GENERAL DESCRIPTION

1.1  ASSEMBLY PROCESSING

The Assembler is a two pass system, or, if both punch and
list output are requested, a three pass sysem on a minimum
configuraton. A pass is defined as a processing cycle of the
source program input.

In the first pass, the Assembler creates a symbol table from
the names used in the source statements. It also checks for
certain possible error conditions and generates diagnostic
messages if necessary.

During pass two, the Assembler again examines each state-
ment in the source program along with the symbol table and
produces the binary program and a program listing. Addition
diagnostic messages may also be producted.

If only one output device is available and if both the binary
output and list output are requested, the listing function is
deferred and performed as pass three.

When using the Assembler with a mass storage device the source
program is written on the device during the first pass; the second
pass of the source is read from the mass storage.

1.2  SYMBOLIC ADDRESSING

Symbols may be used for referring to machine instructions,
data, constants, and certain other pseudo operations. A sym-
bol represents the address for a computer word in memory.
A symbol is defined when it is used as a label for a location in
the program, a name of a common storage segment, the label
of a data storage area or constant, the label of an absolute or
relocatable value, or a location external to the program.

Through use of simple arithmetic operators, symbols may be
combined with other symbols or numbers to form an expres-
sion which may identify a location other than that specifically
named by a symbol. Symbols appearing in operand expres-
sions, but not specifically defined, and symbols that are
defined more than once are considered to be in error by the
Assembler.

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                            [IMAGE]

                       HP ASSEMBLER PROCESSING

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1.3  PROGRAM RELOCATION

Relocatable programs may be relocated in core by the relocating
loader; the location of the program origin and all subsequent
instrucitons is determined at the time the program is loaded.

A relocatable program is assembled assuming a starting
location of zero. All other instructions and data areas are
assembled relative to this zero base. When the program is
loaded, the relocatable operands are adjusted to correspond
with the actual locations assigned by the loader.

The starting locations of the common storage area and the
base page portion of the program are always established by
the loader. References to the common area are common re-
locatable. Reference to the base page portion of the program
are base page relocatable. If a program refers to the common
area or makes use of the base page via the ORB pseudo in-
struction, the program must also be relocatable.

If a program is to be relocatable, all subprograms comprising
the program must be relocatable; all memory reference
operands must be relocatable expressions or literals, or have
an absolute value of less than 100(8).

1.4  PROGRAM LOCATION COUNTERS

The Assembler maintains a counter, called the program loca-
tion counter, that assigns consecutive memory addresses to
source statements.

The initial value of the program location counter is estab-
lished according to the use of either the NAM or ORG psuedo
operation at the start of the program. The NAM operation
causes the program location counter to be set to zero for a
relocatable program; the ORG operation specifies the absolute
starting location for an absolute program.

Through use or the ORB pseudo operation a relocatable pro-
gram may specify that certain operations or data areas be
allocated to the base page. If so, a separate counter, called
the base page location counter, is used in assigning these
locations.

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1.5  ASSEMBLY OPTIONS

Parameters specified with the first statement, the control
statement, define the output to be produced by the assembler: [1]

    Absolute - The addresses generated by the Assembler
    are to be interpreted as absolute locations in memory.
    The program is a complete entity; external symbols,
    common storage references, and entry points are not
    permitted.

    Relocatable - The program may be located anywhere in
    memory. All operands which arefer to memory locations
    are adjusted as the program is loaded. Operands, other
    than those referring to the first 64 locations, must be re-
    locatable expressions. Subprograms may contain external
    symbols and entry points, and may refer to common
    storage.

    Binary output - An absolute or relocatable program is
    to be punched on the paper tape.

    List output - A program listing is produced either during
    pass two or pass three.

    Table print - List the symbol table at the end of the first
    pass.

    Selective assembly - Sections of the program may be
    included or excluded at assembly time depending on the
    option used.





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[1] See Chapter 5 for complete details.

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