CHAP 6

• Computer program


  A set of instructions that tells the computer what to do
• Computer programming languages


  • Fortran
  • C
  • C++
  • Java
  • Python
• Machine language


  • Computer hardware operates only with binary numbers (1s and 0s)
  • Microprocessors designed to understand (or execute) a set of instructions where each instruction is given a binary code (also called machine code)
  • It is not possible to develop program directly at this level
• Assembly language


  One level above machine language<|>Uses alphabetic abbreviations, called mnemonics, so that it is easier to understand and remember<|>These mnemonics are also decided by the manufacturer
• Assembly language


  • Easier to understand by humans but not computers
  • Still needs to be translated to machine code (using an ASSEMBLER) to execute on microprocessor
• High level languages


  Use English statements and known mathematical symbols<|>A single high level statement may be composed of many assembly commands<|>Program written in High level language cannot be directly executed, it still needs to be translated to final machine code
• Assembly language


  • The most basic language that the microprocessor can understand
  • Exploring assembly language will give us a better understanding of how the microprocessor works
  • In some cases, part of the high level program can employ assembly language program to optimize code or to interact directly with hardware (e.g. to access microprocessor registers directly)
• Instruction Groups


  • Movement Instructions
  • Arithmetic Instructions
  • Logical Instructions
  • Jump instructions
• Byte


  8 bits
• Word


  16 bits or 2 bytes
• Double word


  32 bits or 4 bytes
• Little endian


  The least significant byte is stored in the lowest-numbered memory location, and the most significant byte is stored in the highest
• 8086 Registers


  • AX
  • BX
  • CX
  • DX
  • CS
  • DS
  • SS
  • ES
  • SP
  • BP
  • SI
  • DI
  • FLAGS
  • IP
• Registers


  • Provide a scratch pad so that the processor can keep data on a temporary basis
  • Used to keep the data handy to avoid costly memory accesses
• Instruction Pointer Register (IP)

  A 16-bit register used in conjunction with the code segment register (CS) to generate a 20-bit physical address (CS:IP)<|>Not directly accessible by a programmer but can be altered through the use of a number of instructions, e.g. JMP, CALL, INT, etc.
• Segment Registers (CS,DS,SS,ES)


  Required because the total addressable RAM space is 1 Mb = 2^20 in size whereas all of the pointer registers are only 16-bits
• Generating Memory Addresses


  1. Physical address = (Segment Register * 16) + Pointer Register

  2. More than one segment & pointer combination can point to the same physical address
• Memory Segments


  Code: for program instructions<|>Data: for data<|>Stack: for subroutines<|>Extra: additional for data
• Default Segment Usage


  IP and CS: Instructions

  SP and SS: Stack

  BP and SS: Data

  BX, SI, DI and DS: Data
• Flags


  CF - Carry Flag

  ZF - Zero Flag
• put in AX address


  Physical location of the memory
• SI paired with data segment


  Compiler will decide which data is assigned to which memory
• DS and SI are decided by the compiler, so memory should not crash
• Segment Over-Ride


  Physical address is ES:SI or ES * 10h + SI
• With CF = 1
• Adding two numbers


  1. Carry cannot be 2

  2. Carry will be 0 or 1
• Extra 1 will raise carry flag
• CF - Carry


  Indicates if there is a carry from the addition
• ZF - Zero


  Indicates if the result of an arithmetic operation is zero
• 2EH - 2EH


  • ZF = 1
• 2EH - 05H


  • ZF = 0
• Is it possible to add and have ZF = 1?
• F F

  0 1 +


  • With CF = 1 and ZF = 1
• Addressing Modes


  • Register
  • Immediate
  • Direct
  • Register indirect
  • Indexed
  • Register Relative
  • Based Indexed
  • Relative based Indexed
• Move instruction


  Moves data from source to destination
• MOV AX,BX


  • Register to register
• MOV AX,0005H


  • Data to register
• MOV AX,[5000H]


  • Memory to register
• MOV AX,[BX]


  • Register indirect
• MOV AX,[SI]


  • Indexed