[Year 12 SofDev] Data Type Sizes

Mon Jun 3 12:50:23 EST 2013

Thanks Robert & Kevork,
Very much appreciated!

Damien Atkinson-Buck
Head of Learning Area: Technology/Arts

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From: Kevork Krozian [mailto:kevork at edulists.com.au]
Sent: Friday, 31 May 2013 10:49 PM
To: 'Year 12 Software Development Teachers' Mailing List'
Subject: Re: [Year 12 SofDev] Data Type Sizes

Hi Folks,

Interesting discussion Damien and Robert.
I think the answers have to do with the width of the address bus of the architecture of the machine, see http://en.wikipedia.org/wiki/Address_bus .
The width of the address bus ( number of wires/bits ) determines the size of the addressable unit of memory, retrieved or stored, known as the word size, see http://en.wikipedia.org/wiki/Word_(data_type) . So an 8 bit address bus uses a word size of 8 bits, 16 uses a 16 bit word etc.
Therefore, while it may be possible to represent a character with 8 bits depending on the encoding used ( ASCII, EBCDIC) , the overriding factor is the CPU architecture that determines the addressable memory size, hence 8, 16, 32 and 64 bit word sizes would use different storage sizes for the same amount of data as either a single word size or multiples of the basic word size.
As the references quoted here indicate, while there are exceptions ( why keep things simple ??? ) , the general outline is the underlying principle of units of memory used for storage of data and different data types.

As for the Study Design purpose I don't think the physical size of memory in the implementation is what is described, rather the number of characters used for each data item. Eg. postcode is 4 characters,  name is 15 characters, state is 3 characters , age is an integer etc.

Happy to hear any other views.

Kevork Krozian
Edulists Creator Administrator
www.edulists.com.au<http://www.edulists.com.au>
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From: sofdev-bounces at edulists.com.au<mailto:sofdev-bounces at edulists.com.au> [mailto:sofdev-bounces at edulists.com.au] On Behalf Of Robert Timmer-Arends
Sent: Friday, 31 May 2013 8:06 PM
To: Year 12 Software Development Teachers' Mailing List
Subject: Re: [Year 12 SofDev] Data Type Sizes

Hello Damien

First, I'm not sure that how the various data types are physically stored is part of the study design - at least, I can't see it anywhere.

Second, one reason for the different storage sizes is probably historical as much as anything. For example, C was developed on PDP-11 which was a 16-bit minicomputer, so 2 bytes as basic int storage makes sense. C++, coming along later as it did, was probably first developed on a 32-bit machine. I wouldn't be surprised if VB.NETs owes its 2-byte integers to its ancestor quickBASIC, originally written for 8088/86 machines.The other number types would have a similar heritage.

As for char, 1 byte accomodates ASCII, while 2 bytes accomodates Unicode. I don't know why, but MS was one of the early adopters of Unicode, possibly because of its flagship product MS Word, so building chars on 2 bytes would have been a logical step for them.

The boolean data type is the interesting one. In principle it only needs one bit, but fetching a single bit from memory can be very inefficient, depending on how the memory system is designed. That leads to 1 byte storage on older machines since it's more efficient to store a byte than a bit (0000 0000 or 1111 1111), although I'm not sure how that translates to 32-bit machines!

Regards
Robert T-A

----- Original Message -----
From: ATKINSON-BUCK, Damien<mailto:Damien.ATKINSON-BUCK at ivanhoe.com.au>
To: Year 12 Software Development Teachers' Mailing List(sofdev at edulists.com.au)<mailto:sofdev at edulists.com.au)>
Sent: Friday, May 31, 2013 3:42 PM
Subject: [Year 12 SofDev] Data Type Sizes

Hi folks,
            I've been working through various data dictionaries with my students and the issue of data type sizes, somebody pointed out a difference between the text and what VB.NET state, so I decided to do a bit of research into various languages and see what they store data as. Attached are the results. It's OK for SACs as we can base that on the language we're using, but just wondering what the official VCAA size might be - and if anyone can explain the reasons for the difference (I assume it's due to various compilation methods, but the brain is too fried to go into too much). Also, anyone out there using the languages that I have blank fields on that could fill them in?
Cheers
Damien

Damien Atkinson-Buck
Head of Learning Area: Technology/Arts
p: +61 3 9490 3848

e: damien.atkinson-buck at ivanhoe.com.au<mailto:damien.atkinson-buck at ivanhoe.com.au>

f: +61 3 9490 3490

w: www.ivanhoe.com.au<http://www.ivanhoe.com.au>

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