• The symbols defined in Table 3-1 cab be used to specify microinstructions in symbolic form.
  • Symbols are used in microinstructions as in assembly language
  • The simplest and most straightforward way to formulate an assembly language for a microprogram is to define symbols for each field of the microinstruction and to give users the capability for defining their own symbolic addresses.
  • A symbolic microprogram can be translated into its binary equivalent by a microprogram assembler.

Sample Format

Five fields:           label; micro-ops; CD; BR; AD

  • The label field: may be empty or it may specify a symbolic address terminated with a colon
  • The microoperations field: of one, two, or three symbols separated by commas , the NOP symbol is used when the microinstruction has no microoperations
  • The CD field: one of the letters {U, I, S, Z} can be chosen where 
  • U: Unconditional Branch
  • I: Indirect address bit
  • S: Sign of AC
  • Z: Zero value in AC
  • The BR field: contains one of the four symbols {JMP, CALL, RET, MAP}
  • The AD field: specifies a value for the address field of the microinstruction with one of {Symbolic address, NEXT, empty}

o When the BR field contains a RET or MAP symbol, the AD field is left empty

Fetch Subroutine

During FETCH, Read an instruction from memory and decode the instruction and update PC.

  • The first 64 words are to be occupied by the routines for the 16 instructions.
  • The last 64 words may be used for any other purpose.

o A convenient starting location for the fetch routine is address 64.

  • The three microinstructions that constitute the fetch routine have been listed in three different representatives.
    • The register transfer representation:


  • The symbolic representation:


  • The binary representation: