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TMA1271 - Introduction to Machine Architecture

Trimester 3 Year 2000/2001

Tutorial 3 
 
 

1. Programming model of 8085 Microprocessor.

2. Addressing Modes.

3. Instruction Set.

4. Instruction Format.

Note for the Instructor:

In this tutorial, we should cover the addressing modes and instruction set of the 8085. You may follow the above guideline. In the next tutorials, we will cover the assembly language related to 8085.


 

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. 

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.  

      This programming model will be used in subsequent tutorials to examine how these registers are affected after the execution of an instruction. 

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.  
 

INSTRUCTION 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: 

 
 

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
 

Some important aspects of the instruction set are noted below: 

  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.
 
 

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: 

 

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  
 

   
 

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

where port is an 8-bit device address.    (Port) <-- A.  Since the byte is not the data but points directly to where it is located this is called direct 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.

opcode + data byte + data byte 

For example: 

Task

Opcode

Operand

Binary code

Hex Code

  
 
 
 

Transfer the program sequence to the memory location 2085H.

JMP

2085H

  

C3 

85 

20

First byte 

Second Byte 

Third Byte

 

This instruction would require three memory locations to store in memory. 
 

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.