CPS120: Introduction to Computer Science

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CPS120 Introduction to Computer Science

• Low-Level Programming Languages

Nell Dale John Lewis
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Bits, Bytes and Characters

• Bytes are frequently used to hold individual
  characters in a text document. In the ASCII
  character set, each binary value between 0 and
  127 is given a specific character.
• Most computers extend the ASCII character set to
  use the full range of 256 characters available in
  a byte.
• The upper 128 characters handle special things
  like accented characters from common foreign
  languages

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ASCII

• Each byte contains not a letter but a number --
  the number is the ASCII code corresponding to the
  character (see below). So on disk, the numbers
  for the file look like this
• F o u r a n d s e v e n
• 70 111 117 114 32 97 110 100 32 115 101 118 101
  110
• By looking in the ASCII table, you can see a
  one-to-one correspondence between each character
  and the ASCII code used.
• Note the use of 32 for a space -- 32 is the ASCII
  code for a space.
• We could expand these decimal numbers out to
  binary numbers (so 32 00100000) if we wanted to
  be technically correct -- that is how the
  computer really deals with things.

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What the Computer Can Do

• Store, retrieve and process are actions that the
  computer can take on data

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Computer Operations

• A computer is a programmable electronic device
  that can store, retrieve, and process data
• Data and instructions to manipulate the data are
  logically the same and can be stored in the same
  place
• Store, retrieve, and process are actions that the
  computer can perform on data

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Machine Language

• Machine language the instructions built into the
  hardware of a particular computer

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Machine Language

• Every processor type has its own set of specific
  machine instructions
• The relationship between the processor and the
  instructions it can carry out is completely
  integrated
• Each machine-language instruction does only one
  very low-level task

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Pep/7 A Virtual Computer

• A virtual computer is a hypothetical machine
  designed to contain the important features of
  real computers that we want to illustrated

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Features in Pep/7

• Pep/7 has 32 machine-language instructions
• The memory unit is made up of 4,096 bytes of
  storage (0-4095 decimal)
• The word length in Pep/7 is 16 bits
• Pep/7 has seven registers, four of which we focus
  on at this point
• The program counter (PC) (contains the address
  of the next instruction to be executed)
• The instruction register (IR) (contains a copy
  of the instruction being executed)
• The index register (X register) holds data
• The accumulator (A register) holds data

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Features in Pep/7
Pep/7s architecture
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Instruction Format

• There are two parts to an instruction
• The 8-bit instruction specifier (1rst byte)
• Indicates the operation to be carried out
• And optionally, the 16-bit operand specifier (2nd
  and 3rd bytes)
• Holds the operand itself or an address

The Pep/7 instruction format
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Instruction Format

• The instruction specifier is made up of several
  sections
• The operation code
• The register specifier
• 0 for A 1 for X
• The addressing-mode specifier
• 00 immediate mode last two bytes have operand
• 01 direct mode last two bytes contain an address

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Instruction Format

• The operation code specifies which instruction is
  to be carried out
• The 1-bit register specifier is 0 if register A
  (the accumulator) is involved in the operation
  and 1 if register X (the index register) is
  involved
• The 2-bit addressing-mode specifier says how to
  interpret the operand part of the instruction

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Instruction Format
Difference between immediate-mode and direct-mode
addressing
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Some Sample Instructions
Subset of Pep/7 instructions
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A Program Example

• Lets write "Hello" on the screen

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Pep/7 Simulator

• A program that behaves just like the Pep/7
  virtual machine behaves
• To run a program, we enter the hexadecimal code,
  byte by byte with blanks between each

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Assembly Language

• Assembly languages assign mnemonic letter codes
  to each machine-language instruction
• The programmer uses these letter codes in place
  of binary digits
• A program called an assembler reads each of the
  instructions in mnemonic form and translates it
  into the machine-language equivalent

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Hello -- Assembly

• CHARO CH/ J Output 'H'
• CHARO Ce/ J Output 'e'
• CHARO Cl/ J Output 'l'
• CHARO Cl/ J Output 'l'
• CHARO Co/ J Output 'o'
• STOP
• END

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Pep/7 Assembly Language
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Figure 7.5 Assembly Process
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A New Program
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Our Completed Program
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Testing

• Test plan a document that specifies how many
  times and with what data the program must be run
  in order to thoroughly test the program
• A code-coverage approach designs test cases to
  ensure that each statement in the program is
  executed
• Data-coverage testing is another approach it
  designs test cases to ensure that the limits of
  the allowable data are covered