More on Assembler

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More on Assembler

• Semester 1, Week 11

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Output from use of Assembler (1 of 2)

• The output from an Assembler may consist of
• a listing of the original Assembly Language
  program and the machine code generated,
• a listing of errors encountered in the Assembly
  process, with or without suggested causes for the
  error,
• a symbol table,
• / continued

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Output from use of Assembler (2 of 2)

• A cross-reference table showing where each symbol
  is to be found within the program, a summary of
  external references used within the program, and
  whether these are resolved or unresolved,
• a list of subroutines and macros and their
  lengths in bytes.
• Any use of subroutines requires a defined
  mechanism of subroutine linkage to be built into
  the Assembly language.

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Structure of Assembly Programs

• When you first look at an Assembly language
  program you might see comments and instructions.
  Like any programming, it takes a little while to
  distinguish what is documentation and what is
  code.
• As well as these two sections of a program there
  may also be 'calls' to other programs (including
  macros) or 'includes'.

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Structure of Assembly Programs (2)

• In some Assemblers there is a possibility that
  libraries of other code might be made open -
  included - for the use of the program that
  follows the 'include' statement.

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Comments

• Comments are most usually denoted by the use of
  semicolon () or a hash ().
• You might see a heading like this
• 
  
• 
  
• Art's Assembler Code
  
• Program 1
  
• 
  
• 
  
• Below this line is a list of code.

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Comments

• Or something like this
• This is an Assembler program
• Written by Art
• Date 28 Nov 2006
• --- oOo ---
• Below this line is a list of code.

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More on Program Structure

• In assembly language code lines have two parts
• ltinstructiongt ltparametergt, ltparametergt
• . Part 1 . Part 2 ..
• The first part is the name of the instruction
  which is to be executed. The second part has the
  parameters of the command. For example
• MOV AL, 25
• In the above example, we are using the
  instruction 'MOV'. This particular line of code
  means "move the value 25 to the AL register".

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More on Program Structure (2)

• In this case 'AL' is the name of a particular
  register - AL and AH combined make up the AX
  register. (Remember, there are usually 32
  registers and they all are named, labelled or
  numbered to allow an Assembler programmer to use
  them specifically.)
• Is that '25' an actual decimal value of 25? In
  this case, YES because this is an example. Most
  Assembly Languages use Hexidecimal numbering for
  calculations and addressing.

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More on Program Structure (3)

• Another example
• ADD AH BH
• Here 'ADD' is the command to be executed - in
  this case an addition. "AH" as well as "BH" are
  the parameters. If we assume that AH and BH are
  the labels for parts of the AX and BX registers
  then the contents of BH will be added to the
  contents of AH and the value is usually stored in
  the first register of the two parameters - in
  this case, AH.

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More on Program Structure (4)

• The name of the instructions in many Assembly
  Languages is made of two, three or four letters.
• These instructions are also called mnemonic names
  or operation codes, since they represent a
  function the processor will perform.
• Sometimes instructions are used as follows
• ADD AL, 170
• The brackets in the second parameter indicate
  that the programmer is working with the content
  of the memory cell number 170 and not with the
  170 as a value. This is known as direct
  addressing.

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Mnemonics

• Coming up in two slides is a list of instruction
  mnemonics used by many Assemblers.
• Each listed instruction has one or two examples
  but there may be several more examples of each
  type of instruction.
• In actual programs there will most often be
  numbers or addresses pertaining to the mnemonic
  that follow the three- or four-letter instruction.

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Mnemonics (2)

• Also, there may be variations on the instruction.
  
• An example of that is MOV since this instruction
  does not need parameters when it appears as MOVS.
  
• In this case the 'S' is a dedicated register for
  this type of instruction.

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Instruction Types

• Transfer instructions
• Transfer instructions are used to move the
  contents of the operators.
• MOV (parameters)
• MOVS

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Instruction Types (2)

• Loading instructions
• Loading instructions are used to load bytes or
  groups of bytes into a register.
• LODS

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Instruction Types (3)

• Stack instructions
• Stack instructions allow the 'stack' to be used
  to store or retrieve data.
• POP (parameter)
• PUSH (parameter)

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Instruction Types (4)

• Logic instructions
• Logic instructions are used to perform logic
  operations on the operators.
• AND OR TEST

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Instruction Types (5)

• Arithmetic instructions
• Arithmetic instructions are used to perform
  arithmetic operations on the operators.
• ADD (parameters)
• SUB (parameters)
• MUL (parameters)
• DIV (parameters)

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Instruction Types (6)

• Jump instructions
• Jump instructions are used to transfer the flow
  of the process to the indicated operator.
• JMP (parameter)

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Instruction Types (7)

• Loop instructions
• Loop instructions are used transfer the process
  flow, conditionally or unconditionally, to a
  destiny, repeating this action until the counter
  is zero.
• LOOP (parameter)

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Instruction Types (8)

• Counting instructions
• Counting instructions are used to decrease or
  increase the content of the counter or counters.
• INC (parameter)
• DEC (parameter)

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Instruction Types (9)

• Comparison instructions
• Comparison instructions are used to compare
  operators.
• CMP (parameters)

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Instruction Types (10)

• Flag instructions
• Flag instructions directly affect the content of
  the flag or flags.
• CLI (CLear Instruction flag)
• STI (STart Instruction flag)

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NASM - Microsoft's Assembler

• The Netwide Assembler, NASM, is an 80x86
  assembler designed for portability and
  modularity. It supports a range of object file
  formats, including
• Linux a.out and ELF,
• (ELF Executable and Linkable Format object
  files),
• NetBSD/FreeBSD,
• (BSD Berkley Software Distribution)
• COFF, (Common Object File Format)
• Microsoft 16-bit OBJ, (Object files)
• Win32 and
• rdf (relocatable dynamic object file format)

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NASM - Microsoft's Assembler (2)

• NASM will output plain binary files.
• Its syntax is designed to be simple and easy to
  understand, similar to Intel's but less complex.
• It supports Pentium, P6 and MMX opcodes and has
  macro capability.

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NASM - Microsoft's Assembler (3)

• The Netwide Assembler came about because there
  was a need for a good free x86-series assembler
  that system programmers (or anybody) could use.

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NASM Versus Others

• Comparing other assemblers to NASM
• A86 is good, but not free. It is DOS only. Users
  have to pay for 32-bit capability.
• AS86 is Linux-specific It does not have much
  documentation.
• Gas is free, and ports over DOS and Unix, but it
  is not very useful since error checking is
  minimal. The syntax is difficult to work with.
  You cannot write 16-bit code using this assembler.

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NASM Versus Others (2)

• MASM (Microsoft Macro Assembler) is expensive.
  It runs under DOS only.
• TASM (Turbo Assembler) is user friendly. Its
  syntax is much like MASM's. It is expensive. It
  is DOS-only.

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NASM with Operating Systems

• NASM can be used with the following operating
  systems
• Windows
• DOS (Disk Operating System)
• Linux
• BSD (Berkley Software Distribution)
• QNX (A real time Operating System based on Unix)

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NASM with Operating Systems (2)

• Can you assemble and use the same NASM code
  across multiple Operating Systems? A generic NASM
  subroutine will assemble and be usable under
  every operating system it supports.

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NASM with Operating Systems (3)

• NASM has the widest set of books that teaching
  assembly language programming to beginners in a
  non-DOS environment.
• NASM should be compatible with two other 'asm'
  assemblers MASM and TASM. It is not compatible
  with FASM (Flat Assembler), though.

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MASM

• The name MASM originally referred to Macro
  Assembler but over the years it has become
  synonymous with Microsoft Assembler.
• MASM, as a programming tool, had its earliest
  version in 1981.
• MASM has Intel syntax for writing x86 assembler
  and it is a de facto standard in terms of its
  compatibility with Microsoft object file formats.
• But

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MASM (2)

• Many people do not like code that is not open
  source. (MASM is not open source.)
• MASM is not the fastest assembler available.
• MASM is not the most powerful assembler
  available. (E.g. TASM is faster.)
• Because using MASM involves the use of MS-DOS
  operating system it is of little use to
  programmers wanting to learn or use assembly
  language on modern Operating Systems.

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TASM

• Turbo Assembler is, like many of them, a
  stand-alone assembler.
• It includes all the tools needed to create and
  debug assembly programs for 16 and 32 bit DOS and
  Windows platforms, including Windows 3.X, Win95,
  Win98, and NT. Some of the tools included are
  assemblers, linkers, console style debuggers, and
  resource compilers. Each of these tools comes in
  a 16 bit and a 32 bit version.
• It is Borland Turbo Assembler. It has full Intel
  chip set support-from 8086 to Pentium.

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TASM (2)

• Specifications include
• Up to 48,000 lines-per-minute assembly
• Full 8088, 8086, 80286, 80386, i486, and Pentium
  support
• IDEAL and MASM assembly modes
• Interface support for C, C, Pascal, FORTRAN,
  and COBOL
• Multi-pass assembler with forward-reference
  resolution
• Fast 16- and 32-bit Turbo Linker
• Turbo Debugger for DOS and Windows

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FASM

• Flat Assembler is an Intel x86 macro Assembler
  for MSDOS, Win32 and Linux systems that accepts
  16 bit and 32 bit 80x86/Pentium code, MMX, SSE,
  SSE2 instructions and macros.
• It has support for code optimisation and can
  generate binary files, MZ and PE executables.
• FASM Is not widely used since it is not as
  flexible as NASM and MASM.