Unit 3 assembler and processor

Unit 3 assembler and processor

• 1.
  Assembly Language Prakash Khaire UTU
• 2.
  Assembly Language  Aprogramming language that is one step away from machine language.  Each assembly language statement is translated into one machine instruction by the assembler.  Programmers must be well versed in the computer's architecture, and, undocumented assembly language programs are difficult to maintain.  It is hardware dependent; there is a different assembly language for each CPU series.
• 3.
  Assembly Language  Itwas introduced in 1952  Helped to overcome machine language programming Definition : “A language that allows instructions and storage locations to be represented by letters and symbols instead of numbers is called assembly language or symbolic language”
• 4.
  Assembly Language Model … add r1,r2 sub r2,r3 PC cmp r3,r4 ALU bne I1 Registers Memory sub r4,1 I1: jmp I3 …
• 5.
  Assembly Language Instructions Builtfrom two pieces Add R1, R3, 3 Opcode Operands What to do with Where to get the data data and put (ALU operation) the results
• 6.
  Types of Opcodes Arithmetic,logical add, sub, mult and, or Cmp Memory load/store ld, st Control transfer jmp bne
• 7.
  Operands Each operand takenfrom a particular addressing mode: Examples: Register add r1, r2, r3 Immediate add r1, r2, 10 Indirect mov r1, (r2) Offset mov r1, 10(r3) PC Relative beq 100 Reflect processor data pathways
• 8.
  Advantages over MachineLanguage Easier to understand and use Easier to locate and correct errors. Easier to modify No worry about addresses Easily relocatable Efficiency of machine language
• 9.
  Limitations of AssemblyLanguage Machine Dependent Knowledge of hardware required Machine level coding
• 10.
  Architecture of 8086
• 11.
  Registers in 8086CPU  There are four general purpose registers  AX [divided into AH/AL] – the accumulator register  BX [divided into BH/BL] – the base address register  CX [divided into CH/CL] – the count register  DX [divided into DH/DL] – the data register Other registers  SI – source index register  DI – destination index register  BP – Base pointer  SP – Stack pointer
• 12.
  Registers in 8086CPU  This registers are of 16 bits and divided into two 8 bit registers  For Example If AX = 0011000000111001b then AH = 00110000b and AL=001111001b  Registers are located in CPU,  Segment Registers  Used for special purpose pointing at accessible blocks of memory  It stores any data  Four segment registers during execution  CS – Points the segment storing the current program  DS – Points the segment where variable are defined  ES – Extra Segment register, depends on the usage of coder  SS – Points to the segment containing the stack  Along with general purpose registers, segment registers work to access any memory value.
• 13.
  Flags  FLAG :The FLAG register is the status register in the Intel 8086 microprocessor that contains the current state of the microprocessor. This register is 16 bits wide.  carry flag (CF)- indicates a carry after addition or a borrow after subtraction, also indicates error conditions.  parity flag (PF)- is a logic “0” for odd parity and a logic “1” for even parity.  auxiliary carry flag (AF)- important for BCD addition and subtraction; holds a carry (borrow) after addition (subtraction) between bits position 3 and 4. Only used for DAA and DAS instructions to adjust the value of AL after a BCD addition (subtraction).  zero flag (ZF)- indicates that the result of an arithmetic or logic operation is zero.  sign flag (SF)- indicates arithmetic sign of the result after an arithmetic operation.  overflow flag (OF)- a condition that occurs when signed numbers are added or subtracted. An overflow indicates that the result has exceeded the capacity of the machine.
• 14.
  Offset  The offsetaddress in an 8086/8088 is the logical address that the program "thinks about" when it addresses a location in memory.  The Execution Unit (EU or CPU) is responsible for generating the offset address.  The Bus Interface Unit (BIU), on the other hand, takes the offset address and adds it to four times the selected segment register value in order to determine a real address, which is now 20-bits in length.
• 15.
  Registers in 8086CPU  Programmers access various memory locations on combining BX, SI, DI and BP registers.  The value in segment registers[CS, DS, SS,ES] is called “segment”  The value in purpose registers [BX, SI, DI, BP] is called “offset”  For Example  DS contains value 1234h and SI contains the value 7890h it can also be recorded as 1234:7890  Two special purpose registers  IP – Instruction Register  Flags Register – determines the current state of the processor
• 16.
  Registers in 8086CPU  IP works along with CS register and points to the currently executing instruction  Flags register is modified by CPU, while performing mathematical operations
• 17.
  Assembler Design Approach Assemblergenerates object code. Role of assembler – Translates the source code into target code During this translation process it uses various table like Machine Opcode Table Symbol table Literal table
• 18.
  Assembler Design Approach MachineOpcode Table  It holds opcode used by the processor for different instructions mnemonics Mnemonic size Opcode Holds various instructions Size of instructions in bytes Size of instructions in bytes
• 19.
  Symbol Table  Itis data structure used by assmebler  It keeps into account attributes of the identifier and other information.  Attributes : type, value, scope and address  It also performs the function of book keeping.  It is composed of multiple word entries in fixed format Name Value Type
• 20.
  One Pass Assembler Doeseverything in one pass Problem: How do we handle forward references? Could eliminate forward references easy for data – just define the data areas before they are referenced not easy in code – how do we handle selection or loop statements which have forward jumps? Two types of one pass assemblers: produce object code directly in memory produce object program for later execution
• 21.
  One Pass Assembler Assembler generates object code instructions as it scans source program  If an operand symbol has not yet been defined  operand address is set to 0 in instruction  symbol is entered into the symbol table (unless it is already present)  entry is flagged to indicate the symbol is undefined  address of instruction is added to list of forward references associated with this symbol  When symbol definition is encountered  forward reference list is scanned, and proper address is inserted in any instructions previously generated (in memory)
• 22.
  One pass assemblerthat produces object programs  Use the same procedure  When the definition of a symbol is encountered  if instruction which made the forward reference is still in memory, then fix it  if not, the instruction has already been written out in a Text record, so generate a new Text record with the correct operand address  (could also use a modification record)  The loader will fix up the address field
• 23.
  One pass assemblerthat produces object programs  Problem : Forward Reference  It be eliminated by declaring variable before using them  However, elimination can’t be done easily because sometimes program needs a forward jump.