Microinstruction fields and sequencing techniques

Fields of a microinstruction

A microinstruction (20 bits in Mano's design) has:

• F1, F2, F3 (3 bits each): microoperation fields. Each selects a microoperation to execute at the same time as the other two (for example F1 = 110 PCTAR, F2 = 100 READ, F3 = 101 INCPC).
• CD (2 bits): condition field. 00 = U (unconditional), 01 = I (indirect bit), 10 = S (sign bit of AC), 11 = Z (AC zero).
• BR (2 bits): branch field: 00 JMP, 01 CALL, 10 RET, 11 MAP.
• AD (7 bits): address field, giving the branch address.

 | F1(3) | F2(3) | F3(3) | CD(2) | BR(2) | AD(7) |

Sequencing techniques

Sequencing means selecting the address of the next microinstruction. The techniques are:

1. Incrementing CAR: the microinstructions of a routine are stored in sequence, so CAR ← CAR + 1 is the default.
2. Unconditional branching: BR = JMP with CD = U, so CAR ← AD.
3. Conditional branching: CD selects a status bit; if it is 1, CAR ← AD, otherwise CAR ← CAR + 1.
4. Mapping: at the end of fetch, the opcode is mapped to the routine address (0 + opcode + 00).
5. Subroutine call and return: CALL saves CAR + 1 in the subroutine register SBR and jumps to AD; RET loads CAR ← SBR. A common routine (like INDRCT) can be shared by many routines.

   opcode --> mapping ------+
   AD field ----------------+--> [ MUX ] --> CAR --> Control mem
   SBR ----------------------+      ^
   CAR + 1 -------------------------+
                          select by  CD, BR, status

Sequencing lets the control unit branch on status bits and reuse common routines, which keeps the microprogram short.