TK2633-Lecture3-Introduction to 8085 Assembly Language Programming(2)

7/22/2019 TK2633-Lecture3-Introduction to 8085 Assembly Language Programming(2)

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TK 2633

Microprocessor & Interfacing

Lecture 3: Introduction to 8085 Assembly Language Programming

(2)

1

Prepared By: Associate Prof. Dr Masri Ayob



Instruction, Data Format, And Storage

Instruction is a command to the microprocessor to

perform a given task on specified data.

Each instruction has two parts:

• one is the task to be performed, called the operation

code (op-code),• 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 an 8-bit (or 16-bit)

address. In some instructions, the operand is

implicit.Prepared by: Dr Masri Ayob 2



Instruction, Data Format, And Storage

In the 8085, “byte” and “word” are synonymousbecause it is an 8-bit microprocessor.

However, instructions are commonly referred to in

terms of bytes rather than words.

• 1-byte instructions



Prepared by: Dr Masri Ayob 3



1-byte instructions

It includes the opcode and the operand in thesame byte. For example:

Prepared by: Dr Masri Ayob - TK21234



2-byte instructions

The first byte specifies the operation code andthe second byte specifies the operand. For

example:




3-byte instructions

The first byte specifies the operation code and the

following two bytes specify the 16-bit address.The second byte is the low-order address and the

third byte is the high-order address.

For example:

Prepared by: Dr Masri Ayob – TK26336



Addressing Modes

Instructions can be categorized according to theirmethod of addressing the hardware registers

and/or memory.

• Implied Addressing

• Register Addressing• Immediate Addressing

• Direct Addressing

• Register Indirect Addressing

• Combined Addressing Modes.




Implied Addressing

The addressing mode of certain instructions isimplied by the instruction’s function.

• For example:

• the STC (set carry flag) instruction deals only

with the carry flag• the DAA (decimal adjust accumulator) instruction

deals with the accumulator.




Register Addressing

Quite a large set of instructions call for registeraddressing.

We must specify one of the registers A through E,

H or L as well as the operation code.

The accumulator is implied as a second operand.

• For example, the instruction CMP E may be

interpreted as 'compare the contents of the E

register with the contents of the accumulator.




Register Addressing

Most of the instructions that use registeraddressing deal with 8-bit values.

However, a few of these instructions deal with 16-

bit register pairs.

• For example, the PCHL instruction exchanges thecontents of the program counter with the contents

of the H and L registers.




Immediate Addressing

These instructions have data assembled as a partof the instruction itself.

• For example, the instruction CPI 'C' may be

interpreted as ‘compare the contents of the

accumulator with the letter C.• When assembled, this instruction has the

hexadecimal value FE43.

• Hexadecimal 43 is the internal representation for

the letter C.




Immediate Addressing

• For example, the instruction CPI 'C' may beinterpreted as ‘compare the contents of the

accumulator with the letter C.

• When this instruction is executed, the processor

fetches the first instruction byte and determines

that it must fetch one more byte.

• The processor fetches the next byte into one of

its internal registers and then performs the

compare operation.




Direct Addressing

These instructions require three bytes of storage:one for the instruction code, and two for the 16-bit

address.

Example:

• JMP 1000H causes a jump to the hexadecimaladdress 1000H by replacing the current contents of

the PC with the new value 1000H.




Register Indirect Addressing

These instructions reference memory via aregister pair.

For example: MOV M,C

• moves the contents of the C register into the

memory address stored in the H and L register pair.

The instruction LDAX B loads the accumulator

with the byte of data specified by the address in

the B and C register pair .




Combined Addressing Modes

Some instructions use a combination ofaddressing modes.

• A CALL instruction, for example, combines direct

addressing and register indirect addressing.

• The direct address in a CALL instruction specifiesthe address of the desired subroutine;

• the register indirect address is the stack pointer.

The CALL instruction pushes the current contents

of the program counter into the memory locationspecified by the stack pointer. .




Timing Effects of Addressing Modes

Addressing modes affect both the amount of timerequired for executing an instruction and the

amount of memory required for its storage.

For example, instructions that use implied or

register addressing, execute very quickly sincethey deal directly with the processor’s hardware or

with data already present in hardware registers.

• Most important, however is that the entire

instruction can be fetched with a single memory

access. The number of memory accesses required

is the single greatest factor in determining

execution timing.

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Timing Effects of Addressing Modes

More memory accesses require more executiontime.

• A CALL instruction for example, requires five

memory accesses: three to access the entire

instruction and two more to push the contents ofthe program counter onto the stack.




Data Format

The 8085 is an 8-bit microprocessor, and itprocesses only binary numbers.

We need to code binary numbers into different

media.

Common codes and data formats are ASCII, BCD,signed integers, and unsigned integers:

• ASCII Code - 7-bit alphanumeric code that

represents decimal numbers, English alphabets,

and nonprintable characters such as carriage

return. Extended ASCII is an 8-bit code.

• BCD Code - Binary-coded decimal; it is used for

decimal numbers.




Data Format

Signed Integer - A signed integer is either apositive number or a negative number.

• In an 8-bit processor, the most significant digit, D7,

is used for the sign; 0 represents the positive sign

and I represents the negative sign.• The remaining seven bits, D6—D0, represent the

magnitude of an integer. Therefore, the largest

positive integer that can be processed by the 8085

at one time is 0111 1111 (7FH); the remaining Hex

numbers, 80H to FFH, are considered negative

numbers.




Data Format

Unsigned Integers - An integer without a sign canbe represented by all the 8 bits in a

microprocessor register.

• Therefore, the largest number that can be

processed at one time is FFH.• However, this does not imply that the 8085

microprocessor is limited to handling only 8-bit

numbers. Numbers larger than 8 bits (such as 16-bit

or 24-bit numbers) are processed by dividing themin groups of 8 bits.




How To Write, Assemble, And Execute

a Simple Program

A program is a sequence of instructions written totell a computer to perform a specific function.

The instructions are selected from the instruction

set of the microprocessor.

To write a program:

• divide a given problem in small steps in terms of the

operations the 8085 can perform;

• then translate these steps into instructions.




Example: Writing a simple program

PROBLEM STATEMENT• Write instructions to load the two hexadecimal

numbers 32H and 48H in registers A and B,

respectively.

• Add the numbers, and display the sum at the LEDoutput port PORT.





PROBLEM ANALYSIS• 1. Load the numbers in the registers.

• 2. Add the numbers.

• 3. Display the sum at the output port PORT.









ASSEMBLY LANGUAGE PROGRAM• To write an assembly language program, we need to

translate the blocks shown in the flowchart into

8085 operations and then, subsequently, into

mnemonics.

• By examining the blocks, we can classify them into

three types of operations: Blocks 1 and 3 are copy

(data transfer) instruction.

• Block 2 is an arithmetic operation;

• Block 4 is a machine-control operation.

• To translate these steps into assembly and machine

languages, we should review the instruction set.













Storing in memory and converting from hex codeto binary code.

To store the program in R/W memory of a single-

board microcomputer and display the output, we

need to know the memory addresses and theoutput port address.





• Let us assume that R/W memory ranges from 2000H

to 2OFFH, and the system has an LED output port

with the address 01H. Now, to enter the program:

• Reset the system by pushing the RESET key.

• Enter the first memory address using Hex keys

where the program should be stored. Let us

assume it is 2000H.

• Enter each machine code by pushing Hex keys.

For example, to enter the first machine code,

push the 3, E, and STORE keys. (The STORE keymay be labeled differently in different systems).

• Repeat Step 3 until the last machine code, 76H.

• Reset the system.





In this illustrative example, the program will bestored in memory as follows:





EXECUTING THE PROGRAM• To execute the program, we need to tell the

microprocessor where the program begins by

entering the memory address 2000H.

• Now, we can push the Execute key (or the keywith a similar label) to begin the execution.

• As soon as the Execute function key is pushed,

the microprocessor loads 2000H in the program

counter, and the program control is transferred

from the Monitor program to our program.

• The microprocessor begins to read one machine

code at a time, and when it fetches the complete

instruction, it executes that instruction.




Overview Of The 8085 Instruction Set

The 8085 microprocessor instruction set has 74operation codes that result in 246 instructions.

The following notations are used in the description

of the instructions:


R = 8085 8-bit register (A,B,C,D,E,H,L)

M = Memory register (location)

Rs

=Register source (A,B,C,D,E,H,L)

Rd=Register destination (A,B,C,D,E,H,L)

Rp=Register pair (BC,DE,HL,SP)

() = Contents of



Data Transfer (Copy) Instructions

These instruction perform the following 6operations:

• Load an 8-bit register

• Copy from register to register

• Copy between I/O and accumulator• Load 16-bit number in a register pair

• Copy between register and memory

• Copy between registers and stack memory




Data Transfer (Copy) Instructions




Arithmetic Instructions



Arithmetic Instructions




Logic and Bit Manipulation Instructions

ANDOR

X-OR (Exclusive OR)

Compare

Rotate Bits




Branch Instructions




Machine Control Instructions

These instructions affect the operation of theprocessor.

• HLT Stop processing and wait

• NOP Do not perform any operation




Common Errors

LDA 205lH: Not entering the code of the 16-bitaddress in reversed order.

Forgetting to enter the code for the operand, such

as 205lH.

MOV B, A: Assuming that this copies from B to A.

Incrementing the address in decimal, from 2039H

to 2040H.

HLT: Not terminating a program.Confusing the entering of Hex code in memory as

executing a program.




Summary

The 8085 microprocessor operations are classifiedinto five major groups: data transfer (copy),

arithmetic, logic, branch, and machine control.

An instruction has two parts:

• opcode (operation to be performed)• operand (data to be operated on) - The operand can

be data (8- or 16-bit), address, or register, or it can

be implicit.




Summary

The method of specifying an operand (directly,indirectly, etc.) is called the addressing mode.

The instruction set is classified in three groups

according to the word size: 1-, 2-, or 3-byte

instructions.To write an assembly language program, divide

the given problem into small steps in terms of the

microprocessor operations, translate these steps

into assembly language instructions, and then

translate them into the 8085 machine code.




Thank youQ & A

TK2633-Lecture3-Introduction to 8085 Assembly Language Programming(2)

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