Computer Architecture I
Unit 4
Microprogrammed Control
Third Stage
Asst.Lec Hayder Ali Hussein
Computer Engineering Department/ AL-Farabi University College
Outline
Looping
• Control Memory
• Address Sequencing
• Microinstruction Code Format
Control Memory
Section - 1
Control Memory
A computer that employs a microprogrammed control unit will have two separate memories: a
main memory and a control memory.
The control memory holds a fixed microprogram that can not be altered by the occasional user.
The microprogram consists of microinstructions that specify various internal control signals
for execution of register microoperation.
Microinstructions generates the microoperations to fetch instruction from main memory; to
evaluate the effective address, to execute the operation specified by the instruction, and to
return control to the fetch of next instruction.
Microprogrammed Control Organization
External Control
input word
Next- Control
Control Control
address data
address memory
generator register
register (ROM)
(Sequencer)
Next-address information
Address Sequencing
Section - 2
Address Sequencing
Microinstructions are stored in control memory in groups, with each group specifying a routine.
The transformation from the instruction code bits to an address in control memory where the
routine is located is referred to as a mapping process.
The address sequencing capabilities required in a control memory are:
1. Incrementing of the control address register.
2. Unconditional branch or conditional branch, depending on status bit conditions.
3. A mapping process from the bits of the instruction to an address for control memory.
4. A facility for subroutine call and return.
Address Sequencing
Instruction code
Mapping
logic
Status Branch Mux
bits Multiplexer
logic select
Subroutine
register
(SBR)
Clock
Control address register
(CAR)
Incrementer
Control memory
Select a status bit
Branch address
Microoperations
Microinstruction Code Format
Section - 3
Microinstruction Code Format
3 3 3 2 2 7
F1 F2 F3 CD BR AD
F1, F2, F3: Microoperation fields
CD: Condition for branching
BR: Branch field
AD: Address field
Symbols & Binary Code for Microinstruction Fields
F1 Microoperation Symbol F2 Microoperation Symbol
000 None NOP 000 None NOP
001 AC AC + DR ADD 001 AC AC - DR SUB
010 AC 0 CLRAC 010 AC AC DR OR
011 AC AC + 1 INCAC 011 AC AC DR AND
100 AC DR DRTAC 100 DR M[AR] READ
101 AR DR(0-10) DRTAR 101 DR AC ACTDR
110 AR PC PCTAR 110 DR DR + 1 INCDR
111 M[AR] DR WRITE 111 DR(0-10) PC PCTDR
Symbols & Binary Code for Microinstruction Fields
F3 Microoperation Symbol
000 None NOP
001 AC AC + DR XOR CD Symbol Comments
010 AC AC’ COM 00 U-1 Unconditional branch
011 AC shl AC SHL 01 I - DR(15) Indirect address bit
100 AC shr AC SHR 10 S - AC(15) Sign bit of AC
101 PC PC + 1 INCPC 11 Z - AC = 0 Zero value in AC
110 PC AR ARTPC
111 Reserved
BR Symbol Function
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
Thank you
