What is the Format of Microinstruction in Computer Architecture?
Microinstruction Format
A microinstruction format includes 20 bits in total. They are divided into four elements as displayed in the figure.
F1, F2, F3 are the micro-operation fields. They determine micro-operations for the computer.
CD is the condition for branching. They choose the status bit conditions.
BR is the branch field. It determines the type of branch.
AD is the address field. It includes the address field whose length is 7 bits.
The micro-operations are divided into three fields of three bits each. These three bits can define seven different
micro-operations. In total there are 21 operations as displayed in the table.
Symbols with their Binary Code for Microinstruction Fields:
Name: Code Symbol
F1 000 None NOP
001 AC ← AC + DR ADD
010 AC ← 0 CLRAC
011 AC ← AC + 1 INCAC
100 AC ← DR DRTAC
101 AR ← DR(0 − 10) DRTAR
110 AR ← PC PCTAR
111 AC ← AC + DR WRITE
F2 000 None NOP
001 AC ← AC + DR SUB
010 AC ← AC ∨ DR OR
011 AC ← AC ∧ DR AND
100 DR ← M[AR] READ
101 DR ← AC ACTDR
Name: Code Symbol
110 DR ← DR + 1 INCDR
111 DR(0 − 10) ← PC PCTDR
F3 000 None NOP
001 AC ← AC ⊕ DR XOR
010 AC ← AC′ COM
011 AC ← shl AC SHL
100 AC ← shr AC SHR
101 PC ← PC + 1 INCPC
110 PC ← AR ARTPC
111 DR(0 − 10) ← PC Reserved
As shown in the table, each microinstruction can have only three micro-operations, one from each field. If it uses
less than three, it will result in more than one operation using the no operation binary code.
Condition Field
A condition field includes 2 bits. They are encoded to define four status bit conditions. As stated in the table, the first
condition is always a 1, with CD = 0. The symbol that can indicate this condition is ‘U’. The table displays the
multiple condition fields and their summary in an easy manner.
Condition Field Symbols and Descriptions
Condition Symbol Comments
00 Always = 1 U Unconditional Branch
01 DR (15) I Indirect address bit
10 AC (15) S Sign bit of AC
11 AC = 0 Z Zero value in AC
As shown in the table, when condition 00 is connected with BR (branch) field, it results in an unconditional branch
operation. Then the execution is read from memory the indirect bit I is accessible from bit 15 of DR. The status of the
next bit is supported by the AC sign bit. If all the bits in AC are 1, then it is indicated as Z (its binary variable whose
value is 1). The symbols U, I, S, and Z can indicate status bits while writing microprograms.
Branch Field
The BR (branch) field includes 2 bits. It can be used by connecting with the AD (address) field. The reason for
connecting with the AD field is to select the address for the next microinstruction. The table illustrates the various
branch fields and their functions.
Branch Field Symbols and Descriptions
BR Symbol Comments
00 JMP CAR ←AD if condition = 1
CAR←CAR + 1 if condition = 0
01 CALL CAR ←AD , SBR ← CAR +1, if condition = 1
CAR←CAR + 1 if condition = 0
10 RET CAR ←SBR (Return from subroutine)
11 MAP CAR(2-5) ←DR(11-14), CAR(0,1,6) ←0
As shown in the table, when BR = 00, a JMP operation is implemented and when BR = 01, a subroutine is called.
The only difference between the two instructions is that when the microinstruction is saved, the return address is saved
in the Subroutine Register (SBR).
These two operations are dependent on the CD field values. When the status bit condition of the CD field is defined
as 1, the address that is next in order is transferred to CAR. Else, it gets incremented. If the instruction needs to return
from the subroutine, its BR field is determined as 10.
This results in the transfer of the return address from SBR to CAR. The opcode bits of instruction can be mapped
with an address for CAR if the BR field is 11. They are present in DR (11 - 14) after instruction is read from
memory. The last two conditions in the BR fields are not dependent on the CD and AD field values.
