Transcript The Research Process

TK 2633
Microprocessor & Interfacing
Lecture 3: Introduction to 8085
Assembly Language Programming
(2)
Prepared By: Associate Prof. Dr Masri Ayob
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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.
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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:
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2-byte instructions
The first byte specifies the operation code and
the second byte specifies the operand. For
example:
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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:
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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 16bit 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 Execute
a 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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Example: Writing a simple program
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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Example: Writing a simple program
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Example: Writing a simple program
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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Example: Writing a simple program
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Example: Writing a simple program
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Example: Writing a simple program
Storing in memory and converting from hex code
to binary code.
To store the program in R/W memory of a singleboard microcomputer and display the output, we
need to know the memory addresses and the
output port address.
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Example: Writing a simple program
• 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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Example: Writing a simple program
In this illustrative example, the program will be
stored in memory as follows:
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Example: Writing a simple program
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:
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
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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
• NOP
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Stop processing and wait
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 you
Q&A
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