Microprogram and Symbolic Microprogram for FETCH Operation

What is a Microprogram?

A microprogram is a sequence of microinstructions stored in a special memory called the Control Memory (CM) or Control Store, which is used to implement the control unit of a computer.

• Each microinstruction specifies one or more microoperations to be performed in a single clock cycle.
• A microprogram corresponds to a machine language instruction (like ADD, LOAD, etc.) and defines how that instruction is executed at the register-transfer level.
• The technique of using microprograms to implement the control unit is called microprogramming, introduced by Maurice Wilkes.

Key Points:

FETCH Operation (Instruction Fetch Cycle)

The FETCH cycle retrieves an instruction from memory and prepares the CPU to execute it. The steps involved are:

1. Transfer the content of PC to MAR (Memory Address Register)
2. Initiate a memory read; increment PC
3. Transfer the fetched instruction from MDR to IR (Instruction Register)
4. Decode the instruction and determine the next microprogram address

Symbolic Microprogram for FETCH

Label       Microoperations                     Comments
-------     ----------------------------        -------------------------
FETCH:      MAR ← PC                            Transfer PC to MAR
            MDR ← M[MAR], PC ← PC + 1          Read memory; increment PC
            IR ← MDR                            Load instruction into IR
            DECODE IR; Go to next routine       Decode opcode; branch

Step-by-step Explanation:

Summary

A microprogram is a set of microinstructions stored in control memory that implements a machine instruction by specifying register-level operations step by step. The FETCH microprogram reads the instruction from memory using PC → MAR → MDR → IR, increments PC, and decodes the opcode to branch to the appropriate execution routine.

Flags in 8085 Microprocessor

What is a Flag?

A flag is a single-bit flip-flop (register) that reflects the status (condition) of the result after the execution of an arithmetic or logical operation. Flags are part of the Program Status Word (PSW) or Flag Register in the 8085 microprocessor.

The 8085 has an 8-bit flag register, but only 5 bits are used as flags.

The 5 Flags of 8085 Microprocessor

1. Sign Flag (S) -- Bit 7

• Set to 1 if the result of an operation is negative (i.e., the Most Significant Bit of the result is 1).
• Set to 0 if the result is positive.
• Example: If result = 10000001 (negative in 2's complement), S = 1.

2. Zero Flag (Z) -- Bit 6

• Set to 1 if the result of an operation is zero.
• Set to 0 if the result is non-zero.
• Example: If A = 05H and we subtract 05H, result = 00H, so Z = 1.

3. Auxiliary Carry Flag (AC) -- Bit 4

• Set to 1 if there is a carry out from bit 3 to bit 4 (carry from lower nibble to upper nibble) during an arithmetic operation.
• Primarily used for BCD (Binary Coded Decimal) arithmetic operations.
• Example: 0FH + 01H = 10H, carry from bit 3 to bit 4, so AC = 1.

4. Parity Flag (P) -- Bit 2

• Set to 1 if the result contains an even number of 1s (even parity).
• Set to 0 if the result contains an odd number of 1s (odd parity).
• Example: Result = 00000011 (two 1s, even), so P = 1.

5. Carry Flag (CY) -- Bit 0

• Set to 1 if there is a carry out from the MSB (bit 7) during addition, or a borrow during subtraction.
• Set to 0 if there is no carry/borrow.
• Example: FFH + 01H = 100H, carry generated, so CY = 1.

Flag Register Layout (8-bit)

X = Not used (undefined bits)

Summary Table