Assembly Language for the Basic Computer

The basic unit of an assembly language program is a line of code. The specific language is defined by a set of rules that specify the symbols that can be used and how they may be combined to form a line of code.

Rules of the Language
  • Each line of an assembly language program is arranged in three columns called fields which specify the following information:
  1. The label field may be empty or it may specify a symbolic address.
  2. The instruction field specifies a machine instruction or a pseudo instruction.
  3. The comment field may be empty or it may include a comment.
  • A symbolic address, which is terminated by a comma, consists of one, two, or three, but not more than three alphanumeric characters. The first character must be a letter; the next two may be letters or numerals. A symbolic address in the instruction field specifies the memory location of an operand.
  • The instruction field in an assembly language program may specify one of the following items:
  1. A memory reference instruction (MRI)
          E.g. ADD OPR
          ADD PTR I
      2. A register reference or input-output instruction (non MRI)
          E.g. CLA
      3. A pseudo instruction with or without an operand
          E.g. ORG 0
          END
  • A MRI occupies two or three symbols separated by spaces. The first must be a three letter symbol defining an MRI operation code, the second is symbolic address and the third symbol, which may or may not be present, is the letter I. If I is missing, it denotes direct address instruction. If I is present, it denotes indirect address instruction.
  • A non MRI is defined as an instruction that does not have an address part. It consists of only operation code.
  • A pseudo instruction is not a machine instruction but rather an instruction to the assembler giving information about some phase of the translation.
Definition of Pseudo instructions
  • A line of code may or may not have a comment, but if it has, it must be preceded by a slash for the assembler to recognize the beginning of a comment field. Comments are inserted for explanation purposes only and are neglected during the binary translation process.
  • Example:
Assembly Language Program to Subtract Two Numbers

Listing of Translated Program

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