Computer Instruction Format

  • The computer instruction format is depicted in 3-5(a).
  • It consists of three fields:
    • A 1-bit field for indirect addressing symbolized by I
    • A 4-bit operation code (opcode)
    • An 11-bit address field
  • Fig 3-5(b) lists four of the 16 possible memory-reference instructions.

                         Fig. 3-5 (a): Instruction Format

                                 Fig 3-5 (b): Four Computer Instructions

Microinstruction Format

  • The microinstruction format for the control memory is shown in 3-6.
  • The 20 bits of the microinstruction are divided into four functional parts.
    • The three fields F1, F2, and F3 specify microoperations for the computer.
    • The CD field selects status bit conditions.
    • The BR field specifies the type of branch.
    • The AD field contains a branch address.

                         Fig 3-6: Microinstruction code format

Microoperations


  • The three bits in each field are encoded to specify seven distinct microoperations as listed in Table 3-1.
  • No more than three microoperations can be chosen for a microinstruction, one from each field.
  • If fewer than three microoperations are used, one or more of the fields will use the binary code 000 for no operation.
  • It is important to realize that two or more conflicting microoperations cannot be specified e.g. 010 001 000
  • Each microoperation in Table 3-1 is defined with a register transfer statement and is assigned a symbol for use in a symbolic microprogram.

   Table 3-1 : Symbols and Binary code for Microinstruction Fields

Condition and Branch Field

  • The CD field consists of two bits which are encoded to specify four status bit conditions as listed in Table 3-1.
  • The BR field consists of two It is used, in conjunction with the address field AD, to choose the address of the next microinstruction.
    • The jump and call operations depend on the value of the CD field.
    • The two operations are identical except that a call microinstruction stores the return address in the subroutine register SBR.
    • Note that the last two conditions in the BR field are independent of the values in the CD and AD fields.