undefined | unit 3 macro processors

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UNIT - 3

Macro Processors

Q1) Explain Macros facility?

A1)

Writing a macro is another way of ensuring modular programming in assembly language.

A macro is a sequence of instructions, assigned by a name and could be used anywhere in the program.

In NASM, macros are defined with %macro and %endmacro directives.

The macro begins with the %macro directive and ends with the %endmacro directive.

The Syntax for macro definition −

%macro macro_name number_of_params

%endmacro

Where, number_of_params specifies the number parameters, macro_name specifies the name of the macro.

The macro is invoked by using the macro name along with the necessary parameters. When you need to use some sequence of instructions many times in a program, you can put those instructions in a macro and use it instead of writing the instructions all the time.

For example, a very common need for programs is to write a string of characters in the screen. For displaying a string of characters, you need the following sequence of instructions −

mov edx,len ;message length

mov ecx,msg ;message to write

mov ebx,1 ;file descriptor (stdout)

mov eax,4 ;system call number (sys_write)

int 0x80 ;call kernel

In the above example of displaying a character string, the registers EAX, EBX, ECX and EDX have been used by the INT 80H function call. So, each time you need to display on screen, you need to save these registers on the stack, invoke INT 80H and then restore the original value of the registers from the stack. So, it could be useful to write two macros for saving and restoring data.

We have observed that, some instructions like IMUL, IDIV, INT, etc., need some of the information to be stored in some particular registers and even return values in some specific register(s). If the program was already using those registers for keeping important data, then the existing data from these registers should be saved in the stack and restored after the instruction is executed.

Example

Following example shows defining and using macros −

; A macro with two parameters

; Implements the write system call

%macro write_string 2

mov eax, 4

mov ebx, 1

mov ecx, %1

mov edx, %2

int 80h

%endmacro

section .text

global _start ;must be declared for using gcc

_start: ;tell linker entry point

write_string msg1, len1

write_string msg2, len2

write_string msg3, len3

mov eax,1 ;system call number (sys_exit)

int 0x80 ;call kernel

section .data

msg1 db 'Hello, programmers!',0xA,0xD

len1 equ $ - msg1

msg2 db 'Welcome to the world of,', 0xA,0xD

len2 equ $- msg2

msg3 db 'Linux assembly programming! '

len3 equ $- msg3

When the above code is compiled and executed, it produces the following result −

Hello, programmers!

Welcome to the world of,

Linux assembly programming!

Q2) Explain Macro definition and call, Macro Expansion?

A2)

Definition: macro

A macro name is that of associate abbreviation, that stands for a few of the connected lines of the code. Macros are helpful for that of the subsequent purposes:

To modify and to scale back that of the quantity of the repetitive cryptography

To reduce errors caused by that of the repetitive cryptography

To make that of associate program a lot of clear.

A macro consists of the name, set of the formal parameters and also the body of code. the utilization of that of the macro name with set of the particular parameters is to switch by a number of the code generated by its body. this can be called macro enlargement.

Macros permit that of a coder to outline that of the pseudo operations, usually operations that are generally the fascinating, aren't enforced as that of the a part of the processor instruction, and may be enforced as that of a sequence of directions. every use of a macro generates new of the program instructions; the macro has that of the result of automating writing of that of the program.

Macros may be outlined that are utilized in several of the programming languages, like C, C++ etc. Example macro therein of the C programming. Macros are ordinarily utilized in that of the C to outline little snippets of code. If the macro has the parameters, they're substituted into that of the macro body throughout expansion; therefore, a C macro will mimic that of a C perform. the standard reason for doing this can be to avoid that of the overhead of a call within the straightforward cases, wherever the code is light-weight enough that call overhead has that of a big impact on performance.

For example,

#define scoop (a, b) a>b? A: b

Defines the macro scoop, taking 2 of the arguments a and b. This macro is also called like several C perform, exploitation identical syntax. Therefore, afterward of the pre-processing

z = max(x, y);

Becomes z = x>y? X:y;

While this use of macros is extremely necessary for that of the C, as an example to outline type-safe generic data-types or that of the debugging tools, it's conjointly slow, rather inefficient, and should cause that of variety of pitfalls.

C macros are capable of that of the mimicking functions, making new syntax at intervals a number of the constraints, moreover as increasing into that of the discretionary text (although the compiling program would require that text to be valid C ASCII text file, as an alternative comments), however they need a number of the constraints as that of a programming construct. Macros that mimic functions, as an example, may be referred to as like real functions, however that of a macro can't be passed to it of the perform employing a function pointer, since that of the macro itself has no address.

In programming languages, like C or that of the programing language, a reputation that defines a collection of the commands that are substituted for that of the macro name where the name seems therein of a program (a method referred to as macro expansion) once the program is compiled or the assembled. Macros are kind of like that of the functions therein they'll take arguments and in that they're calls to it of the lengthier sets of directions. in contrast to functions, macros are replaced by that of the the particular commands they represent once the program is ready for the execution. perform directions are traced into a program just once.

Macro enlargement.

A macro decision results in that of the macro enlargement. throughout macro enlargement, the macro statement is replaced by the sequence of the assembly statements.

In the higher than program a macro decision is shown therein of the center of the figure. that's INITZ. that is named throughout that of the program execution? each macro begins with the MACRO keyword at the start and ends thereupon of the ENDM (end macro).whenever a macro is named the complete of the code is substituted within the program wherever it's called. that the resultant of the macro code is shown on the correct most aspect of the figure. Macro job in high level programming languages

(C programming)

#define max(a,b) a>b?a:b

Main ()

The higher than program was written exploitation C programming statements. Defines the macro scoop, taking 2 arguments a and b. This macro is also referred to as like several C perform, exploitation identical syntax. Therefore, when preprocessing

Becomes z = x>y ? x: y;

After macro enlargement, the entire code would seem like this.

#define max(a,b) a>b?a:b

main()

Q3) Explain Nested Macro calls?

A3)

A macro body may contain that of the more macro definitions. However, these nested macro definitions aren't valid till the insertion macro has been that of the expanded! meaning, the insertion of that of the macro should are referred to as, before the nested macros that may be known as.

Example 1:

A macro, which might be wont to outline that of the macros thereupon of the capricious names, could look as that of the follows:

DEFINE MACRO MACNAME

MACNAME MACRO

DB 'I am the macro &MACNAME;.'

ENDM

In order to not overload the instance with "knowhow", the nested macro solely introduces itself kindly thereupon of an acceptable character string within the ROM. The call

DEFINE Obiwan

Would outline the macro

Obiwan MACRO

DB 'I am the macro Obiwan.'

ENDM

And the decision

DEFINE Skywalker

Would outline the subsequent macro:

Skywalker MACRO

DB 'I am the macro Skywalker.'

ENDM

Example 2:

A macro is to insert a variable range of NOPs into the program. For this, a macro with a nested REPT block appears to be best-suited:

REPEAT MACRO NOPS

REPT NOPS

NOP

ENDM

The macro decision

REPEAT 4

Results in one thing like that:

Line I Addr Code supply

9+ one N 0004 REPT four

10+ one NOP

11+ one ENDM

12+ a pair of 0000 00 NOP

13+ a pair of 0001 00 NOP

14+ a pair of 0002 00 NOP

15+ a pair of 0003 00 NOP

Q4) Explain Advanced macro facilities?

A4)

Advanced macro facilities are aimed to supporting semantic expansion.

Used for:

Performing conditional expansion of model statements and

In writing expansion time loops.

These facilities can be grouped into following.

1. Facilities for alteration of flow of control during expansion.

2. Expansion time variables.

3. Attributes of parameters.

1. ALTERATION OF FLOW OF CONTROL DURING EXPANSION

Two features are provided to facilitate alteration of flow of control during expansion.

1. Expansion time sequencing symbol

2. Expansion time statements

AIF,

AGO and

ANOP

A sequencing symbol (SS) has the syntax. <Ordinary string>

As SS is defined by putting it in the label field of statement in the macro body.

It is used as an operand in an AIF or AGO statement to designate the destination of an expansion time control transfer.

AIF STATEMENT

An AIF statement has the syntax AIF (<expression>) <sequencing symbol>

Where <expression> is a relational expression involving ordinary strings, formal parameters and their attributes and expansion time variables.

If the relational expression evaluates to true, expansion time control is transferred to the statement containing <sequencing symbol> in its label field.

AN AGO STATEMENT

An AGO statement has the syntax AGO <sequencing symbol>

Unconditionally transfers expansion time control to the statement containing <sequencing symbol> in its label field.

AN ANOP STATEMENT

An ANOP statement is written as <sequencing symbol> ANOP

And simply has the effect of defining the sequencing symbol.

2. EXPANSION TIME VARIABLE

Expansion time variables (EV’s) are variables which can only be used during the expansion of macro calls.

A local EV is created for use only during a particular macro call.

A global EV exists across all macro calls situated in a program and can be used in any macro which has a declaration for it.

Local and global EV’s are created through declaration statements with the following syntax: LCL <EV specification> [, <EV specification>] GBL <EV specification> [, <EV specification>]

<EV specification> has the syntax &<EV name> where <EV name> is an ordinary string.

Values of EV‟s can be manipulated through the pre-processor statement SET.

A SET statement is written as<EV specification> SET <SET-expression>

Where <EV specification> appears in the label field and SET in the mnemonic field.

A SET statement assigns the value of <SET-expression> to the EV specified in <EV specification>.

The value of an EV can be used in any field of a model statement, and in the expression of an AIF statement.

A call on macro CONSTANTS is expanded as follows.

The local EV A is created.

The first SET statement assigns the value „1‟ to it. The first DB statement thus declares a byte constant „1‟.

The second SET statement assigns the value „2‟ to A

And the second DB statement declares a constant „2‟.

3. ATTRIBUTES OF FORMAL PARAMETERS

An attribute is written using the syntax <attribute name>‟ <formal parameter spec>

And represents information about the value of the formal parameter,

I.e. about the corresponding actual parameter.

The type, length and size attributes have the names T,L and S

EXAMPLE

Here expansion time control is transferred to the statement having .NEXT in its label field only if the actual parameter corresponding to the formal parameter A has the length of „1‟.

Q5) Explain Design of macro pre-processor?

A5)

The macro pre-processor accepts that of an assembly program containing the definitions and the calls

And then it translates it into that of an assembly program which does not contain any of the macro definitions and the calls.

The program form output by that of the macro pre-processor that can be handed over to that of an assembler to obtain that of the target program.

$C:\Users\Mansi\Downloads\content writing\System Programming AKU University\m.jpg$

We begin the design by that of the listing all tasks involved in that of the macro expansion.

1. Identify macro calls in that of the program.

2. Determine the values of that of the formal parameters.

3. Maintain the values of that of the expansion time variables declared in a macro.

4. Organize expansion time of that of the control flow.

5. Determine the values of that of the sequencing symbols.

6. Perform expansion of that of a model statement.

Following 4 step procedures is to be followed to arrive at a design specification for each of the task.

Identify that of the information necessary to perform each of the tasks.

Design that of a suitable data structure to record that of the information.

Determine that of the processing necessary to obtain that of the information.

Determine that of the processing necessary to perform that of the task.

IDENTIFYMACROCALLS

A table which is called the Macro Name Table (MNT) is designed to hold that of the names of all macros defined in that of a program.

A macro name is entered in that of this table when macro definition is to be processed.

While processing that of a statement in that of the source program, the pre-processor compares that of the string found in its mnemonic field with that of the macro names in MNT.

A match indicates that of the current statement is that of a macro call.

2. DETERMINETHEVALUESOFFORMALPARAMETERS

A table called that of the Actual Parameter Table (APT) is designed to hold that of the values of formal parameters during that of the expansion of a macro call.

Each entry in the table is that of a pair (<formal parameter name>, <value>).

Two items of that of the information are needed to construct this of the table, the names of formal parameters and that of the default values of keyword parameters.

A table called that of the Parameter Default Table (PDT) is used for each macro.

It would contain pairs of that of the form (<formal parameter name>, <default value>)

If a macro call statement does not specify that of a value for some parameter par, its default value would be copied from PDT to APT.

3. MAINTAINEXPANSIONTIMEVARIABLES

An Expansion time of that of the Variable Table (EVT) is maintained for this purpose.

The table contains pairs of that of the form (<EV name>, <value>).

The value field of that of a pair is accessed when that of a pre-processor statement or a model statement under that of an expansion refers to an EV.

4. ORGANIZEEXPANSIONTIMECONTROLFLOW

The body of that of a macro, i.e. the set of the pre-processor statements and that of the model statements in it, is stored in that of a table called the Macro Definition Table (MDT) for use during that of the macro expansion.

The flow of control during that of the macro expansion determines when that of a model statement is to be visited for that of the expansion.

For this purpose MEC (Macro Expansion Counter) is to be initialized to the first statement of that of the macro body in that of the MDT.

It is updated after that of the expanding a model statement of that of on processing a macro pre-processor statement.

5. DETERMINEVALUESOFSEQUENCINGSYMBOLS

A Sequencing Symbols that of the Table (SST) is maintained to hold this type of information.

The table contains pairs of that of the form (<sequencing symbol name>, <MDT entry #>)

Where <MDT entry #> is the number of the MDT entry which contains that of the model statement defining the sequencing symbol.

This entry is made on encountering that of a statement which contains that of the sequencing symbol in its label field (for back reference to symbol) or on encountering that of a reference prior to its definition (forward reference).

6. PERFORMEXPANSIONOFAMODELSTATEMENT

This is trivial task given that of the following:

1. MEC points to the MDT entry that of containing the model statement.

2. Values of formal parameters and that of the EV‟s are available in the APT and the EVT, respectively

3. The model statement defining that of a sequencing symbol can be identified from that of the SST

Expansion of a model statement is to be achieved by that of the performing a lexical substitution for that of the parameters and EV‟s used in the model statement.

Q6) Write The Sequence Of Instructions To Perform The Operation Beta = Alpha+5 Using Sic/xe Instructions?

A6)

23
LDA ALPHA
ADD Si
STA BETA
... ...
ALPHA RESW 1
BETA RESW 1

Q7) What Is The Use Of Td Instruction In Sic Architecture?

A7)

The test device (TD) instruction tests whether the addressed device is ready to send or receive a byte of data. The condition code is set to indicate the result of this test. Setting of < means the device is ready to send or receive, and = means the device is not ready.

Q8) Define The Basic Functions Of Assembler?

A8)

Translating mnemonic operation codes to their machine language equivalents.

Assigning machine addresses to symbolic labels used by the programmer.

Q9) What Is Meant By Assembler Directives Give Example?

A9)

These are the statements that are not translated into machine instructions, but they provide instructions to assembler itself.

example S1ART,END.BYTE.WORD.RESW and RESB.

Q10) What Are Forward References?

Q10)

It is a reference to a label that is defined later in a program.

Consider the statement

10 1000 STL RETADR
……
……
8o 1036 RETADR RESW 1

The first instruction contains a forward reference RETADR. If we attempt to translate the program line by line, we will unable to process the statement in line10 because we do not know the address that will be assigned to RETADR .The address is assigned later(in line 80) in the program.

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