Macro Data Structure

1. Macro Definition Table (MDT)

• Purpose: Stores the body of all macro definitions (i.e., instructions between MACRO and MEND).

• Each line of a macro definition is stored in sequence.

Example:

MACRO
INCR &ARG
   ADD AREG, &ARG
MEND

MDT:

Index	Instruction
0	INCR &ARG
1	ADD AREG, &ARG
2	MEND

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2. Macro Name Table (MNT)

• Purpose: Holds the names of defined macros and their starting index in MDT.

• Used to quickly locate macro bodies during expansion.

MNT:

Index	Macro Name	MDT Index
0	INCR	0

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3. Macro Definition Table Counter (MDTC)

• Purpose: Points to the next available entry in the MDT.

• Updated as macro lines are inserted.

• Starts from 0 and increments with every new line added.

Example:

• Before macro insertion: MDTC = 0

• After above macro: MDTC = 3

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4. Macro Name Table Counter (MNTC)

• Purpose: Points to the next available entry in the MNT.

• Incremented each time a new macro is defined.

Example:

• After defining INCR, MNTC = 1

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5. Argument List Array (ALA)

• Purpose: Maintains a list of formal parameters during macro definition and replaces them with actual parameters during macro expansion.

• Ensures proper parameter substitution.

Macro Call:

INCR NUM1

ALA During Expansion:

Index	Formal Arg	Actual Arg
0	&ARG	NUM1

• During expansion, &ARG is replaced by NUM1 in the instructions.

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Summary Table:

Structure	Purpose
MDT	Stores macro body (excluding MACRO keyword)
MNT	Stores macro names and corresponding MDT index
MDTC	Tracks next available index in MDT
MNTC	Tracks next available index in MNT
ALA	Manages argument substitution