- 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: