Assembly Language Programming

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Assembly Language Programming

• By Maria Ramila I. Jimenez
• Faculty ICS33

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Assembler Directives

• Assembler directives are instructions to the
  assembler, and are not translated into machine
  instructions. The use of directives gives the
  programmer some control over the operation of the
  assembler, increasing flexibility in the way
  programs are written. The following is a list of
  the common pseudo-ops.

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Assembler Directives

• EQUATEis used to make programs easier to write.
  The EQU directive creates absolute symbols and
  aliases by assigning an expression or value to
  the declared variable name. Its format is,
• name EQU expression
• Consider the following statement.
• NUMBER1 EQU 36H
• The assembler will replace every occurrence of
  the label NUMBER1 with the value its been equated
  to, ie, 36 hexadecimal.

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Assembler Directives
The statement LDAA NUMBER1 will be interpreted
by the assembler as LDAA 36H An absolute symbol
represents a 16bit value an alias is a name that
represents another symbol. The declared name must
be unique, one that has not been previously
declared. The redefining of a previous symbol is
normally not allowed. NUM1 EQU 20H... ...NUM1
EQU 30H error
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Assembler Directives

• ORIGINThis specifies the address to be used for
  the generation of code. Subsequent instructions
  and data address's begin at the new value.
  Normally, it is used to set the start address of
  the program, but can also set the location
  counter to the value specified.
• ORG 120HLDAA FFH

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Assembler Directives

• ORIGINThe statement LDAA FFH begins at byte
  120h.
• ORG 2start LDAA 34H
• The instruction associated with the label start
  is declared to start at the address 2bytes beyond
  the current value of the location counter
  (specified by ).

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Assembler Directives

• END and Optional Start AddressThe END directive
  specifies the end of the assembly language source
  listing. It may be followed by an optional entry
  address. The optional entry address is used by
  LOADERS to initialize the Program Counter before
  running the program. If no entry address is
  specified, execution will start at the first
  location allocated by the assembler.
• END

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Assembler Directives

• In this example, the END directive informs the
  assembler that there is no more source
  statements.
• ORG 0100Hstart LDAA 3FHJMP startEND start
• In this example, the END directive also specifies
  that the entry point to the program is the label
  start, whose address is 0100H.