Architecture of 8085 microprcessor, Slides of Microprocessors

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• Architecture of
• Block Diagram of

Units of 8085

1. Processing Unit
2. Storage and Interface Unit
3. Instruction Unit
4. Interrupt and Serial input/output unit

1. Processing Unit

 ALU

 Accumulator

 Temporary Register

 Flags

Accumulator

 (^) The accumulator is an 8-bit register that is a part of processing unit.  (^) 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.

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.

General Purpose Registers

 (^) There are six 8-bit general purpose registers B, C, D, E, H & L.  (^) General purpose registers are used for temporary storage of data and intermediate results while the processor is executing the program.  (^) Two eight bit registers can be combined for handling 16-bit data and Combination of two 8-bit registers is known as pair.  (^) Valid register pairs are B-C, D-E, H-L.  (^) The H-L pair is used to address memories.

Stack pointer

 (^) The stack pointer is a 16-bit register which basically serves two purposes. a) Points towards the stack memory. Initially it indicates the beginning of the stack memory. Whenever something is added to the stack, the stack pointer is decremented and whenever something is removed from the stack the stack pointer is incremented. Hence the stack pointer always points to the top of the stack. b) Stack pointer also points towards the memory location where the μP has to go after attending an interrupt or a subroutine; therefore it acts as a bookmark.

Increment/Decrement address

latch

 (^) It is another 16-bit internal register latch available in the register section for internal operations and is not accessible to the user.  (^) It selects an address to be sent out from the program counter, from the stack pointer, or from one of the 16-bit register pairs.  (^) It latches this address onto the address lines for the required time.  (^) This 16-bit circuit is used to increment or decrement the contents of program counter or stack pointer as a part of execution of instructions related to them

Address /Data buffer and Address buffer

 (^) Address/Data Bus Buffer (AD0 to AD7) is a 8-bit bidirectional buffer.  (^) Address Bus buffer( A8 to A15) is a 8-bit unidirectional buffer.  (^) The contents of the stack pointer and program counter are loaded into the address buffer and address-data buffer.  (^) These buffers are then used to drive the external address bus and address-data bus.  (^) As the memory and I/O chips are connected to these buses, the CPU can exchange desired data to the memory and I/O chips  (^) The address-data buffer is not only connected to the external data bus but also to the internal data bus which consists of 8-bits.  (^) The address data buffer can both send and receive data from internal data bus

Instruction Register and Decoder

 (^) The first word of an instruction is the operation code, i.e., binary code for that instruction.  (^) In the first machine cycle of any instruction μ𝑝 fetches the instruction from the fetches the instruction from the memory.  (^) The op-code representing the instruction to be executed is fetched from the (program) memory location pointed to by (PC) and loaded into the instruction register (IR).  (^) The IR passes this op-code to the instruction decoder which interprets this op- code appropriately in order to decide what operation needs to be done for executing this instruction.  (^) The instruction decoder tells the control unit the type of instruction to be executed; the number of machine cycles necessary to execute the instruction etc.

Contd.

 (^) E.g. if the instruction is ADI 04H, then the first binary code read by the μ𝑝 fetches the instruction from the is C6H into the (IR).  (^) After decoding this, the decoder will recognize that another memory read cycle is required to read 04H to be added to the number in the accumulator.  (^) The decoder will direct the control circuit to send out another memory read pulse and transfers the data coming on the data bus into the temporary register (Temp), so that it can be added to the accumulator.  (^) When the addition is completed the control circuit directs the result back to the accumulator.  (^) The program counter is then incremented to point the next memory address and send out another memory read pulse to read the μ𝑝 fetches the instruction from the code of next instruction from memory.

Contd.

 (^) Timing and control unit is a very important unit as it synchronizes the registers and flow of data through various registers and other units.  (^) It provides timing & control signals necessary to all the operations in the microcomputer.  (^) This unit consists of an oscillator and controller sequencer which sends control signals needed for controls the flow of data between the microprocessor and peripherals (input, output & memory).  (^) The oscillator generates two-phase clock signals which aids in synchronizing all the registers of 8085 microprocessor.

Signals that are associated with

Timing and control unit are:

1. Control Signals: READY, RD(active low), WR (active low), ALE 2. Status Signals: S0, S1, IO/M(active low) 3. DMA Signals: HOLD, HLDA 4. RESET Signals: RESET IN(active low), RESET OUT