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

Pep/7’s architecture

Features in Pep/7

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 (2 nd and 3 rd bytes) • Holds the operand itself or an address

The Pep/7 instruction format

11

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