Symbolic Microinstructions | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

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

• The symbolic representation:

• The binary representation:

• 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.

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.

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.