Microinstruction Format | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Microinstruction Format
Computer Instruction Format 

• The computer instruction format is depicted in Fig. 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.

Microinstruction Format

• The microinstruction format for the control memory is shown in Fig. 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.

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 simultaneously. 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.

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 bits. 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.