Assembler

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Assembler

• Semester 1, Week 10

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Machine Code

• The combinations of 1s and 0s established in
  flip-flops and latches are the instructions and
  data of software.
• Those combinations are identifiable as machine
  code.

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Machine Code (2)

• The computer's microprocessor reads in and
  handles a certain number of 0's and 1's at a
  time.
• For example, it may be designed to read 32 binary
  digits at a time.
• Because it is designed to know how many bits (and
  which bits) tell it what operation to do, it can
  look at the right sequence of bits and perform
  the next operation. Then it reads the next
  instruction, and so on.

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Machine Code (3)

• Microprocessors (CPUs based on a single
  integrated circuit) may be classified according
  to the number of machine-code instructions that
  they are capable of obeying
• CISC (complex instruction set computer)
  microprocessors support up to 200 instructions,
  whereas
• RISC (reduced instruction set computer)
  microprocessors support far fewer instructions
  but execute programs more rapidly.

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Machine Code Example

• ADD AX, BX (Assembly Language) is encoded as
• The instruction in Hex is 03C3h

Machine Code Instruction
Assembly Instruction
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Machine Code Example (2)

• while SUB BL, DL is encoded as
• which is 2ADAh
• (001010 1 0 11 011 101 is the machine code.)

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Assembly, Language, Assembler

• First of all let us distinguish between terms
  associated with low-level instructions in
  comparison with 3rd Generation Language. The
  terms are
• High-Level Language,
• the compiler,
• Assembly,
• Assembly Language,
• Assembler.

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High-Level Language

• High-level language
• A programming language that is described as 3rd
  Generation Language (3 GL) or above 4 GL is
  usually defined as a high-level language.
• Examples of 3 GL are CoBOL, Pascal, C, C

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Compiler

• The compiler is, itself, a type of system
  software program that takes, for example, Pascal
  instructions and finds their machine code
  equivalents to (eventually) reduce 3 GL
  instructions to binary digits.
• With binary as instruction code, the central
  processing unit will be able to process the
  original instructions.

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Assembly

• Assembly
• Assembly is the term used to describe the
  translation process of high-level language to
  machine code i.e. the production, in machine
  code form, of a program that is written in a
  symbolic (higher-level) language.

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Assembly Language

• Assembly language
• The instructions that convert high-level
  language instructions into bit patterns (binary
  instruction code) are collectively known as
  Assembly language.
• As an example Pascal is a high-level language
  and is reduced to 'machine-manageable'
  instructions - i.e. Assembly language
  instructions - by using a compiler.

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Assembler

• An Assembler is the equivalent of a compiler for
  Assembly language.
• The Assembler (symbolic assembly program) is the
  program which the computer uses to produce a
  machine language (Machine code) program from a
  program written in a symbolic (or high-level)
  language.

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Assembler (2)

• An Assembler converts programs, written in a
  symbolic Assembly language, into binary words in
  'machine code'.
• Because such languages are machine-oriented,
  Assemblers, too, are machine oriented.

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Pedants Corner

• If you hear or read the term 'Assembler' to
  describe the code then that is a bad
  computer-world habit of talking about Assemblers
  (the conversion programs) and Assembly Language
  (the code) in general terms.

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Source and Object

• Source program
• Source program is the name given to the
  program written in symbolic language. (Source
  code can be high-level or low-level, actually. In
  any case, they are written by a programmer.)
• Object program
• Object program is the name given to the
  program in machine language produced by the
  Assembly process. (Consider that object code is
  made by the compiler or assembler, so appear to
  be made by the computer, itself.)

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Programmers Use of Assembler

• Because Assembly, to some extent, 'deconstructs
  instructions' a person can become familiar with
  the way programs work at a 'machine level'.
• Many programmers still prefer to write programs
  in Assembly language because this language gives
  them close control over their hardware and
  efficient execution in terms of timing and
  internal memory.

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Programmers Use of Assembler (2)

• Like machine language, Assembly languages are
  usually designed for a specific machine and
  specific microprocessors.
• For instance, there is a specific Assembly
  language associated with the Intel 80386 chip
  used in an IBM microcomputer.

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Programmers Use of Assembler (3)

• In general, there is a one-to-one correspondence
  between Machine Language (made up of lines of 0s
  and 1s) and Assembly language (made up of
  mnemonics and parameters).
• Each operation in Assembly corresponds to a
  machine operation.

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Programmers Use of Assembler (4)

• As a low level language, Assembly language does
  make use of certain mnemonics (e.g. LOAD, SUM)
  and assigns addresses and storage locations
  automatically.

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Programmers Use of Assembler (5)

• While Assembly language gives programmers great
  control, it is costly in terms of programmer
  time, difficult to read and debug and (many would
  say) difficult to learn.
• Assembly language is used primarily today in
  systems software.

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Quick Example Line of Assembler Code

• AR 5, 3
• This Assembly language command adds the contents
  of Register 3 to the contents of Register 5 and
  stores the result in Register 5.

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Information for Assembler

• Assemblers process three types of information
• absolute quantities - operation codes and
  numerical constants.
• relocatable quantities - instruction labels or
  working stores within the routine, i.e. items
  with a unique position relative to the start of
  the routine.
• cross-reference symbols.

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What the Assembler Does (1 of 2)

• The Assembler
• Translates symbolic operation codes into machine
  code and symbolic addresses into actual machine
  addresses.
• Includes the necessary linkage for closed
  subroutines.
• Allocates areas of main storage.
• Will indicate invalid source language
  instructions.

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What the Assembler Does (2 of 2)

• Produces the object program on disk or tape.
• Produces a printed listing of the object program
  together with comments.
• Notice that the fact that one symbolic
  instruction is translated into one machine
  instruction (i.e. one for one as mentioned
  earlier) is one feature which distinguishes a
  low-level language from a high-level language.

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One-Pass, Two-Pass

• There are two types of Assembler
• the one-pass (or load-and-go) Assembler, and
• the two-pass Assembler.

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One-Pass Assembler

• In the one-pass Assembler, the program is
  assembled in memory and is then executed.
• Any library routines to which reference is made
  are automatically incorporated.

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Two-Pass Assembler

• The two-pass Assembler, in the first pass, builds
  the local symbol table of labels used within the
  program or routine and the global symbol table of
  external names.
• Examples of those symbols and names include the
  references to other programs and subprograms
  those assembled with the program, or those in the
  program and routine library.

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Two-Pass Assembler (2)

• The Assembler carries out the translation using
  these tables in the second pass.
• External references are found, thereafter, by
  searching the index of the program library,
  locating, retrieving and appending these programs
  to the routine in memory.

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Loader

• Because it is easier to write a program by
  subdividing into subprograms or subroutines, most
  Assemblers have been written to assemble
  subroutines or subprograms and then combine them
  into a single entity.
• The output of the Assembler is in binary-symbolic
  or semi-compiled form and it is converted into
  'absolute' machine code by a loader.

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More Assembler Information

• The Assembler also produces the following
  information
• Relocation data / relocatable addresses
• Global (external) symbol tables
• Diagnostics for syntax errors encountered in the
  Assembly process cross reference tables

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Assembler Facilities

• The facilities offered by Assemblers include
• Conditional Assembly
• Code is included or ignored depending on
  conditions that prevail at assembly time. Some
  assemblers permit conditional or unconditional
  jumps, loops or subroutine calls to be performed.

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Assembler Facilities (2)

• Macros
• When a program written in an Assembly Language
  is assembled, the Assembler program treats every
  instruction as data, from which it produces a
  string of numbers that form a machine code
  instruction. There is no reason, however, why one
  instruction that a programmer has written, should
  be turned into only one machine code instruction.

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Macros of Assembly Language

• When an Assembler uses 12 hexadecimal digits,
  occupying 6 bytes, these can represent one
  Assembly Language instruction and one machine
  code instruction (where the machine code is made
  up of pure hex, octal or binary notation).

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Macros (2)

• If the Assembler produces 24, 36 or 144 digits,
  one Assembly language instruction would become 2,
  3 or 12 (or more) machine code instructions
• 2 machine code instructions 2 x 12 hex digits
  (24 digits)
• 3 machine code instructions 3 x 12 hex digits,
  (36 digits)
• 12 machine code instructions 12 x 12 hex
  digits, (144 digits)

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Macros (3)

• An Assembly Language instruction that generates
  more than one machine code instruction is called
  a macro instruction.
• In Assembler, a macro definition defines how to
  expand a single language statement or computer
  instruction into a number of instructions.

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Macros (4)

• A macro incorporates into the user's program a
  procedure that would carry out the same process
  in any other program that included it.
• For example, the IBM macro, DUMP causes the CPU
  to print out, line by line, the contents of every
  location in memory and then perform all the
  operations necessary to terminate the program
  currently in the computer's memory.

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Macros (5)

• A macro of this type is a complicated operation
  one Assembly language instruction has been
  converted into many machine code instructions.
• Assembler macros generate instructions that will
  be run in line with the rest of a program.

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A Frequent Macro

• The most frequent use of macro instructions is in
  Input/Output.
• Before a record can be read into the memory of
  the computer, there are many checks that need to
  be performed to ensure that the correct disk is
  being used, or that all error conditions have
  been covered, etc.

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A Frequent Macro (2)

• Perhaps records have been put in 'blocks' to save
  time on Input/Output (I/O) and the user needs to
  be certain that the correct record is extracted
  from a block.

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A Frequent Macro (3)

• The programming involved in this sort of
  operation is long and complex. However, if these
  instructions are written as a macro, they need
  only be written once, then everyone writing
  similar programs (say, among a group of
  programmers,) can use them.
• Other features include subroutines, local and
  global variables, comments and relocatability.

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End of Part One

• More on Assembler next week!