HLASM Language Reference

Coding Made Easier
– A control program
– Storage allocation
– Input and output facilities
– Linker
Coding Made Easier
It can be very difficult to write an assembler language program using only machine
instructions. The assembler provides additional functions that make this task
easier. They are summarized below.
Symbolic Representation of Program Elements
Symbols greatly reduce programming effort and errors. You can define symbols to
represent storage addresses, displacements, constants, registers, and almost any
element that makes up the assembler language. These elements include
operands, operand subfields, terms, and expressions. Symbols are easier to
remember and code than numbers; moreover, they are listed in a symbol cross
reference table, which is printed in the program listings. Thus, you can easily find a
symbol when searching for an error in your code. See page 29 for details about
symbols, and how you can use them in your program.
Variety in Data Representation
You can use decimal, binary, hexadecimal, or character representation of machine
language binary values in writing source statements. You select the representation
best suited to the purpose. The assembler converts your representations into the
binary values required by the machine language.
Controlling Address Assignment
| If you code the correct assembler instruction, the assembler computes the relative
| offset, or displacement from a base address, of any symbolic addresses you
specify in a machine instruction. It inserts this displacement, along with the base
register assigned by the assembler instruction, into the object code of the machine
instruction.
At execution time, the object code of address references must be in
| relative-immediate or base-displacement form. The computer obtains the required
address by adding the displacement to the base address contained in the base
| register, or from the relative-immediate offset of the instruction.
Relocatability
The assembler produces an object module that is independent of the location it is
initially assigned in virtual storage. That is, it can be loaded into any suitable virtual
storage area without affecting program execution. This is made easier because
most addresses are assembled in their base-displacement form.
Sectioning a Program
You can divide a source module into one or more control sections. After assembly,
you can include or delete individual control sections from the resulting object
module before you load it for execution. Control sections can be loaded separately
into storage areas that are not contiguous. A discussion of sectioning is contained
in “Source Program Structures” on page 51.
Chapter 1. Introduction 9