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.
- 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.
- 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.