Assembler Notes
Assembler is system software which is used to convert an
assembly language program to
its equivalent object code. The input to the assembler is a
source code written in assembly
language (using mnemonics) and the output is the object
code. The design of an
assembler depends upon the machine architecture as the
language used is mnemonic
language.
1. Basic Assembler Functions:
The basic assembler functions are:
Translating mnemonic language code to its equivalent object
code.
Assigning machine addresses to symbolic
labels.
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Source Program
-Mnemonic opcode
-Symbol
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• The design of assembler can be to perform the following: –
Scanning (tokenizing)
–
Parsing (validating
the instructions) –
Creating the symbol table –
Resolving the forward references –
Converting into the machine language
• The design of assembler in other words:
– Convert mnemonic operation codes to their machine language
equivalents
– Convert symbolic
operands to their equivalent machine addresses
– Decide the proper instruction format Convert the data
constants to internal machine
representations
– Write the object program and the assembly listing
So for the design of the assembler we need to concentrate on
the machine architecture of
the SIC/XE machine. We need to identify the algorithms and
the various data structures
to be used. According to the above required steps for
assembling the assembler also has
to handle assembler directives, these do not generate the
object code but directs the
assembler to perform certain operation. These directives
are: • SIC Assembler Directive:
– START: Specify name & starting address.
– END: End of the program, specify the first execution
instruction.
– BYTE, WORD, RESB, RESW
– End of record: a null char(00)
End of file: a zero length record
The assembler design can be done:
Single pass assembler
Multi-pass assembler
Single-pass Assembler:
In this case the whole process of scanning, parsing, and
object code conversion is
done in single pass. The only problem with this method is
resolving forward reference.
This is shown with an example below:
10 1000 FIRST STL RETADR 141033
--
--
--
--95 1033 RETADR RESW 1
In the above example in line number 10 the instruction STL
will store the linkage
register with the contents of RETADR. But during the
processing of this instruction the
value of this symbol is not known as it is defined at the
line number 95. Since I single- pass assembler the scanning, parsing and object
code conversion happens simultaneously.
The instruction is fetched; it is scanned for tokens, parsed
for syntax and semantic
validity. If it valid then it has to be converted to its
equivalent object code. For this the
object code is generated for the opcode STL and the value
for the symbol RETADR need
to be added, which is not available.
Due to this reason usually the design is done in two passes.
So a multi-pass
assembler resolves the forward references and then converts
into the object code. Hence
the process of the multi-pass assembler can be as follows:
Pass-1
Assign addresses to all the statements
Save the addresses assigned to
all labels to be used in Pass-2
Perform some processing of assembler
directives such as RESW, RESB to find
the length of data areas for assigning the address values. Defines the symbols in the symbol table(generate the symbol table)
Pass-2
Assemble the instructions
(translating operation codes and looking up addresses).
Generate data values defined by BYTE,
WORD etc.
Perform the processing of the assembler directives not done during pass-1.
Write the object program and assembler
listing.
Two Pass Assembler
Mostly assembler are
designed in two passes(stages), therefore, they are called Two-Pass Assemblers.
'Re pass-wise grouping of tasks in a two pass assembler is given below:
Pass I
- Separate the symbols, mnemonic op-code and operational
fields.
- Determine the storage requirement for every assembly language
statement and up date the location counter.
- Build the symbol table. (Table that is used to store
each label and its corresponding value).
Pass II
FUNCTION
The program of figure 5,
although, written in a hypothetical assembler language, contains the basic
elements which need to be translated into machine language. (It is not
essential for students to understand the meaning of each statement of the
program.) For ease of reference, each instruction is defined by a line number, which
is not part of the program. Each instruction in our language contains either an
operation specification (lines 1- 15) or a storage specification (lines 16-
21). An operation specification is a symbolic operation code, which may be
preceded by a label and must be followed by 0, 1, or two operand
specifications, as appropriate to the operation. A storage specification is a
symbolic instruction to the assembler. In our assembler language, it must be
preceded by a label and must be followed, if appropriate, by a constant FIXED.
Labels and operand specifications are symbolic addresses; every operand
specification must appear somewhere in the program as a label.
Line
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Label
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Operation
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Operand 1
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Operand 2
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1
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COPY
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ZERO
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OLDER
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2
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COPY
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ONE
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OLD
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3
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READ
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LIMIT
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4
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WRITE
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OLD
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5
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FRONT
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LOAD
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OLDER
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6
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ADD
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OLD
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7
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STORE
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NEW
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8
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SUBST
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LIMIT
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9
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BRPOS
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FINAL
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10
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WRITE
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NEW
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11
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COPY
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OLD
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OLDER
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12
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COPY
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NEW
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OLD
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13
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JMP
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FRONT
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14
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FINAL
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WRITE
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LIMIT
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15
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STOP
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16
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ZERO
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CONST
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0
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17
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ONE
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CONST
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18
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OLDER
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SPACE
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19
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OLD
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SPACE
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20
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NEW
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SPACE
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21
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LIMIT
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SPACE
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