Microprogram and Symbolic Microprogram for FETCH Operation

What is a Microprogram?

A microprogram is a sequence of microinstructions stored in a special memory called control memory (or control store). Each microinstruction specifies one or more micro-operations (elementary hardware operations) to be performed in a single clock cycle.

Key points:

• Microprogramming is a technique used to implement the control unit of a CPU.
• The concept was introduced by Maurice Wilkes.
• Each machine instruction is interpreted by executing a corresponding sequence of microinstructions.
• Microinstructions are stored in ROM-based control memory.
• A microprogram is analogous to a program, but operates at the register-transfer (hardware) level.

Microprogrammed Control Unit uses control memory to generate control signals, rather than hardwired logic.

Symbolic Microprogram for FETCH Operation

The FETCH cycle retrieves an instruction from memory and places it in the Instruction Register (IR), then increments the Program Counter (PC).

Registers Used:

Symbolic Microprogram (FETCH Cycle):

Step    Micro-operation                     Comment
------  ----------------------------------  ---------------------------
T0:     MAR ← PC                           Transfer PC content to MAR
T1:     MBR ← M[MAR], PC ← PC + 1         Read memory into MBR;
                                            Increment PC simultaneously
T2:     IR ← MBR                           Load instruction into IR

Explanation of Each Step:

1. T0: MAR ← PC

   • The content of the Program Counter is transferred to the Memory Address Register.
   • This tells the memory which address to read from.

2. T1: MBR ← M[MAR], PC ← PC + 1

   • The instruction stored at the address in MAR is fetched from memory into MBR.
   • Simultaneously, PC is incremented to point to the next instruction.
   • These two micro-operations can occur in parallel (same clock cycle).

3. T2: IR ← MBR

   • The fetched instruction is transferred from MBR to the Instruction Register (IR).
   • After this, the DECODE phase begins.

Summary Flow:

PC --> MAR --> Memory Read --> MBR --> IR
                                PC + 1 --> PC

The FETCH microprogram typically requires 3 micro-steps (T0, T1, T2) and forms the first part of the instruction cycle in any CPU.

Short Notes (Any Two): 5 Marks

a) DMA (Direct Memory Access)

DMA is a technique that allows peripheral devices to transfer data directly to/from memory without involving the CPU for each byte of transfer.

Key Points:

• Normally, the CPU handles all data transfers between I/O devices and memory, which wastes CPU time.
• In DMA, a special hardware unit called the DMA Controller (DMAC) takes over the system bus temporarily.
• The CPU sends the starting address, block size, and direction (read/write) to the DMAC, then releases the bus.
• The DMAC performs the transfer and sends an interrupt to the CPU when done.

Steps in DMA Transfer:

1. CPU initializes the DMAC (address, count, mode).
2. DMAC sends a Bus Request (BR) to the CPU.
3. CPU grants the bus via Bus Grant (BG) and suspends its activity.
4. DMAC transfers data directly between memory and I/O device.
5. After transfer, DMAC releases the bus and interrupts the CPU.

Advantages:

• Very fast data transfer.
• CPU is free to do other tasks.
• Suitable for high-speed devices like disk drives, video cards.

b) Flags in 8085 Microprocessor

The 8085 microprocessor has a special 8-bit Flag Register (also called the Program Status Word) that reflects the result of arithmetic and logical operations.

The 8085 has 5 flags:

Flag Register Bit Layout:

D7   D6   D5   D4   D3   D2   D1   D0
 S    Z    0   AC    0    P    1   CY

Importance:

• Flags are used by conditional jump instructions (e.g., JZ, JNZ, JC, JNC) to control program flow.
• They are automatically set or reset after arithmetic/logical operations.

c) Instruction Set

An instruction set (also called Instruction Set Architecture, ISA) is the complete collection of instructions that a processor can understand and execute.

Key Points:

• It defines the interface between hardware and software.
• Each instruction tells the CPU what operation to perform, on what data, and where to store the result.

Types of Instructions:

Instruction Format:

An instruction typically has:

• Opcode: specifies the operation
• Operand(s): specifies the data or address

Types of Instruction Sets:

• RISC (Reduced Instruction Set Computer): fewer, simpler instructions (e.g., ARM)
• CISC (Complex Instruction Set Computer): many complex instructions (e.g., x86, 8085)

Importance:

• Determines the capability and performance of a processor.
• Programmers and compilers use the instruction set to write programs.