Low level languages

Question 1

What is machine language?

Answer 1

The language made up of binary coded instructions that is used directly by the computer.

Question 2

What are the characteristics of machine language?

Answer 2

– Every processor type has its own specific set
of machine instructions
– The digital logic of the CPU recognizes the binary
representations of the instructions
– Each machine-language instruction does only
one (typically) very low-level task

Question 3

What is a virtual computer?

Answer 3

A hypothetical machine designed to contain
the important features of a real computer that
we want to illustrate

Question 4

How many instructions does pepe/8 have?

Answer 4

39

Question 5

What is the PC, IR and accumulator?

Answer 5

– 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 accumulator (“A”)
(used to hold data and results of operations)

Question 6

How large is the memory unit?

Answer 6

64Kb

Question 7

What is a register?

Answer 7

A small area of memory in the ALU that holds special data that is usually accessed instantly.

Question 8

What are the two parts of an instruction in pep/8

Answer 8

The instructions specifier(8 bit) and the 16-bit operand specifier.
The instruction specifier indicates which operations to be carried out such as add.
The operand specifier (second and third bytes) hold either the operand itself or the address of where the operand is to be found.

the format of the instruction specifier varies depending on the no. of bits used to represent a particular operation.the first 4/5 bits are is the opcode, if the 5 bit is not used in opcode it selects the register, and the last 3 bits is the addressing mode.

Question 9

What is the addressing mode specifier?

Answer 9

3 bits.
indicates how to interpret the operand part of the instruction.
000 - the operand is in the operand specifier of the instruction. this is called immediate (i)
001 - the operand is in the memory address named in the operand specifier. this is called direct (d)

Question 10

What are unary instructions?

Answer 10

These are instructions that do not have an operand specifier. they are 1 byte long rather then 3.

Question 11

What is another name for the accumulator?

Answer 11

The A register

Question 12

How many bytes long is the:
A register?
Instruction register?
Program counter?

Answer 12

A: 2 bytes
IR: 3 bytes
PC: 2 bytes

Question 13

What does it mean when we say that a computer is a programmable device?

Answer 13

Programmable means that data and instructions are logically the same and are stored in the same
place. The consequence of this fact is that the program the computer executes is not wired into the
hardware but entered from outside.

Question 14

List five operations that any machine language must include.

Answer 14

There must be machine-language instructions to store, retrieve, and process data, to input data and
to output data. These instructions mirror the operations of the von Neumann machine.

Question 15

What is a virtual machine? Discuss this definition in terms of the Pep/8 computer.

Answer 15

A virtual machine is a hypothetical machine designed to illustrate important features of a real computer. The Pep/8 computer is a virtual machine designed to illustrate the features of the von Neumann architecture. It has instructions to store, retrieve, and process data as well as instructions to input and output data.

Question 16

Describe the features of the Pep/8 CPU that we covered in this chapter.

Answer 16

There is one register for arithmetic and logical operations: the A register (the accumulator). There
is a Program Counter that contains the address of the next instruction to be executed and the Instruction Register that contains the instruction being executed. An operand may be immediate (stored in the instruction specifier) or direct (stored in the place named in the instruction specifier).

Question 17

How many mode specifiers are there?

Which ones are relavant and what are they?

Answer 17

8

000: instant meaning the data in the instruction specifier is the data to be used.
001: st

Question 18

How many bits are required to address the Pep/8 memory?

Answer 18

The Pep/8 memory contains 65,536 bytes, so 16 bits are required to address each one.

Question 19

What is a unary instruction?

Answer 19

An instruction that do not have an operand (data to be manipulated). they are only one byte.

Question 20

What is the code for eahc of these instructions and what do they do?:
STOP
LOAD THE OPERAND INTO THE A REGISTER
STORE CONTENTS OF A INTO OPERAND
ADD OPERAND TO A
SUBTRACT OPERAND FROM A
CHARECTER INPUT TO OPERAND
CHARECTER OUTPUT FROM OPERAND

Answer 20

STOP: 0000 - one byte long, so is a unary instruction and halts the program.
LOAD OPERAND INTO A: 1100 - this loads one word (two bytes) into the A register
STORE CONTENTS OF A INTO OPERAND: 0111
ADD OPERAND TO A: 1000
SUBTRACT OPERAND FROM A: 01001
CHARECTER INPUT TO OPERAND:01001 - this allows the program to enter an ASCII charecter from the input device while the program is running. only direct addressing is allowed.
CHARECTER OUTPUT FROM OPERAND: 01010 - this instructions sends an ASCII charecter to an output device while the program is running.

Question 21

What do the following opcodes do?
0000
1100
1110
1000
0111
01001
01010

Answer 21

0000 - halt
1100 - load operand into A register
1110 - store the contents of the A register into the operand
1000 - subtract the opernad from A register
0111 - add the opernad to the a register
01001 - charecter input to the operand
01010 charecter output from the operand

Question 22

Summerise machine code

Answer 22

●Low level instructions given in binary
● Operates directly on hardware
● Very specific operations
● Not very friendly to write or read

Question 23

What is assembly language?

Answer 23

Assembly language

A language that uses mnemonic codes to represent machine-language instructions

Question 24

What is an assembler?

Answer 24

A program that reads each of the instructions in mnemonic form and translates it into the machine-language
equivalent

Question 25

in pep/8 what is a .BLOCK and a .WORD?

Answer 25

● .BLOCK – assigns one or more bytes of memory.
● .WORD – assigns memory, and initialises it to a set
value.