TK 2633
Microprocessor & Interfacing
Lecture 2: Introduction to 8085
Assembly Language Programming
(2)
Prepared By: Associate Prof. Dr Masri Ayob
1
Objectives
Explain the various functions of the registers in the
8085 programming model.
Define the term flag and explain how the flags are
affected.
Explain the terms operation code (Opcode) and
the operand, and illustrate these terms by writing
instructions.
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2
Objectives
Classify the instructions in terms of their word size
and specify the number of memory registers
required to store the instructions in memory.
List the five categories of the 8085 instruction set.
Define and explain the term addressing mode.
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Objectives
Write logical steps to solve a simple programming
problem.
Draw a flowchart from the logical steps of a given
programming problem.
Translate the flowchart into mnemonics and
convert the mnemonics into Hex code for a given
programming problem.
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The 8085 Hardware/Programming Model
A model is a conceptual representation of a real
object.
The microprocessor can be represented in terms
of its hardware (physical electronic components)
and a programming model (information needed to
write programs).
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8085 Hardware Model
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8085 Programming Model
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8085 Flag Register
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8085 Hardware Model
Two major segments:
• One segment includes the arithmetic/logic unit
(ALU) and an 8-bit register called an accumulator,
instruction decoder, and flags.
• The second segment shows 8-bit and 16-bit
registers.
• Both segments are connected with various internal
connections called an internal bus.
• The arithmetic and logical operations are performed
in the ALU. Results are stored in the accumulator,
and flip-flops, called flags, are set or reset to reflect
the results
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8085 Hardware Model
There are three buses:
• a 16-bit unidirectional address bus to send out
memory addresses;
• an 8-bit bidirectional data bus, and a control bus to
transfer data, and.
• the control bus for timing signals.
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8085 Programming Model
The model includes six 8-bit registers (B, C, D, E,
H & L), one accumulator, and one flag register.
It also has two 16-bit registers:
• the stack pointer (SP);
• the program counter (PC).
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General-purpose Registers
The 8085 has six general-purpose registers to
store 8-hit data;
• B, C, D, E, H, and L.
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.
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Accumulator
The accumulator is an 8-bit register that is part of
the 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.
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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;
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Flags
The most commonly used flags are Zero, Carry,
and Sign. The microprocessor uses these flags to
test data conditions.
These flags have critical importance in the
decision-making process of the microcessor.
• E.g., the instruction JC (Jump On Carry) is
implemented to change the sequence of a program
when the CY flag is set.
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Flags
The following flags are set or reset after the
execution of an arithmetic or logic operation; data
copy instructions do not affect any flags. See the
instruction set (Appendix F) to find how flags are
affected by an instruction.
• Z-Zero: The Zero flag is set to 1 when the result is
zero; otherwise it is reset.
• CY - Carry: If an arithmetic operation results in a
carry, the CY flag is set; otherwise it is reset.
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Flags
• S - Sign: The Sign flag is set if bit D7 of the result =
1; otherwise it is reset.
• P - Parity: If the result has an even number of 1s, the
flag is set; for an odd number of 1s, the flag is reset.
• AC - Auxiliary Carry: In an arithmetic operation,
when a carry is generated by digit D3 and passed to
digit D4, the AC flag is set. This flag is used
internally for BCD (binary-coded decimal)
operations; there is no Jump instruction associated
with this flag.
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Example
Instruction:
ADD B
Register contents before
instruction
Register contents after
instruction:
A
9A h
A
23 h
B
89 h
B
89 h
Flag
80 h
Flag
10 h
1001 1010
1000 1001
0010 0011
Flag: S=0, Z=0, AC=1 , P=0 and C=1,
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Note: All flags are
modified to reflect the
result of the addition.
Program COUNTER (PC) AND STACK POINTER (SP)
These are two 16-bit registers used to hold
memory addresses.
PC:
• The function of the PC 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.
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Program COUNTER (PC) AND STACK POINTER (SP)
SP:
• It points to a memory location in R/W memory,
called the stack.
• The beginning of the stack is defined by loading a
16-bit address in the stack pointer.
• The PC will automatically update when calling to
/returning from Subroutines.
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Stack
The stack is one of the most important things you
must know when programming.
• Think of the stack as a deck of cards. When you put
a card on the deck, it will be the top card. Then you
put another card, then another.
• When you remove the cards, you remove them
backwards, the last card first and so on.
• The stack works the same way, you put (push)
words (addresses or register pairs) on the stack and
then remove (pop) them backwards.
• That's called LIFO, Last In First Out.
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Stack
There are instructions that allow you to modify SP
contents but you should NOT change the contents
of that register if you don't know what you're doing!
• PUSH
• POP
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8085 Instruction Set
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.
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8085 Instruction Set
Can be classified into the following five functional
categories:
•
•
•
•
•
data transfer (copy) operations,
arithmetic operations,
logical operations,
branching operations, and
machine-control operations.
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Data Transfer (copy) Instructions
Copies 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.
• The term transfer is misleading; it creates the
impression that the contents of a source are
destroyed when, in fact, the contents are retained
without any modification.
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Data Transfer (copy) Instructions
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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 register C). The
instruction DAD is an exception; it adds 16-bit data
directly in register pairs.
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Arithmetic Operations
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 complement,
and the results, if negative, are expressed in 2’s
complement. No two other registers can be
subtracted directly.
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Arithmetic Operations
Lncrement/Decrement - The 8-bit contents of a register or
a memory location can be incremented or decremented by
1.
• Similarly, the 16-bit contents of a register pair (such
as BC) can be incremented or decremented by I.
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.
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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 EXORed 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.
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Logical Operations
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.
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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 programming.
These instructions test for a certain condition (e.g.,
Zero or Carry flag) and alter the program sequence
when the condition is met.
• This set includes an instruction called
unconditional jump.
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Branching Operations
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.
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Machine Control Operations
These instructions control machine functions such
as Halt, Interrupt, or do nothing.
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Review of the 8085 Operations
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.
Arithmetic and logical operations are performed
with the contents of the accumulator, and the
results are stored in the accumulator (with some
exceptions).
• The flags are affected according to the results.
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Review of the 8085 Operations
Any register including memory can be used for
increment and decrement.
A program sequence can be changed either
conditionally or by testing for a given data
condition.
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Thank you
Q&A
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