Introduction to 8085 Architecture and Programming Part 2-Microprocessor-Article, Summaries of Microprocessors

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5. The 8085 Programming Model

In the previous tutorial we described the 8085 microprocessor registers in reference to the internal data operations. The same information is repeated here briefly to provide the continuity and the context to the instruction set and to enable the readers who prefer to focus initially on the programming aspect of the microprocessor.

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.

ACCUMULATOR A (8) (^) FLAG REGISTER

B (8)

D (8)

H (8)

Stack Pointer (SP) (16)

Program Counter (PC) (16)

C (8)

E (8)

L (8)

Data Bus Address Bus

8 Lines Bidirectional 16 Lines unidirectional

6. 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 addressing.
2. Register addressing.
3. Direct addressing.
4. Indirect addressing.

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.

7. 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.

1. In data transfer, the contents of the source are not destroyed; only the contents of the destination are changed. The data copy instructions do not affect the flags. 2. Arithmetic and Logical operations are performed with the contents of the accumulator, and the results are stored in the accumulator (with some expectations). The flags are affected according to the results. 3. Any register including the memory can be used for increment and decrement. 4. A program sequence can be changed either conditionally or by testing for a given data condition.

8. 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:

Task Opcode Operand Binary Code

Hex Code

Load an 8-bit data byte in the accumulator.

MVI A, Data 3E

Data

First Byte

Second Byte

Assume that the data byte is 32H. The assembly language instruction is written as

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.

ADI data A <-- A + data

OUT port

0011 1110

DATA

ADD D

OUT PORT

HLT

Write an assembly program to multiply a number by 8

Program

MVI A, 30H RRC RRC RRC OUT PORT HLT

Write an assembly program to find greatest between two numbers

Program

MVI B, 30H MVI C, 40H MOV A, B CMP C JZ EQU JC GRT OUT PORT HLT

EQU: MVI A, 01H OUT PORT HLT

GRT: MOV A, C OUT PORT HLT