Fig: Functional Block Diagram of 8085 Microprocessor
1. ALU
- The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’, ‘AND’, ‘OR’ etc.
- Uses data from memory and from Accumulator to perform arithmetic operation and always stores result of operation in Accumulator.
- The ALU consists of accumulator, flag register and temporary register.
a. Accumulator
- The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator.
- The accumulator is also identified as register A.
b. Flag register
- 8085 has 8-bit flag register. There are only 5 active flags.
- Flags are flip-flops which are used to indicate the status of the accumulator and other register after the completion of operation.
- These flip-flops are set or reset according to the data condition of the result in the accumulator and other registers.
i. Sign flag(S):
- Sign flag indicates whether the result of a mathematical or logical operation is negative or positive.
- If the result is negative, this flag will be set (i.e. S=1) and if the result is positive, the flag will be reset (i.e. S=0).
ii. Zero flag (Z):
- Zero flag indicates whether the result of a mathematical or logical operation is zero or not.
- If the result of current operation is zero, the flag will be set (i.e. Z=1) otherwise the flag will be reset (Z=0).
- This flag will be modified by the result in the accumulator as well as in the other register.
iii. Auxiliary carry flag (AC):
- In operation when a carry is generated by bit D3 and passes on to bit D4, the AC flag will be set otherwise AC flag will be reset.
- This flag is used only internally for BCD operation and is not available for the programmer to change the sequence of program with the jump instruction.
iv. Parity flag (P):
- This flag indicates whether the current result is of even parity (no. of 1’s is even) or odd parity (no. of 1’s is odd).
- If even parity, P flag will be set otherwise reset.
v. Carry flag (CY):
- This flag indicates whether during an addition or subtraction operation carry or borrow is generated or not.
- If carry or borrow is generated, the flag will be set otherwise reset.
2. Timing and control unit
- This unit produces all the timing and control signal for all the operation.
- This unit synchronizes all the MP operations with the clock and generates the control signals necessary for communication between the MP and peripherals.
3. Instruction register and decoder
- The instruction register and decoder are part of ALU. When an instruction is fetched from memory, it is loaded in the instruction register.
- The decoder decodes the instruction and establishes the sequence of events to follow.
- The IR is not programmable and cannot be accessed through any instruction.
4. Register array
- The register unit of 8085 consists of
-Two internal registers W and Z
-Two 16-bit address registers PC (program counter) and SP (stack pointer)
-One increment/decrement counter register
-And, one multiplexer (MUX)
- The six general-purpose registers are used to store 8-bit data. They can be combined as register pairs BC, DE, and HL to perform some 16-bit operations.
- The two internal registers W and Z are used to hold 8-bit data during the execution of some instructions, CALL and XCHG instructions.
- SP is 16-bit registers used to point the address of data stored in the stack memory. It always indicates the top of the stack.
- PC is 16-bit register used to point the address of the next instruction to be fetched and executed stored in the memory.
5. System bus
a. Data bus
- It carries ‘data’, in binary form, between MP and other external units, such as memory.
- Typical size is 8 or 16 bits.
b. Address bus
- It carries ‘address’ of operand in binary form.
- Typical size is 16-bit.
c. Control Bus
- Control Bus are various lines which have specific functions for coordinating and controlling MP operations.
- E.g.: Read/Write control line.
6. Interrupt Control
- Interrupt is a signal, which suspends the routine what the MP is doing, brings the control to perform the subroutine, completes it and returns to main routine.
- May be hardware or software interrupts. Some interrupts may be ignored (maskable), some cannot (non-maskable).
- E.g. INTR, TRAP, RST 7.5, RST 6.5, RST 5.5
7. Serial I/O Control
- The MP performs serial data input or output (one bit at a time). In serial transmission, data bits are sent over a single line, one bit at a time.
- The 8085 has two signals to implement the serial transmission: SID (serial input data) and SOD (serial output data).
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