Chapter 4 Macro Processors

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Chapter 4 Macro Processors

• System Software
• Chih-Shun Hsu

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Introduction

• A macro represents a commonly used group of
  statements in the source programming language
• The macro processor replaces each macro
  instruction with the corresponding group of
  source language statement, this is called
  expanding macros
• The functions of a macro processor essentially
  involve the substitution of one group of
  characters or lines for another

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Macro Definition and Expansion

• The MACRO statement identifies the beginning of a
  macro definition
• The symbol in the label field is the name of the
  instruction
• The entries in the operand field identify the
  parameter of the macro instruction
• Each parameter begins with the character
• The MEND assembler directive marks the end of the
  macro definition
• A macro invocation statement gives the name of
  the macro instruction being invoked and the
  arguments to be used in expanding the macro

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Use of macros in a SIC/XE Program(3/1)
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Use of macros in a SIC/XE Program(3/2)
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Use of macros in a SIC/XE Program(3/3)
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Program with Macro Expanded(3/1)
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Program with Macro Expanded(3/2)
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Program with Macro Expanded(3/3)
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Macro Processor Data Structures

• The macro definitions themselves are stored in
  definition table (DEFTAB), which contains the
  macro prototype and the statements that make up
  the macro body
• The macro names are entered into NAMTAB, which
  serves as an index to DEFTAB
• For each macro instruction defined NAMTAB
  contains pointers to the beginning and end of the
  definition in DEFTAB
• The third data structure is an argument table
  (ARGTAB), which is used during the expansion of
  macro invocations
• When a macro invocation statement is recognized,
  the arguments are stored in ARGTAB according to
  their position in the argument list

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Macro Processor Data Structures
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Algorithm for a One-pass Macro Processor(3/1)
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Algorithm for a One-pass Macro Processor(3/2)
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Algorithm for a One-pass Macro Processor(3/3)
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Machine-Independent Macro Processor Features

• Concatenation of Macro Parameters
• Generation of Unique Labels
• Conditional Macro Expansion
• Keyword Macro Parameters

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Concatenation of Macro Parameters(2/1)

• Most macro processors allow parameters to
  concatenated with other character strings
• If similar processing is to be performed on each
  series of variables, the programmer might want to
  incorporate this processing in to a macro
  instruction
• The body of the macro definition might contain a
  statement like LDA XID1 in which the
  parameter ID is concatenated after the character
  string X and before the character string 1
• If the macro definition contained both ID and
  ID1 as parameters, the situation would be
  ambiguous
• Most macro processors deal with this problem by
  providing a special concatenation operator (e.g.
  ?)
• LDA XID?1

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Concatenation of Macro Parameters(2/2)
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Generation of Unique Labels

• Relative addressing in a source statement may be
  acceptable for short jumps such as JEQ -3
• For longer jumps spanning several instructions,
  such notation is very inconvenient, error-prone
  and difficult to read
• Allow the creation of special types of labels
• Each symbol beginning with has been modified by
  replacing with xx, where xx is a two character
  alphanumeric counter of the number of macro
  instructions expanded
• For the first macro expansions, xx will have the
  value AA
• For succeeding macro expansions, xx will be set
  to AB, AC, etc

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Generation of Unique Labels with Macro
Expansion(2/1)
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Generation of Unique Labels with Macro
Expansion(2/1)
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Conditional Macro Expansion(2/1)

• Most macro processors can modify the sequence of
  statements generated for a macro expansion,
  depending on the arguments supplied in the macro
  invocation
• The IF statement evaluates a Boolean expression
  that is its operand
• If the value of this expression is TRUE, the
  statements following the IF are generated until
  an ELSE is encountered
• Otherwise, these statements are skipped, and the
  statements following the ELSE are generated
• The ENDIF statement terminates the conditional
  expression that was begun by the IF statement

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Conditional Macro Expansion(2/2)

• The macro processor must maintain a symbol table
  that contains the values of all macro-time
  variables used
• Entries in this table are made or modified when
  SET statements are processed
• The implementation outlined above does not allow
  for nested IF structures
• WHILE a macro-time looping statement
• The WHILE statement specifies that the following
  lines, until the next ENDW statement, are to be
  generated repeatedly as long as a particular
  condition is true
• The macro-time variable CTR is used to count the
  number of times the lines following the WHILE
  statement have been generated

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Use of Macro-time Conditional Statements(5/1)
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Use of Macro-time Conditional Statements(5/2)
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Use of Macro-time Conditional Statements(5/3)
RDBUFF F3, BUF, RECL, 04, 2048
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Use of Macro-time Conditional Statements(5/4)
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Use of Macro-time Conditional Statements(5/5)
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Use of Macro-time looping Statements(2/1)
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Use of Macro-time looping Statements(2/2)
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Keyword Macro Parameters

• Positional parameter parameters and arguments
  were associated with each other according to
  their positions in the macro prototype and the
  macro invocation statement
• Keyword parameters each argument value is
  written with a keyword that named the
  corresponding parameter
• Each parameter name is followed by an equal sign,
  which identifies a keyword parameter
• The parameter is assumed to have the default
  value if its name does not appear in the macro
  invocation statement

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Use of Keyword Parameters in Macro
Instructions(3/1)
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Use of Keyword Parameters in Macro
Instructions(3/2)
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Use of Keyword Parameters in Macro
Instructions(3/3)
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Macro Processor Design Options

• Recursive Macro expression
• General-Purpose Macro Processors
• Macro Processing within Language Translators

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Recursive Macro Expansion

• Macro within macro can be solved if the macro
  processor is being written in a programming
  language that allows recursive calls
• The compiler would be sure that previous value of
  any variables declared within a procedure were
  saved when that procedure was called recursively
• If would take care of other details involving
  return from the procedure

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Example of Nested Macro Invocation(2/1)
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Example of Nested Macro Invocation(2/2)
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General-Purpose Macro Processors(2/1)

• Advantages of general-purpose macro processors
• The programmer does not need to learn about a
  different macro facility for each compiler or
  assembler languagethe time and expense involved
  in training are eliminated
• The costs involved in producing a general-purpose
  macro processor are somewhat greater than those
  for developing a language-specific processor
• However, this expense does not need to be
  repeated for each language the result is
  substantial overall saving in software
  development cost

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General-Purpose Macro Processors(2/2)

• A general-purpose facility must provide some way
  for a user to define the specific set of rules to
  be followed
• Comments should usually be ignored by a macro
  processor, however, each programming language has
  its own methods for identifying comments
• Each programming language has different
  facilities for grouping terms, expressions, or
  statementsa general-purpose macro processor
  needs to taking these grouping into account
• Languages differ substantially in their
  restrictions on the length of identifiers and the
  rules for the formation of constants
• Programming languages have different basic
  statement formssyntax used for macro definitions
  and macro invocation statements

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Macro Processing within Language Translators(2/1)

• The macro processor reads the source statements
  and performs all of its functions, the output
  lines are passed to the language translator as
  they are generated
• The macro processor operates as a sort of input
  routine for the assembler or compiler
• The line-by-line approach avoids making an extra
  pass over the source program, so it can be more
  efficient than using a macro preprocessor
• Some of the data structures required by the macro
  processor and the language translator can be
  combined
• A line-by-line macro processor also makes it
  easier to give diagnostic messages that are
  related to the source statement containing the
  error

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Macro Processing within Language Translators(2/2)

• An integrated macro processor can potentially
  make use of any information about the source
  program that is extracted by the language
  translator
• An integrated macro processor can support macro
  instructions that depend upon the context in
  which they occur
• Line-by-line macro processors must be specially
  designed and written to work with a particular
  implementation of an assembler or compiler, which
  results in a more expensive piece of software
• The assembler or compiler will be considerably
  larger and more complex than it would be
• The additional complexity will add to the
  overhead of language translation

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MASM Macro Processor(2/1)

• The macro processor of MASM is integrated with
  Pass 1 of the assembler
• MASM generates the unique names of local labels
  in the form ??n, where n is a hexadecimal number
  in the range 0000 to FFFF
• .ERR signals to MASM that an error has been
  detected
• EXITM directs MASM to terminate the expansion of
  the macro
• is a concatenation operator

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MASM Macro Processor(2/2)

• is a macro comment, serves only as
  documentation for the macro definition
• is an ordinary assembler language comment,
  included as part of the macro expansion
• IRP sets the macro-time variable to a sequence
  of values specified in ltgt
• The statements between the TRP and the matching
  ENDM are generated once for each value of the
  variable

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Examples of MASM Macro and Conditional
Statements(3/1)
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Examples of MASM Macro and Conditional
Statements(3/2)
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Examples of MASM Macro and Conditional
Statements(3/3)
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Example of MASM Iteration Statement
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ANSI C Macro Language

• In the ANSI C language, definitions and
  invocations of macros are handled by a
  preprocessor, which is generally not integrated
  with the rest of the compiler
• define ABSDIFF(X,Y) XgtY ? X-Y Y-X
• define DISPLAY(EXPR) printf(EXPRd\n,EXPR)
• Macro in ANSI C may contain definitions or
  invocations of other macros
• DISPLAY(ABSDIFF(3,8))?ABSDIFF(3,8)5
• The ANSI C preprocessor also provides conditional
  compilation statements
• ifndef BUFFER_SIZE
• define BUFFER_SIZE 1024
• endif

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The ELENA Macro Processor

• Macro definitions in ELENA are composed of a
  header and a body
• The header consists of keywords and parameter
  markers, which are identified by the character
• At least one of the first two token s in a macro
  header must be a keyword, not a parameter marker
• The macro processor appends a numeric value to
  create unique labels
• ELENA provides macro-time variables and
  macro-time instructions that can be used to
  control the macro expansion
• The IF statement is a macro-time conditional go
  to statement
• The macro is identified by the sequence of
  keywords that appear in its header

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Examples of ELENA Macro Definition and
Invocation(2/1)
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Examples of ELENA Macro Definition and
Invocation(2/2)
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Example of ELENA Macro-time Instructions(2/1)
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Example of ELENA Macro-time Instructions(2/1)