Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

Computer Architecture & Organisation (CAO)

Computer Science Engineering (CSE) : Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

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Symbolic Microinstructions

  • 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.

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.
    • 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 representations.
    • The register transfer representation

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

  • The symbolic representation:

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

  • The binary representation:

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

  • Control Storage:  128  20-bit words
  • The first 64 words: Routines for the 16 machine instructions 0, 4, 8, …, 60 gives four words in control memory for each routine.
  • The last 64 words: Used for other purpose (e.g., fetch routine and other subroutines)
  • The execution of the third (MAP) microinstruction in the fetch routine results in a branch to address 0xxxx00, were xxxx are the four bits of the operation code. e.g. ADD is 0000
  • In each routine we must provide microinstructions for evaluating the effective address and for executing the instruction.
  • The indirect address mode is associated with all memory-reference instructions.
  • A saving in the number of control memory words may be achieved if the microinstructions for the indirect address are stored as a subroutine.
  • This subroutine, INDRCT, is located right after the fetch routine, as shown in Table 3-2. 
  • Mapping: OP-code XXXX into 0XXXX00, the first address for the 16 routines are 0(0 0000 00), 4(0 0001 00),  8, 12, 16, 20, ..., 60
  • To see how the transfer and return from the indirect subroutine occurs:
    • MAP microinstruction caused a branch to address 0
    • The first microinstruction in the ADD routine calls subroutine INDRCT when I=1
    • The return address is stored in the subroutine register SBR.
    • The INDRCT subroutine has two microinstructions:
      • INDRCT:  READ     U   JMP   NEXT
      • DRTAR   U    RET
    • Therefore, the memory has to be accessed to get the effective address, which is then transferred to AR.
    • The execution of the ADD instruction is carried out by the microinstructions at addresses 1 and 2
    • The first microinstruction reads the operand from memory into DR.
    • The second microinstruction performs an add microoperation with the content of DR and AC and then jumps back to the beginning of the fetch routine.

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

Binary Microprogram ·

  • The symbolic microprogram must be translated to binary either by means of an assembler program or by the user if the microprogram is simple.
  • The equivalent binary form of the microprogram is listed in Table 7-3.
  • Even though address 3 is not used, some binary value, e.g. all 0’s, must be specified for each word in control memory.
  • However, if some unforeseen error occurs, or if a noise signal sets CAR to the value of 3, it will be wise to jump to address 64.

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

Symbolic Microinstructions Computer Science Engineering (CSE) Notes | EduRev

Control Memory

  • When a ROM is used for the control memory,the microprogram binary list provides the truth table for fabricating the unit.
    • To modify the instruction set of the computer, it is necessary to generate a new microprogram and mask a new ROM.
  • The advantage of employing a RAM for the control memor y is that the microprogram can be altered simply by writing a new pattern of 1’s and 0’s without resorting to hardware procedure.
  • However, most microprogram systems use a ROM for the control memory because it is cheaper and faster than a RAM.
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