1. MICROPROGRAM CONTROL

2. A control word (CW) is a word whose individual bits represent various control signals
Sequence of CWs correspond to the control sequence of machine instruction constitute microroutine
Individual CW in the microroutine are referred as microinstruction

3. The microroutines for all instrution in the instruction in the instruction set of computer are stored in special memory called control store
Control unit generate control signals for any instruction by sequentially reading CW of microroutine from control store

4. μPC – is used to read CW sequentially from control store

7. μPC is incremented in following situation
New instruction-IR, μPC will be loaded with starting address of microroutine
Branch instruction is encountered& branch condition is satisfied, μPC is loaded with branch address
End microinstruction is encountered , μPC is loaded with address of first CW in microroutine

9. Sequence of control signals to be generated by the controller can be stored in special ROM also called control memory(CM)
Output of CMDR provides desired control signals
Control ROM is sequentially accessed by incrementing CMAR

10. MICROINSTRUCTION
Assigning 1 bit to each control signal results in long microinstruction bcoz number of required signal is usually large
Usually only few bits are set to 1 in any microinstrution
Space is poorly used
To avoid this,signals can be grouped such that mutually exclusive signals are placed in same group

11. MICROPROGRAM SEQUENCING
microprogram is is easy to write bcoz software techniques is used
But 2 disadvantages are encountered
Separate microroutine for each instruction lead to large control store
Execution time is longer bcoz it takes more time to carry out branch instruction

12. ENCODING OF CONTROL SIGNALS

13. Encoding of control signals
Different formats of microinstruction depending on the encoding of control signals.
Horizontal/vertical formats
Functional encoding
Resource encoding
Direct versus indirect encoding

14. Horizontal/Vertical Formats

16. Vertical organization:
Horizontal Format:
Minimal encode scheme,in which many resource can be controlled with single microinstruction
Large number of control fields
Characterized by limited encoding and ability to express high degree of parallelism
Vertical organization:
Highly encode scheme ,specify small number of control function

17. limited number of control fields
Characterized by high degree of encoding and limited ability to express parallelism

18. VERTICAL MICROINSTRUCTION FORMAT-CONTROL FIELDS

19. FUNCTIONAL ENCODING

23. Functional Encoding
The micro operations executed in a data path of a CPU may be categorized under different function names such as Shift function, Add function, Logical functions, Input-Output, etc
Multiple control bits associated to control a specified function

24. RESOURCE ENCODING

25. Resource encoding
The CPU data path consists of a set of interconnected RTL components.
Each of these components or a subset of such components viewed as a resource.
If control signals associated with such a resource are mutually exclusive, then they can be encoded in a single control field.

26. Pg-352 2nd para pal chaudhuri book

28. DIRECT Vs INDIRECT ENCODING

30. Direct->control field directly generates control signals via decoder
Indirect->control field A serves as a field decode logic which further steers.decode first level decode logic of another field(B)
Indirect,which have different types of control information from same control field

31. MICROINSTRUCTION WITH NEXT ADDRESS FIELD

33. μAR-Microinstruction Address Register
Μar which is loaded from next address field
Next address bits are via OR gates to μAR,so that address can be modified on basis of the data in IR,external i/p,condition codes
Decoding circuit generates starting address of given microroutine in basis if OP code in IR

34. PREFETCHING MICROINSTRUCTION
Drawback of microprogram control leads to slower operating speed bcoz of the time take to fetch microinstruction
Faster operation is achieved if nect microinstruction is prefetched while the current one is being executed

35. EMULATION
REFER hamacher book

36. Micro instruction execution
Two basic actions
Generating next microinstruction address and fetching micro program instruction from control memory by this address
Executing the micro operations controlled by different signals encoded in various control fields of the microinstruction
Decoding and applying control signals on the CPU data path
Executing the intended micro operations controlled by the signals
Storing the output in the destination register specified in the micro operation on the CPU data path

37. Mono phase and Poly phase operation
In a micro operation cycle, a micro program control unit implements the sequence of actions under 4 different phases.
If the micro instruction cycle is controlled by a single clock pulse which synchronizes all the control signals ,then the mode is termed as mono phase operation
If each of the phases is controlled by a different phase of a clock ,the mode is referred as poly phase operation

38. Micro-programmed Control Unit -Advantages
1. A micro-programmed control unit is flexible and allows designers to incorporate new and more powerful instructions as VLSI technology increases the available chip area for the CPU
2. allows any design errors discovered during the prototyping stage to be removed

39. Microprogrammed Control Unit - Disadvantages
1. requires several clock cycles to execute each instruction, due to the access time of the microprogram memory
2. Occupies a large portion (typically 55%) of the CPU chip area