler. 30

31 ... Pass Two (2) Pass two of a simple assembler. 31

32 The Symbol Table a) During pass one, the assembler accumulates information about symbols and their values that must be stored in the symbol table for lookup during pass two. b) All the organizing methods attempt to simulate an associative memory, which conceptually is a set of pairs (symbol, value). c) The simplest implementation is to implement the symbol table as an array of pairs. On average, half of the table will have to be searched on each lookup. d) Binary search A table of size n entries can be searched in d) Binary search A table of size n entries can be searched in about log 2 n attempts. 32

33 The Symbol Table (1) Hash coding. (a) Symbols, values, and the hash codes derived from the symbols. 33

34 The Symbol Table (2) Hash coding. (b) Eight-entry hash table with linked lists of symbols and values. 34

35 a) With n symbols and k slots in the hash table, the average list will have length n/k. b) By choosing k approximately equal to n, symbols can be located with only about one lookup on the average. c) By adjusting k we can reduce table size at the expense of slower lookups. 35

36 Linking and Loading a) Most program consists of more than on e procedure. b) Compilers and assemblers generally translate one procedure at a time and put the translated output on disk. c) Before the program can be run, all the translated procedures must tbe found and dlinked dtogether th properly. d) Programs that perform these functions are called linkers. 36

37 Linking and Loading Generation of an executable binary program from a collection of Generation of an executable binary program from a collection of independently translated source procedures requires using a linker. 37

38 Linker a) The linker s function is to collect procedures translated separately and link them together to be run as an executable binary program. b) The translation from source procedure to object module represents a change of level. The linking process, however, does not represent a change of level, since both the linker s input and the linker s output are programs for the same virtual machine. c) The two-step process of translating and linking can save a great deal of time during the development of a program. 38

39 Tasks Performed by the Linker (1) Each module has its own address space, starting ti at 0. 39

40 Tasks Performed by the Linker (2) Each module has its own address space, starting ti at 0. 40

41 Tasks Performed by the Linker (3) Each module has its own address space, starting ti at 0. 41

42 Tasks Performed by the Linker (4) Each module has its own address space, starting ti at 0. 42

43 Tasks Performed by the Linker (5) The object modules of Fig after being positioned in the binary image but before being relocated and linked. 43

44 Tasks Performed by the Linker (6) The same object modules after linking and after relocation has been performed. Together they form an executable binary program, ready to run 44

45 Linking Steps a) It constructs a table of all the object modules and their lengths. b) Based on this table, it assigns a starting address to each object module. c) It finds all the instructions that reference memory and adds to each a relocation constant equal to the starting address of its module. d) It finds all the instructions that reference other procedures d) ds a e s uc o s a e e e ce o e p ocedu es and inserts the address of these procedures in place. 45

46 Structure of an Object Module The internal structure of an object module produced by a translator. 46

47 Dynamic Linking a) A more flexible way to link separately compiled procedures is to link each procedure at the time it is first called. b) This process is known as dynamic linking. 47

48 Binding Time and Dynamic Relocation The relocated binary program of Fig 7 15(b) moved up 300 addresses The relocated binary program of Fig. 7-15(b) moved up 300 addresses. Many instructions now refer to an incorrect memory address. 48

49 Dynamic Linking in MULTICS (1) Before EARTH is called. 49

50 Dynamic Linking in MULTICS (2) After EARTH has been called and linked. 50

51 Dynamic Linking in Windows Use of a DLL file by two processes. 51

The Assembly Language Level. Chapter 7

The Assembly Language Level. Chapter 7 The Assembly Language Level Chapter 7 Definitions Translator Converts user program to another language Source language Language of original program Target language Language into which source code is converted