Computer skills Pointer, record, storage, file system

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Computer skills

Pointer, record, storage, file system

Jakub Yaghob

Variable

Variable

Named storage location which contains some value of a type

Storage location → address

var i : integer; f : real; b : array[1..5] of integer;

I

F

B[1]

B[2]

B[3]

B[4]

B[5]

4B

8B

4B

4B

4B

4B

4B

A

A+16

A+20

A+24

A+28

A+4

A+12

A+32

Alignment

Modern CPUs require data in memory aligned to their size

E.g. integer (4B) must have address aligned to 4

I

F

B[1]

B[2]

B[3]

B[4]

B[5]

4B

8B

4B

4B4B4B

4B

var

i

: integer; f : real;

b : array[1..5] of integer;

4B

A

A+16

A+20

A+24

A+28

A+4

A+8

A+32

A+36

Record/structure

A group of related data

Access to record fields

r.age

var r : record age : integer;

name,surname : string;

sex : char;

end;

Record/structure – example 1

var

r : record

i : integer; d : real;

end;

I

D

8B

4B

A

A+4A+12

I

D

8B

4B

A

A+4

A+16

4B

A+8

Sizeof

Size of a type in bytes

Predefined for base types

array [LB..UB] of T

(UB-LB+1) x sizeof(T)record - not enough, must be aligned 

Record/structure – example 2

var

r : record

i : integer; d : real; c : char;

end;

I

D

8B

4B

A

A+4A+12

I

D

8B

4B

A

A+4

A+16

4B

A+8

C

A+13

C

A+17

1

B

1

B

I

D

8B

4B

A

A+4

A+16

4B

A+8

C

A+17

1

B

A+24

7

B

Pointer

Address type for storing an address

Typeless

pointerPointer to a typePascal

Declarationvar p : ^integer;Dereferencingp^Address of@i

Pointer – example

var

i,j : integer; p1,p2 : ^integer;

begin

p1:=@

i; p2:=@j; write(p1^);

p1:=p2; write(p1^);end

I

J

P1

P2

Pointer – advanced example

type

node = record

i : integer; next : ^node;

end;

var

list : ^node;

LIST

I

NEXT

INEXT

I

NEXT

nil

LIST

I

NEXT

I

NEXT

I

NEXT

nil

External storage

Capacity larger then operating memory

Slower then operating memory

Not directly addressable by CPU

Sectors512B x 4KAddressingLBA x CHSHDDRotating platters, heads, clustersSSDFlash modules

Multiples of bytes

Decimal

Binary

1000

kBkilobyte1024KiB kibibyte10002MBmegabyte

10242MiB

mebibyte1000

3GBgigabyte10243GiBgibibyte10004

TBterabyte10244TiBtebibyte10005

PBpetabyte10245PiBpebibyte10006

EBexabyte10246EiBexbibyte10007ZB

zettabyte10247ZiBzebibyte10008YB

yottabyte10248YiByobibyte

File

File

Resource for storing data in a storage device

Sequence of bytesText file x binary file

File headerOperationsopenread/writeseekclose

File system

File system

File naming

File storage/placementFree space managementDirectory

Helps to organize filesMany known filesystemsFAT, NTFS, EXT2, XFS, …

Data structure examples

FAT16 directory entry

0

1

23456789

AB

CD

EF0File name

ExtensionAtrCreate T

10Cre DAcc D

Update DTClusterFile size

Data structure examples

IP packet header