Page 69 - DCAP507_SYSTEM_SOFTWARE

Unit 4: Design of Assembler

Notes

In preserving with the purpose of pass 1 of an assembler (define symbols and literals), we can
generate the symbol and literal tables displayed below.

Symbol Table

Literal Table

We scan the program above along with a local counter. For every symbol in the label field we
make an entry in the symbol table. For the symbol PRGAM2 its value is its relative location. We
modernize the location counter, observing that the every instruction is 1 word long. Continuing,
we discover that the next four symbols are defined by the pseudo-op EQU. These symbols are
entered into the symbol to and the connected values given in the argument fields of the EQU
statements entered.
Observe that none of the Pseudo-ops encountered effect location counter as they do not effect in
any object code. So the location counter has a value 8 when LOOP is encountered. Thus, LOOP is
entered into the symbol table with a value 2. It is a relocatable variable. Its length is 1 word as
it signifies a location that will enclose an instruction. All other symbols are entered in like
manner. In the same pass all literals are renowned and entered into a literal table. The first
literal is in statement 10 and its value is the address of the location that comprises the literal.
Because this is the first literal, it will have the first address the literal area. The literal table will
be positioned at the end of the program.

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