Write short notes on following: i) Semeconductor memories ii) Hardwired control unit iii) RISC and CISC iv) Micro instruction formats

i) Semiconductor Memories

Semiconductor memories are storage devices made from semiconductor-based integrated circuits. These devices store data in digital electronic devices such as computers, smartphones, and many other types of electronic systems. There are two main types of semiconductor memories: volatile and non-volatile.

Volatile Memory:

• Data is lost when power is turned off.
• Examples include RAM (Random Access Memory), DRAM (Dynamic RAM), and SRAM (Static RAM).

Non-Volatile Memory:

• Data is retained even when power is turned off.
• Examples include ROM (Read-Only Memory), PROM (Programmable ROM), EEPROM (Electrically Erasable PROM), and Flash memory.

Characteristics of Semiconductor Memories:

Feature	Description
Access Time	The time interval between the read/write request and the availability of the data.
Density	The amount of data that can be stored in a given physical area of the memory chip.
Volatility	Whether or not the memory retains data without power.
Durability	The number of read/write cycles the memory can endure before failure.
Cost	The expense associated with manufacturing or purchasing the memory.

Example: A DDR4 SDRAM is a type of volatile semiconductor memory used in modern computers for temporary data storage during operation.

ii) Hardwired Control Unit

A hardwired control unit is a type of control unit in a computer's central processing unit (CPU) that uses fixed hardware circuits to control the sequence of operations in the processor. It is contrasted with a microprogrammed control unit, which uses software (microcode) to generate control signals.

Characteristics of Hardwired Control Unit:

Feature	Description
Speed	Generally faster than microprogrammed control units because there is no interpretation of microinstructions.
Complexity	More complex to design and modify, as changes require altering the hardware.
Flexibility	Less flexible compared to microprogrammed control units.
Cost	Potentially lower cost for mass production but higher initial design cost.

Example: The control unit within an early microprocessor like the Intel 8080 is a hardwired control unit.

iii) RISC and CISC

RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) are two types of processor architectures.

Differences between RISC and CISC:

Feature	RISC	CISC
Instruction Set Size	Small and simple	Large and complex
Instruction Execution	Single-cycle execution	Multi-cycle execution
Instruction Format	Fixed-size	Variable-size
Focus	Optimizes software	Optimizes hardware
Registers	More general-purpose registers	Fewer registers with special purposes
Memory Access	Load/store architecture	Many instructions can access memory directly

Example:

• RISC: ARM processors used in many smartphones are based on RISC architecture.
• CISC: Intel's x86 processors used in most personal computers are based on CISC architecture.

iv) Microinstruction Formats

Microinstructions are the detailed instructions that a microprogrammed control unit uses to control the processor's data path. The format of a microinstruction defines how the control signals are encoded.

Types of Microinstruction Formats:

1. Horizontal Microinstruction Format:

   • Each bit in the microinstruction directly controls one part of the CPU.
   • Offers high flexibility and speed.
   • Can be very wide, leading to large microinstruction words.

2. Vertical Microinstruction Format:

   • Uses a more compact encoding scheme.
   • Each field in the microinstruction represents a specific control signal or set of signals.
   • Less flexible but more space-efficient than horizontal formats.

Example of a Microinstruction Format:

Field	Purpose
Opcode	Specifies the operation to be performed.
Source Register	Identifies the source register for the operation.
Destination Register	Identifies the destination register for the operation.
ALU Operation	Specifies the operation to be performed by the ALU.
Next Address	Indicates the address of the next microinstruction.

Example: In a hypothetical CPU, a microinstruction might be formatted as follows:

+--------+---------+---------+---------+---------+---------+
| Opcode | Src Reg | Dest Reg| ALU Op  | Next Addr| Control |
+--------+---------+---------+---------+---------+---------+
|  0001  |   0010  |  0011   |  0100   |  010101 |  111000 |
+--------+---------+---------+---------+---------+---------+

This microinstruction could represent an ALU operation where the result of the operation between the contents of source register 2 and destination register 3 is stored back in register 3, and the next microinstruction to execute is at address 010101. The control bits at the end might enable or disable specific parts of the CPU, such as the memory write signal or the instruction register.