HLASM Language Reference

At assembly time: High Level Assembler translates the source statements into
object code. Each source module is assembled into one object module. The
| contents of the object module are relocatable.
At linking time: As specified by linker or binder control statements, the linker or
binder combines the object code of one or more sections into one load module or
program object. It also calculates the addresses needed to accommodate common
sections and external dummy sections from different object modules. In addition, it
calculates the space needed to accommodate external dummy sections.
You can specify the relocatable address of the starting point for program execution
| in a linker control statement or request a starting address in the operand field of an
assembler END instruction.
| At program fetch time: The control program loads the load module or program
| object into virtual storage. All the relocatable addresses are converted to fixed
locations in storage.
| At execution time: The control program passes control to the loaded program
now in virtual storage, and your program is run.
| Sections
| In the load module model, control sections may generate machine language text
| containing instructions and data, or define mappings of storage areas to be
| referenced at execution time. Control sections that generate machine language
| text are called executable control sections, even though they may contain only
| data. Control sections that create only mappings are called reference control
| sections.
| In the program object model, sections may define classes containing
| elements. (Classes are described in “Classes (MVS and CMS)” on page 59.)
| Elements may contain machine language text or define mappings, or both.
| Elements may in turn contain one or more parts, which are described at “Parts
| (MVS and CMS)” on page 61.
| Elements containing machine language text are usually linked in a class comprising
| other elements containing machine language text, and elements defining mappings
| are usually linked in a class with other elements defining mappings.
| The section name is used in binder operations to refer to its entire collection of
| elements and parts, but a program object section is not the same as a load module
| control section. A section name may be referenced as an external name only if
| defined as an entry point in an element belonging to that section. (By default, the
| assembler will generate an entry point in class B_TEXT with the section's name.
| See “Classes (MVS and CMS)” on page 59 for more information.)
| The term “executable” is used to describe both executable control sections in the
| load module model, or sections in the program object model.
| You initiate a section by using the START, CSECT, or RSECT instruction, as
described below:
The START instruction can be used to initiate the first or only section of a
source module. For more information about the START instruction, see
“START Instruction” on page 214.
Chapter 3. Program Structures and Addressing 53