# ALU/FPU register width performance impact on big number co..

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Bubba

March 3, 2005 12:17:43 AM

Archived from groups: comp.sys.ibm.pc.hardware.chips (More info?)

Greetings to everyone,

If we take a processor with 16bit precise ALU and try to compute a

factorial (n!) form 2^80, what happens with ALU then? To be more precise,

does ALU width (precision) limits it from computing numbers higher than it

can allocate and how (if?) does it impacts performance of integer

computing? In other words, how come it is possible to compute very large

number if ALU is not precise enough for them (or my calculator is being

dishonest).

I hope I made my point clearly enough. Thanks in advance.

--

Kupio sam pistolj od svercera na crno,

Na tebe cu rado da potrosim zrno...

Greetings to everyone,

If we take a processor with 16bit precise ALU and try to compute a

factorial (n!) form 2^80, what happens with ALU then? To be more precise,

does ALU width (precision) limits it from computing numbers higher than it

can allocate and how (if?) does it impacts performance of integer

computing? In other words, how come it is possible to compute very large

number if ALU is not precise enough for them (or my calculator is being

dishonest).

I hope I made my point clearly enough. Thanks in advance.

--

Kupio sam pistolj od svercera na crno,

Na tebe cu rado da potrosim zrno...

More about : alu fpu register width performance impact big number

Anonymous

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b
à
CPUs

March 3, 2005 1:46:18 AM

Bubba wrote:

> Greetings to everyone,

>

> If we take a processor with 16bit precise ALU and try to compute a

> factorial (n!) form 2^80, what happens with ALU then? To be more precise,

> does ALU width (precision) limits it from computing numbers higher than it

> can allocate and how (if?) does it impacts performance of integer

> computing? In other words, how come it is possible to compute very large

> number if ALU is not precise enough for them (or my calculator is being

> dishonest).

>

> I hope I made my point clearly enough. Thanks in advance.

The really large numbers (or the really small ones too) are worked on

inside the FPU rather than the ALU. The floating point unit gives up

some level of precision for some flexibility in estimation.

Yousuf Khan

Anonymous

a
b
à
CPUs

March 3, 2005 2:41:47 AM

Bitstring <Xns960DE2CE896C7bubbachipsetone@130.133.1.4>, from the

wonderful person Bubba <nickname@hcp.hr> said

>Greetings to everyone,

>

>If we take a processor with 16bit precise ALU and try to compute a

>factorial (n!) form 2^80, what happens with ALU then? To be more precise,

>does ALU width (precision) limits it from computing numbers higher than it

>can allocate and how (if?) does it impacts performance of integer

>computing? In other words, how come it is possible to compute very large

>number if ALU is not precise enough for them (or my calculator is being

>dishonest).

All sensible math packages (for big numbers) break the calculation up

into smaller sized (i.e. 16 bit, 32 bit, whatever) pieces. Some simple

ones designed for very long numbers actually work in binary coded

decimal, and do long multiplication the 'very hard' way.

BTW, even 16 bit ALUs generally have a 16bit*16bit multiply, which will

give a 32 bit answer (in two registers).

Google is your friend (and yep, this sounds suspiciously like a homework

assignment, so hints is all you get).

--

GSV Three Minds in a Can

SC recommends the use of Firefox; Get smart, or get assimilated.

Anonymous

a
b
à
CPUs

March 3, 2005 11:47:13 AM

In article <Xns960DE2CE896C7bubbachipsetone@130.133.1.4>,

nickname@hcp.hr says...

> Greetings to everyone,

>

> If we take a processor with 16bit precise ALU and try to compute a

> factorial (n!) form 2^80, what happens with ALU then? To be more precise,

> does ALU width (precision) limits it from computing numbers higher than it

> can allocate and how (if?) does it impacts performance of integer

> computing? In other words, how come it is possible to compute very large

> number if ALU is not precise enough for them (or my calculator is being

> dishonest).

The same way you do arithmetic only knowing the 9x9 tables.

--

Keith

Anonymous

a
b
à
CPUs

March 10, 2005 9:58:14 PM

On Wed, 02 Mar 2005 22:46:18 -0500, Yousuf Khan <bbbl67@ezrs.com>

wrote:

>Bubba wrote:

>> Greetings to everyone,

>>

>> If we take a processor with 16bit precise ALU and try to compute a

>> factorial (n!) form 2^80, what happens with ALU then? To be more precise,

>> does ALU width (precision) limits it from computing numbers higher than it

>> can allocate and how (if?) does it impacts performance of integer

>> computing? In other words, how come it is possible to compute very large

>> number if ALU is not precise enough for them (or my calculator is being

>> dishonest).

>>

>> I hope I made my point clearly enough. Thanks in advance.

>

>The really large numbers (or the really small ones too) are worked on

>inside the FPU rather than the ALU. The floating point unit gives up

>some level of precision for some flexibility in estimation.

>

Stirling's approximation.

http://mathworld.wolfram.com/StirlingsApproximation.htm...

RM

!