CPS120: Introduction to Computer Science

Low-Level Programming Languages

Nell Dale • John Lewis

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

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.

What the Computer Can Do

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

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

Machine Language

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

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

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

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

Features in Pep/7

Pep/7’s 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

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

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

Instruction Format

Difference between immediate-mode and direct-mode addressing

Some Sample Instructions

Subset of Pep/7 instructions

A Program Example

• Let’s write "Hello" on the screen

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

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

Hello -- Assembly

CHARO C#H/ J; Output 'H'

CHARO C#e/ J; Output 'e'

CHARO C#l/ J; Output 'l'

CHARO C#l/ J; Output 'l'

CHARO C#o/ J; Output 'o'

STOP

END

Pep/7 Assembly Language

Figure 7.5 Assembly Process

A New Program

Our Completed Program

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