TK 2633 Microprocessor

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TK 2633Microprocessor Interfacing

• Lecture 1 Introduction to 8085 Assembly Language
  Programming (2)

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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 instructions 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.
• 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.
• 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 processors 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.
• 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, D6D0, 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.
• 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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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
  single-board microcomputer and display the
  output, we need to know the memory addresses and
  the output 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 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) RsRegister
source (A,B,C,D,E,H,L) RdRegister
destination (A,B,C,D,E,H,L) RpRegister
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 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.
• 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 youQA