8085 Microprocessor Architecture

1. Internal Architecture of 8085 Microprocessor

8085

8085 Microprocessor Block Diagram

The 8085 is a 8 bit microprocessor. It requires a single +5 volt supply. Its basic clock speed is 3 MHz thus improving on the present 8080’s performance with higher system speed. Also it is designed to fit into a minimum system of three IC’s: The CPU, a RAM/ IO, and a ROM or PROM/IO chip.

The 8085 uses a multiplexed Data Bus. The address is split between the higher 8bit Address Bus and the lower 8bit Address/Data Bus. During the first cycle the address is sent out. The lower 8bits are latched into the peripherals by the Address Latch Enable (ALE). During the rest of the machine cycle the Data Bus is used for memory or l/O data.

The 8085A provides RD’, WR’, and lO/M’  signals for bus control. An Interrupt Acknowledge signal (INTA’) is also provided. Hold, Ready, and all Interrupts are synchronized. The 8085A also provides serial input data (SID) and serial output data (SOD) lines for simple serial interface.

In addition to these features, the 8085A has three maskable, restart interrupts and one non-maskable trap interrupt.

Control Unit

Generates signals within uP to carry out the instruction, which has been decoded. In reality causes certain connections between blocks of the uP to be opened or closed, so that data goes where it is required, and so that ALU operations occur.

Arithmetic Logic Unit

The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’, ‘AND’, ‘OR’, etc. It uses data from memory, Registers and from Accumulator to perform arithmetic and logic operations. Always stores result of operation in Accumulator.

General Purpose Registers

The 8085 Microprocessor has six general-purpose registers to store 8-bit data; these are identified as B, C, D, E, H, and L as shown in the figure. They can be combined as register pairs – BC, DE, and HL – to perform some 16-bit operations. The programmer can use these registers to store or copy data into the registers by using data copy instructions. In addition, it has two 16-bit registers: the stack pointer and the program counter.

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.

Flags

The ALU includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero (Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; they are listed in the Table and their bit positions in the flag register are shown in the Figure below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data conditions.

These flags have critical importance in the decision-making process of the micro- processor. The conditions (set or reset) of the flags are tested through the software instructions. For example, the instruction JC (Jump on Carry) is implemented to change the sequence of a program when CY flag is set. The thorough understanding of flag is essential in writing assembly language programs.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a 16-bit register. The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location.

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a memory location in R/W memory, called the stack. The beginning of the stack is defined by loading 16-bit address in the stack pointer.

Instruction Register/Decoder

It is temporary store for the current instruction of a program. Latest instruction sent here from memory prior to execution. Decoder then takes instruction and ‘decodes’ or interprets the instruction. Decoded instruction then passed to next stage.

Memory Address Register

Holds address, received from PC, of next program instruction. It feeds the address bus with addresses of location of the program under execution.

Control Generator

It Generates signals within microprocessor to carry out the instruction which has been decoded. In reality causes certain connections between blocks of the uP to be opened or closed, so that data goes where it is required, and so that ALU operations occur.

2. 8085 System Bus

Typical system uses a number of buses, collection of wires, which transmit binary numbers, one bit per wire. A typical microprocessor communicates with memory and other devices (input and output) using three buses: Address Bus, Data Bus and Control Bus.

Address Bus

One wire for each bit, therefore 16 bits = 16 wires. Binary number carried alerts memory to ‘open’ the designated box. Data (binary) can then be put in or taken out. The Address Bus consists of 16 wires, therefore 16 bits. Its “width” is 16 bits. Address bus is unidirectional, i.e. numbers only sent from microprocessor to memory, not other way.

Data Bus

Data Bus: carries ‘data’, in binary form, between mP and other external units, such as memory. Typical size is 8 or 16 bits. Size determined by size of boxes in memory and mP size helps determine performance of mP. The Data Bus typically consists of 8 wires. Therefore it can represent 28 combinations of binary digits. Data bus used to transmit “data”, ie information, results of arithmetic, etc, between memory and the microprocessor. Bus is bi-directional. Size of the data bus determines what arithmetic operations can be done.

Control Bus

Control Bus are various lines which have specific functions for coordinating and controlling microprocessor operations. Eg: RD’, WR’ lines. Control whether memory is being ‘written to’ (data stored in memory) or ‘read from’ (data taken out of memory) . May also include clock line(s) for timing/synchronizing, ‘interrupts’, ‘reset’ etc. Typically Microprocessor has 10 control lines. Microprocessor Cannot function correctly without these vital control signals.

3. 8085 Pin Diagram

pin diagram

8085 Microprocessor Pin Diagram

Pin Description

The following describes the function of each pin:

A15 – A8

Address Bus; The most significant 8 bits of the memory address or the 8 bits of I/0 addresses pass through pins.

AD7 – AD0

Multiplexed Address/Data Bus; Lower 8 bits of the memory address (or I/0 address) appear on the bus during the first clock cycle of a machine state. It then becomes the data bus during the second and third clock cycles. 3 stated during Hold and Halt modes.

ALE (Output)

Address Latch Enable: It occurs during the first clock cycle of a machine state and enables the address to get latched into the on chip latch of peripherals. The falling edge of ALE is set to guarantee setup and hold times for the address information. ALE can also be used to strobe the status information.

SO, S1 (Output)

These are status signal. Together with IO/M’, these signals indicate the type of operation going on in the microprocessor.

RD’

READ; indicates the selected memory or 1/0 device is to be read and that the Data Bus is available for the data transfer.

WR’ (Output 3state)

WRITE; indicates the data on the Data Bus is to be written into the selected memory or 1/0 location. Data is set up at the trailing edge of WR’.

READY (Input)

If Ready is high during a read or write cycle, it indicates that the memory or peripheral is ready to send or receive data. If Ready is low, the CPU will wait for Ready to go high before completing the read or write cycle.

HOLD (Input)

HOLD; indicates that another Master is requesting the use of the Address and Data Buses. The CPU, upon receiving the Hold request will relinquish the use of buses as soon as the completion of the current machine cycle. Internal processing can continue. The processor can regain the buses only after the Hold is removed.

HLDA (Output)

HOLD ACKNOWLEDGE; indicates that the CPU has received the Hold request and that it will relinquish the buses in the next clock cycle. HLDA goes low after the Hold request is removed.

INTR (Input)

INTERRUPT REQUEST; is used as a general purpose interrupt. It is sampled only during the next to the last clock cycle of the instruction. If it is active, the Program Counter (PC) will be inhibited from incrementing and an INTA will be issued. During this cycle a RESTART or CALL instruction can be inserted to jump to the interrupt service routine. The INTR is enabled and disabled by software. It is disabled by Reset and immediately after an interrupt is accepted.

INTA’ (Output)

INTERRUPT ACKNOWLEDGE; is used instead of (and has the same timing as) RD during the Instruction cycle after an INTR is accepted. It can be used to activate the 8259 Interrupt chip or some other interrupt port.

RESTART INTERRUPTS

These three inputs have the same timing as INTR except they cause an internal RESTART to be automatically inserted. They are known as vectored interrupts.

RST 7.5: Highest Priority

RST 6.5

RST 5.5: Lowest Priority

TRAP (Input)

Trap interrupt is a non-maskable interrupt. It is recognized at the same time as INTR. It is unaffected by any mask or Interrupt Enable. It has the highest priority of any interrupt.

RESET IN’ (Input)

Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA flip-flops.

RESET OUT (Output)

It indicates CPU is being reset and can be used as a system RESET. The signal is synchronized to the processor clock.

X1, X2 (Input)

Crystal or R/C network connections to set the internal clock generator X1 can also be an external clock input instead of a crystal. The input frequency is divided by 2 to give the internal operating frequency.

CLK (Output)

Clock Output for use as a system clock in different peripherals to synchronize their activity.

IO/M’(Output)

IO/M indicates whether the Read/Write is to memory or I/O devices.

SID (Input)

Serial input data line.

SOD (output)

Serial output data line.

Vcc

It is +5 volt supply.

Vss

It is Ground Reference.

4. The 8085 Programming Model

The 8085 programming model includes six registers, one accumulator, and one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack pointer and the program counter. They are described briefly as follows:

Registers

The 8085 has six general-purpose registers to store 8-bit data; these are identified as B, C, D, E, H, and L as shown in the figure. They can be combined as register pairs – BC, DE, and HL – to perform some 16-bit operations. The programmer can use these registers to store or copy data into the registers by using data copy instructions.

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.

Flags

The ALU includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; their bit positions in the flag register are shown in the Figure below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data conditions.

For example, after an addition of two numbers, if the sum in the accumulator id larger than eight bits, the flip-flop uses to indicate a carry — called the Carry flag (CY) — is set to one. When an arithmetic operation results in zero, the flip-flop called the Zero (Z) flag is set to one. The first Figure shows an 8-bit register, called the flag register, adjacent to the accumulator. However, it is not used as a register; five bit positions out of eight are used to store the outputs of the five flip-flops. The flags are stored in the 8-bit register so that the programmer can examine these flags (data conditions) by accessing the register through an instruction.

These flags have critical importance in the decision-making process of the micro- processor. The conditions (set or reset) of the flags are tested through the software instructions. For example, the instruction JC (Jump on Carry) is implemented to change the sequence of a program when CY flag is set. The thorough understanding of flag is essential in writing assembly language programs.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a 16-bit register. The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a memory location in R/W memory, called the stack. The beginning of the stack is defined by loading 16-bit address in the stack pointer.

5. The 8085 Addressing Modes

The instructions MOV B, A or MVI A, 82H are to copy data from a source into a destination. In these instructions the source can be a register, an input port, or an 8-bit number (00H to FFH). Similarly, a destination can be a register or an output port. The sources and destination are operands. The various formats for specifying operands are called the ADDRESSING MODES. For 8085, they are:

  1. Immediate
  2. Register
  3. Direct
  4. Indirect

Immediate addressing

Data is present in the instruction. Load the immediate data to the destination provided. Example: MVI R,data

Register addressing

Data is provided through the registers. Example: MOV Rd, Rs

Direct addressing

Used to accept data from outside devices to store in the accumulator or send the data stored in the accumulator to the outside device. Accept the data from the port 00H and store them into the accumulator or Send the data from the accumulator to the port 01H.

Example: IN 00H or OUT, 01H

Indirect Addressing

This means that the Effective Address is calculated by the processor. And the contents of the address (and the one following) is used to form a second address. The second address is where the data is stored. Note that this requires several memory accesses; two accesses to retrieve the 16-bit address and a further access (or accesses) to retrieve the data which is to be loaded into the register.

Example MVI A, M

Here M denotes the HL register pair and the operand is located in memory address pointed by the register pair.

6. Instruction Set Classification

An instruction is a binary pattern designed inside a microprocessor to perform a specific function. The entire group of instructions, called the instruction set, determines what functions the microprocessor can perform. These instructions can be classified into the following five functional categories: data transfer (copy) operations, arithmetic operations, logical operations, branching operations, and machine-control operations.

Data Transfer (Copy) Operations

This group of instructions copy data from a location called a source to another location called a destination, without modifying the contents of the source. In technical manuals, the term data transfer is used for this copying function. However, the term transfer is misleading; it creates the impression that the contents of the source are destroyed when, in fact, the contents are retained without any modification. The various types of data transfer (copy) are listed below together with examples of each type:

Types Examples
1. Between Registers. 1. Copy the contents of the register B into register D.
2. Specific data byte to a register or a memory location. 2. Load register B with the data byte 32H.
3. Between a memory location and a register. 3. From a memory location 2000H to register B.
4. Between an I/O device and the accumulator. 4.From        an       input        keyboard         to      the accumulator.

Arithmetic Operations

These instructions perform arithmetic operations such as addition, subtraction, increment, and decrement.

Addition – Any 8-bit number, or the contents of a register or the contents of a memory location can be added to the contents of the accumulator and the sum is stored in the accumulator. No two other 8-bit registers can be added directly (e.g., the contents of register B cannot be added directly to the contents of the register C). The instruction DAD is an exception; it adds 16-bit data directly in register pairs.

Subtraction – Any 8-bit number, or the contents of a register, or the contents of a memory location can be subtracted from the contents of the accumulator and the results stored in the accumulator. The subtraction is performed in 2’s compliment, and the results if negative, are expressed in 2’s complement. No two other registers can be subtracted directly.

Increment/Decrement – The 8-bit contents of a register or a memory location can be incremented or decrement by 1. Similarly, the 16-bit contents of a register pair (such as BC) can be incremented or decrement by 1. These increment and decrement operations differ from addition and subtraction in an important way; i.e., they can be performed in any one of the registers or in a memory location.

Logical Operations

These instructions perform various logical operations with the contents of the accumulator.

AND, OR Exclusive-OR – Any 8-bit number, or the contents of a register, or of a memory location can be logically ANDed, Ored, or Exclusive-ORed with the contents of the accumulator. The results are stored in the accumulator.

Rotate– Each bit in the accumulator can be shifted either left or right to the next position.

Compare– Any 8-bit number, or the contents of a register, or a memory location can be compared for equality, greater than, or less than, with the contents of the accumulator.

Complement – The contents of the accumulator can be complemented. All 0s are replaced by 1s and all 1s are replaced by 0s.

Branching Operations

This group of instructions alters the sequence of program execution either conditionally or unconditionally.

Jump – Conditional jumps are an important aspect of the decision-making process in the programming. These instructions test for a certain conditions (e.g., Zero or Carry flag) and alter the program sequence when the condition is met. In addition, the instruction set includes an instruction called unconditional jump.

Call, Return, and Restart – These instructions change the sequence of a program either by calling a subroutine or returning from a subroutine. The conditional Call and Return instructions also can test condition flags.

Machine Control Operations

These instructions control machine functions such as Halt, Interrupt, or do nothing.

The microprocessor operations related to data manipulation can be summarized in four functions:

  1. copying data
  2. performing arithmetic operations
  3. performing logical operations
  4. testing for a given condition and alerting the program sequence

7. Instruction Format

An instruction is a command to the microprocessor to perform a given task on a specified data. Each instruction has two parts: one is task to be performed, called the operation code (opcode), and the second is the data to be operated on, called the operand. The operand (or data) can be specified in various ways. It may include 8-bit (or 16-bit ) data, an internal register, a memory location, or 8-bit (or 16-bit) address. In some instructions, the operand is implicit.

Instruction word size

The 8085 instruction set is classified into the following three groups according to word size:

  1. One-word or 1-byte instructions
  2. Two-word or 2-byte instructions
  3. Three-word or 3-byte instructions

In the 8085, “byte” and “word” are synonymous because it is an 8-bit microprocessor. However, instructions are commonly referred to in terms of bytes rather than words.

One-Byte Instructions

A 1-byte instruction includes the opcode and operand in the same byte. Operand(s) are internal register and are coded into the instruction.

For example:

Task Op code Operand Binary Code Hex Code
Copy the contents of the accumulator in the register C. MOV C,A 0100 1111 4FH
Add the contents of register B to the contents of the accumulator. ADD B 1000 0000 80H
Invert      (compliment)          each      bit     in             the accumulator. CMA 0010 1111 2FH

These instructions are 1-byte instructions performing three different tasks. In the first instruction, both operand registers are specified. In the second instruction, the operand B is specified and the accumulator is assumed. Similarly, in the third instruction, the accumulator is assumed to be the implicit operand. These instructions are stored in 8- bit binary format in memory; each requires one memory location.

MOV rd, rs

rd <– rs copies contents of rs into rd.

Coded as 01 ddd sss where ddd is a code for one of the 7 general registers which is the destination of the data, sss is the code of the source register.

Example: MOV A,B

Coded as 01111000 = 78H = 170 octal (octal was used extensively in instruction design of such processors).

ADD r

A <– A + r

Two-Byte Instructions

In a two-byte instruction, the first byte specifies the operation code and the second byte specifies the operand. Source operand is a data byte immediately following the opcode. For example to  move immediate data (32H) to the accumulator, the instruction used is:

Mnemonics Hex code
MVI A, 32H 3E 32H

The instruction would require two memory locations to store in memory.

MVI r,data

r <– data

Example: MVI A,30H    coded as 3EH 30H as two contiguous bytes. This is an example of immediate addressing.

Three-Byte Instructions

In a three-byte instruction, the first byte specifies the opcode, and the following two bytes specify the 16-bit address. Note that the second byte is the low-order address and the third byte is the high-order address.

Three byte instructions – opcode + data byte + data byte

LXI rp, data16

rp is one of the pairs of registers BC, DE, HL used as 16-bit registers. The two data bytes are 16-bit data in L H order of significance.

rp <– data16

Example:

LXI H,0520H    coded as 21H 20H 50H in three bytes. This is also immediate addressing.

LDA addr

A <– (addr) Addr is a 16-bit address in L H order. Example: LDA 2134H coded as 3AH 34H 21H. This is also an example of direct addressing.

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