Studying computer architecture without knowledge of C-programming, got some questions

**mrmeng** · 09-18-2010

I'm on a foreign exchange program and studying computer architecture at university. The problem is this university it assumes the students have studied C-programming as their course in basic programming, whereas at my regular university the course in basic programming is ADA.

I am trying to catch up but need some help since I don't really know how C-language is structured.

My current problem:

Shift operations, 32-bit.
Got two functions

int func1 (unsigned word)
{
return (int) ((word<<24) >> 24);
}
int func2 (unsigned word)
{
return ((int) word <<24) >> 24;
}

How does these functions work? I understand the logical shift but not the functions.

My guess is func1 first shift all bits 24 bits to left, and then 24 bits to right, resulting in the 8 LSB will be the same but the 24 MSB will turn into 0's.

Even if my guess is correct I can't see how func2 would work differently. I would greatly appreciate if someone could ellaborate for me.

Examples with 127, 128, 255, 256 as in values?

EDIT: Represented by two's complement

**Salem** · 09-19-2010

> return ((int) word <<24) >> 24;
If the LSByte of word is 0x80 to 0xFF, then the MSBit will end up as the sign bit.

Right-shifting signed numbers is implementation-defined in C, meaning you either get zeros or the sign bit.

The result would be say
0x00000080
or
0xFFFFFF80

func1 as you said will always fill with zeros.

**mrmeng** · 09-19-2010

Thank you, makes sense

**mrmeng** · 09-19-2010

Stumbled upon another problem.

This code is supposed to return 0 when "len" = 0, but it returns memory access exception. Why does this occur and how do I fix it?

float sum_elements(float a[], unsigned len)
{
int i;
float result=0;

for (i=0; i <=len-1; i++)
result += a[i];

return result;
}

**Bayint Naung** · 09-19-2010

Because len is declared as unsigned.

Code:

for (i=0; i <=len-1; i++)      // signed and unsigned comparison

I think you should go and read some books on C...
Btw, valid indices are 0 to array_elements-1 inclusive. So shouldn't you doing for(i = 0; i < len; i++) ?

**Elysia** · 09-19-2010

First, you should read: << !! Posting Code? Read this First !! >>
Second: Unsigned integers can only hold >= 0. Therefore, if len is 0 and you subtract 1, you will get 0xFFFFFFFF, which is -1 in two complement. But since it isn't unsigned, you get the unsigned representation.
Third: <len and <=len - 1 are the same thing. It's a matter of style.

**mrmeng** · 09-19-2010

Originally Posted by Bayint Naung

Because len is declared as unsigned.

Code:

for (i=0; i <=len-1; i++)      // signed and unsigned comparison

I think you should go and read some books on C...

Thank you.

I am studying computer architecture with the 2nd year students and will enroll the course in C programming with the freshmen when they start. I am trying to learn as much as I can on my own but at the moment I need to focus on my other courses, so I greatly appreciate the help since I have very limited knowledge on C.

I'm pretty much studying 24/7, I'm in China so all courses are taught in Chinese. Needless to say it's quite difficult and time consuming, it will be easier when my course in C programming begins but as for now I appreciate all help I can get from this forum, just to keep my head above the water.

I know I am asking newbie questions, please bare with me. My course in programming starts in two weeks

**mrmeng** · 10-16-2010

I got this function, could someone explain to me what it does?

Code:

int sum_array ( int a [N] [N] [N] )
{
    int i, j, k, sum = 0;
    for (i=0; i < N; i++)
         for (j=0; j < N; j++)
              for (k=0; k < N; k++) sum+=a[k] [i] [j]
    return sum;
}

I'm supposed to change the cycle order to make the array elements consistent with the order of access in main memory (current order is from left to right), how do I do this and why?

Thread: Studying computer architecture without knowledge of C-programming, got some questions

Thread Tools

Search Thread

Display

Studying computer architecture without knowledge of C-programming, got some questions

Similar Threads

New Computer = a few questions

Computer will not boot.

Regarding Undergraduate Computer Majors

This is my last night on this computer.

Which distro should I run on my old computer?