Table of Contents

Microprogram Control Unit

As discussed in Control Unit topic, a control unit generates a binary control variable for executing a particular microoperations. The control unit can be a hardwired or microprogrammed control unit.

Figure-1 : Interaction between CU and CPU

A control word can be programmed to perform various operations on components of the system. A control unit whose control word is stored in the control memory which is usually a ROM is called as microprogram control unit. Each control word in the control memory contains within it a microinstruction. Every microinstruction specifies one or more micro-operations.

A microprogram is written for every instruction supported by the CPU. Each instruction of a program causes the corresponding micro-program to be fetched and its control information extracted in a manner that resembles the fetching and execution of a program from the main memory. Since the control signals are embedded into a kind of low-level software, this is also referred as firmware.

General features of MCU:

One time Programmable, programmed during manufacture
Flexible to accommodate new system specification
We can change its functionality, but generally not required once designed
Reusability-code
easier decoding and sequencing logic
Easier to design
cheaper
Slow Speed

Microprogrammed control unit is implemented by writing microprograms in the control memory. These microprograms are a set of microinstructions required for executing the machine instruction. Each microinstruction somehow(implicitly or explicitly ) also specify the address of next microinstruction to be fetched, thus sequencing the microoperations.

Important terms associated with microprogrammed control unit.

Microprogram: The set of related microinstructions forms a microprogram. The assembly instructions are implemented by writing a microprogram. These microprograms can be changed easily, hence microprogramming yields control units that are more flexible than hardwired control unit.
Microinstructions: the control signals to be activated at any time for executing a microoperation is specified by a microinstruction. It specifies one or more microoperations.
Microoperations: These are the operations performed on the contents of the registers. Each microoperation is associated with a group of control lines that must be activated in a prescribed sequence to trigger the microoperations.

Microinstruction Format:

Each control word in the control memory contains within it a microinstruction. Each computer instruction must be microprogrammed. the general format of the 20-bit microinstruction is given below:

The microinstruction is divide in to four fields, these are

Three fields F1, f2, F3 for the microoperations
CD field to select status bit condition
BR field specifies the type or the branch to be used
Adr field is the address field for the microinstructions

Symbol and binary code for Microinstruction fields

F1			F2		F3
000	None	NOP	000	NOP	000	NOP
001	AC <–AC+DR	ADD	001	SUB	001	XOR
010	AC<–0	CLRAC	010	OR	010	COM
011	AC<–AC+1	INCAC	011	AND	011	SHL
100		DRTAC	100	READ	100	SHR
101		DRTAR	101	ACTDR	101	INCPC
110		PCTAR	110	INCDR	110	ARTPC
111		WRITE	111	PCTDR	111	Reserved

Conditional and Branch Microinstructions

CD	Condition	Symbol	Comment	BR	Symbol	Function
00	Always=1	U	Unconditional Bransch	00	JMP	CAR ß AD if condition=1 CAR ßCAR+1 if condition=1
01	DR(15)	I	Indirect address bit	01	CALL	CAR ßAD, SBRßCAR+1 if condition=1D, SBRßCAR+1 if CARßCAR+1 if condition=0
10	AC(15)	S	Sign bit of AC	10	RET	CAR ßSBR(Return from subroutine)
11	AC=0	Z	Zero value in AC	11	MAP	CAR(2-5)ßDR(11-14), CR(0,1,6)ß0

Fetch Routine

Microinstructions for Fetch routine

AR <– PC

DR <– M[AR], PC <PC+1

AR <–DR(0-10), CAR(2-5)<–DR(11-14), CAR(0,1,6)<–0

These instructions are read from the memory into the DR……, the address part is transferred to AR and then the control is transferred to one of the 16 routines by mapping the operation code part of the instruction from DR into CAR.

The fetch routine need theree microinstructions which is written as:

ORG 64

fetch: PCTAR U JMP NEXT

READ, INCPC U JMP NEXT

DRTAR U MAP

The translation of symbolic microprogram to binary produce the following binary microprogram

Binary Address	F1	F2	F3	CD	BR	Addr
1000000	110	000	000	00	00	1000001
1000001	000	100	101	00	00	1000010
1000010	101	000	000	00	11	0000000

Block diagram of MCU

Figure-2 shows various components of a microprogrammed control unit. These are:

Control memory
Address Sequencer
Micro-Instruction Register
Micro-Instruction Decoder

Control Memory

Control memory is in the form of a ROM. It stores microprograms which consist of microinstructions that specifies various internal control signals for executing the microoperations on the contents of the registers.

Mapping of Instructions

Figure – : Mapping of instruction code to microinstruction address

Format of the microinstruction

:The control field part specify the control signals to be activated in response to the assembly instruction and the address field contains the reference to address of the next microinstruction in control memory to be executed. The control memory unit also has a current and the next memory address.

Address Sequencer:

This unit is used to determine the address of the next microinstruction in the control memory.

Various functions of this unit is:

incrementing the control address by ‘1’.
Loading an address into the control address register an address from control memory.
transferring an external address
Loading an initial address to start the control operation

As indicated in the figure the control memory address register (CAR) is connected to the MUX which gets the input from the 2^nd field of the microinstruction (address part) or from the external source of address. The instructions opcode after suitable encoding, provides the starting address of its microprogram.

The external source of address usually denote the starting address of the microprogram for a particular instruction. The control unit also respond to external signal or external conditions which allow the current microinstruction to choose or select one of the two possible address fields

Microprogrammed Control Unit