Mastering AddLen: VAX MACRO Arguments Explained

Mastering AddLen: VAX MACRO Arguments Explained The VAX MACRO assembly language relies heavily on system-defined macros to streamline programming tasks. Among these, the AddLen (Add Length) macro is a fundamental tool used to add a specific value to the length field of a character string descriptor. Understanding its arguments is crucial for precise memory and string manipulation in VAX environments.

Here is a comprehensive breakdown of the AddLen macro, its arguments, and how to use it effectively. What is AddLen?

The AddLen macro modifies a string descriptor by adding a designated increment to its length field. String descriptors on VAX systems typically consist of a 16-bit length field followed by data type and class codes, and a 32-bit pointer to the actual string data. AddLen allows you to adjust this length safely and efficiently. The Arguments Explained

The AddLen macro generally accepts three primary arguments, structured to define the target descriptor, the increment value, and an optional destination.

increment: The value to be added to the current length. This can be a literal value (e.g., #5), a register containing the value, or a memory location.

src_desc: The source string descriptor whose length you want to modify. You pass the address of this descriptor.

dst_desc (Optional): The destination descriptor where the modified length will be stored. If omitted, the macro typically defaults to modifying the source descriptor in place. How It Works Under the Hood

When the VAX MACRO assembler expands the AddLen macro, it generates the underlying machine instructions—usually an ADDW2 (Add Word 2-Operand) or ADDW3 (Add Word 3-Operand) instruction.

Because the length field of a VAX string descriptor is a word (16 bits), the macro specifically targets the first two bytes of the descriptor block. Code Example: In-Place Modification

To add 10 bytes to the length of an existing descriptor named MY_STR_DESC:

.TITLE ADDLEN_EXAMPLE .IDENT /1.0/ ; Define the string descriptor MY_STR_DESC: .WORD 20 ; Current length: 20 bytes .BYTE DSC\(K_DTYPE_T ; Data type (ASCII text) .BYTE DSC\)K_CLASS_S ; Class (Scalar/Fixed) .ADDRESS MY_BUFFER ; Pointer to string data MY_BUFFER: .BLKB 100 ; Allocate 100 bytes of buffer space .ENTRY START, ^M<> ; Execute AddLen AddLen #10, MY_STR_DESC ; The length field in MY_STR_DESC is now 30 RET .END START Use code with caution. Code Example: Creating a New Descriptor Length

If you want to keep the original descriptor intact and output the new length to a second descriptor (NEW_STR_DESC), you provide the third argument: AddLen #15, MY_STR_DESC, NEW_STR_DESC Use code with caution.

This expands to an ADDW3 instruction, leaving MY_STR_DESC at its original length and writing the new sum to the word length of NEW_STR_DESC. Best Practices and Pitfalls

Watch for Word Overflow: The length field is strictly 16-bit unsigned (0 to 65,535). Adding an increment that pushes the total length past 65,535 will cause a word overflow, resulting in corrupted descriptor data.

Buffer Size Validation: AddLen only changes the descriptor metadata. It does not allocate more memory. Ensure the underlying buffer actually has enough physical space to hold the newly claimed length.

Context Alignment: Ensure your descriptor is properly aligned in memory (word or longword boundary) to avoid performance penalties during macro execution.

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