C/C++ Programmer's Guide (G06.25+)

Mixed-Language Programming for TNS Programs
HP C/C++ Programmer’s Guide for NonStop Systems—429301-008
7-9
Memory Models
Memory Models
A TNS C program can use the small-memory model or the large-memory model,
depending on the amount of data storage required. The large-memory model is
recommended and is the default setting. All examples in this subsection illustrate the
large-memory model unless otherwise noted.
A TAL program can use any of the following memory combinations, depending on the
application’s needs:
•
The user data segment
•
The user data segment and the automatic extended data segment
•
The user data segment and one or more explicit extended data segments
•
The user data segment, the automatic extended data segment, and one or more
explicit extended data segments
The following table describes some aspects of memory usage by C and TAL programs.
The far right column refers to the upper 32K-word area of the user data segment.
Any TAL module that uses the upper 32K-word area of the user data segment cannot
run within a TNS C object file that contains the main routine.
Data Model
The size of the C data type int is 16 bits in the 16-bit data model and 32 bits in the
32-bit data model. (The 32-bit data model is also called the wide-data model.) If you
specify the 32-bit data model, the C data type int is 32 bits and the TAL data type INT
corresponds to the C data type short.
Any interface to a TAL routine must be specified in such a way that there is no
possibility for a data-length mismatch. Therefore, in your C program, use short for a
16-bit integer and long for a 32-bit integer and avoid the use of int. Using short in
your C program enables you to use the same declarations regardless of the data
model your program uses. The type short is always a 16-bit integer in the C compiler.
Language
Memory
Model Addressing Data Storage
Upper 32K-Word
Area
TNS C Small 16-bit 32K words Reserved
TNS C Large 32-bit 127.5 MB Reserved
TAL Not
applicable
16-bit or 32-bit 64K words (without the
CRE), plus 127.5 MB in
each extended data
segment that is allocated
Reserved only if
you use the CRE