F453 - low level languages

Question 1

Explain the term opcode

Answer 1

the mnemonic part of the instruction/that indicates what it is to do/code for the operation

Question 2

What is symbolic addressing?

Answer 2

the use of characters to represent the address of a store location

Question 3

What is the purpose of the accumulator?

Answer 3

temporary storage (within ALU)
holds data being processed/used during calculations
deals with the input and output in the processor

Question 4

What is indexed addressing?

Answer 4

uses an index register/IR
…and an absolute address…
…to calculate addresses to be used

Question 5

What is direct addressing?

Answer 5

the instruction gives the address to be used

Question 6

What is relative addressing?

Answer 6

allows a real address to be calculated…
…from a base address…
…by adding the relative address
relative address is an offset
can be used for arrays
can be used for branching

Question 7

What’s immediate addressing?

Answer 7

used in assembly language
uses data in address field…
… as a constant

Question 8

Why is it not possible to use only direct addressing in assembly languages?

Answer 8

number of addresses available is limited…
…by the size of the address field
code is not relocatable/code uses fixed memory locations

Question 9

Explain Mnemonics

Answer 9

a code that is easily remembered…
…used to give the opcode/instruction
e.g. ADD

Question 10

Explain flow control

Answer 10

the order in which instructions are executed
the order may be changed by a jump
instruction/conditional jump instruction

Question 11

How and why is the index register (IR) used?

Answer 11

used in indexed addressing
stores a number used to modify an address…
… which is given in an instruction
allows efficient access to a range of memory locations/by incrementing the value in the IR
eg used to access an array

Question 12

What are the differences between machine code and assembly language?

Answer 12

Machine Code:
written in binary or hex
no translation needed
very difficult to write
Assembly language:
includes mnemonics
includes names for data stores
translated by an assembler
easier to write than machine code, but more difficult than high level language

Question 13

What’s the use of an operand, in an assembly language instruction?

Answer 13

address field (in an instruction)
it holds data…
to be used by the operation given in the opcode
eg in ADD 12, “12” is the operand

Question 14

What’s the difference between direct and indirect addressing?

Answer 14

direct:
the simplest/most common method of addressing
uses the (data in) the address field…without modification
eg In ADD 23, use the number stored in address 23 for
the instruction (accept any valid example)
limits the memory locations that can be addressed *
indirect:
uses the address field as a vector/pointer… to the address to be used
used to access library routines
eg In ADD 23, if address 23 stores 45, address 45 holds the number to be used increases the
memory locations that can be addressed

Question 15

Describe the term assembly language

Answer 15

 a language related closely to the computer being
programmed/low level language/machine specific
 uses descriptive names (for data stores)
 uses mnemonics (for instructions)
 uses labels to allow selection
 each instruction is generally translated into one machine code
instruction
 may use macros

Question 16

Describe the term machine code

Answer 16

 binary notation
 set of all instructions available
 to the architecture/which depend on the hardware design of the
processor
 instructions operate on bytes of data