TK 2633 Microprocessor & Interfacing Lecture 3: Introduction to 8085 Assembly Language Programming (2) 1 Prepared By: Associate Prof. Dr Masri Ayob

TK 2633Microprocessor & Interfacing

Lecture 3: Introduction to 8085 Assembly Language Programming

(2)

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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 - TK2123

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Instruction, Data Format, And Storage

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.• 1-byte instructions• 2-byte instructions• 3-byte instructions


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1-byte instructions

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


2-byte instructions

The first byte specifies the operation code and the second byte specifies the operand. For example:


3-byte instructionsThe 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:

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Addressing Modes

Instructions can be categorized according to their method of addressing the hardware registers and/or memory. • Implied Addressing • Register Addressing • Immediate Addressing• Direct Addressing• Register Indirect Addressing• Combined Addressing Modes.


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Implied Addressing

The addressing mode of certain instructions is implied 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.


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Register Addressing

Quite a large set of instructions call for register addressing.

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.


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Register Addressing

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

However, a few of these instructions deal with 16-bit register pairs. • For example, the PCHL instruction exchanges the

contents of the program counter with the contents of the H and L registers.


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Immediate Addressing

These instructions have data assembled as a part of 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.


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Immediate Addressing

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


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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 hexadecimal

address 1000H by replacing the current contents of the PC with the new value 1000H.


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Register Indirect Addressing

These instructions reference memory via a register 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 .


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Combined Addressing Modes

Some instructions use a combination of addressing modes. • A CALL instruction, for example, combines direct

addressing and register indirect addressing.• The direct address in a CALL instruction specifies

the 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 location specified by the stack pointer. .


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Timing Effects of Addressing Modes

Addressing modes affect both the amount of time required 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 since they 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 execution time. • A CALL instruction for example, requires five

memory accesses: three to access the entire instruction and two more to push the contents of the program counter onto the stack.


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Data Format

The 8085 is an 8-bit microprocessor, and it processes 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.


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Data Format

Signed Integer - A signed integer is either a positive 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.


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Data Format

Unsigned Integers - An integer without a sign can be 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 them in groups of 8 bits.


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How To Write, Assemble, And Executea Simple Program

A program is a sequence of instructions written to tell 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.


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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 LED output port PORT.


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PROBLEM ANALYSIS• 1. Load the numbers in the registers.• 2. Add the numbers.• 3. Display the sum at the output port PORT.


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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.


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Storing in memory and converting from hex code to 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 the output port address.


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• 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 key may be labeled differently in different systems).

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


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In this illustrative example, the program will be stored 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 key with 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.


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Overview Of The 8085 Instruction Set

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

The following notations are used in the description of the instructions:


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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 6 operations:• 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


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Data Transfer (Copy) Instructions


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Arithmetic Instructions

Add

Subtract

Increment (Add 1)

Decrement (Subtract 1)


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Arithmetic Instructions


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Logic and Bit Manipulation Instructions

AND

OR

X-OR (Exclusive OR)

Compare

Rotate Bits


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Branch Instructions


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Machine Control Instructions

These instructions affect the operation of the processor.• HLT Stop processing and wait• NOP Do not perform any operation


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Common Errors

LDA 205lH: Not entering the code of the 16-bit address 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.


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Summary

The 8085 microprocessor operations are classified into 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.


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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.


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Thank youQ & A

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Documents

TK 2633 Microprocessor & Interfacing Lecture 3: Introduction to 8085 Assembly Language Programming (2) 1 Prepared By: Associate Prof. Dr Masri Ayob