Program to write to memory map addresses

I am trying to write a very simple C program in which I am trying to write to the memory on my computer in DOS. This is what I am trying to do.

Say I have the data stored as:

Code:

---

short data=0x1111;

---

and my memory location and size is defined as follow:

Code:

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#define ADDRESSOFMEMORY 0xe0000 //address of memory
#define SIZEOFMEMORY 0x4000  //size of meory

---

Basically I am trying to write the data (0x1111) at the memory map location 0xe000.

How can I do that?

First of all, you probably don't want to do that. You'd only want to do that unless you're doing graphical stuff in mode 13h or something, and not only is that a bad idea, but you also need a DOS compiler to do it.

To answer your question:

Code:

---

short *memory = (short *)ADDRESSOFMEMORY;

*memory = 0x1111;

---

That should work, I think. You'll almost certainly get a segmentation fault if you try it with most modern compilers, however.

Quote:

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Originally Posted by dwks

First of all, you probably don't want to do that. You'd only want to do that unless you're doing graphical stuff in mode 13h or something, and not only is that a bad idea, but you also need a DOS compiler to do it.

To answer your question:

Code:

---

short *memory = (short *)ADDRESSOFMEMORY;

*memory = 0x1111;

---

That should work, I think. You'll almost certainly get a segmentation fault if you try it with most modern compilers, however.

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Thanks for the info. I am in a situation where I have to run a program like this.

Also what if I have to write at 4 consequetive memory locations (say e000h,e0002h, e0004h, e0008h) with the data 0x1111.

For example I have this pseudo code:

Code:

---

for(memoffset=0x0;memoffset<0x8;memoffset+=2) 
    {
        memory[memoffset]=data; 
    }

---

how can I build up a simple program using all the above.

You should be able to figure it out . . . just declare memory as I have done, and then modify it as you have done.

Also remember that memory+1 is one byte beyond memory+0, if you use chars, but it's two bytes ahead, if you use shorts (on a 32-bit machine). So in other words, your loop would probably look like this:

Code:

---

for(memoffset=0x0;memoffset<0x8;memoffset++)

---

Because every time you increment memoffset, you're moving ahead sizeof(short) bytes.

Quote:

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Originally Posted by dwks

You should be able to figure it out . . . just declare memory as I have done, and then modify it as you have done.

Also remember that memory+1 is one byte beyond memory+0, if you use chars, but it's two bytes ahead, if you use shorts (on a 32-bit machine). So in other words, your loop would probably look like this:

Code:

---

for(memoffset=0x0;memoffset<0x8;memoffset++)

---

Because every time you increment memoffset, you're moving ahead sizeof(short) bytes.

---

Thanks for your persistence.

Here is the code that I have written. Although it compiles fine and runs I am not sure if I wrote it correctly to do as I want it to do. A quick review will be really helpful.

Code:

---

//The essentials
#include <stdio.h>
#include <conio.h>
#include <stdlib.h>

#define ADDR 0xe0000


    void main (void)
    {
        //Declaration of Variables     
 
        int offset; 

        //Declare a pointer to the memory
        short *memory = (short *)ADDR;

        short data=0x1234;

        for(offset=0x0;offset<0x8;offset++) 
        {
            memory[offset]=data;
        }       
    }

---

I am new to this area and I am trying to learn.

Thank you once again.

Your logic looks pretty good to me. One thing I forgot to mention is that if you're trying to fill in four consecutive blocks of memory, you'd want to use 0x4 instead of 0x8 in your loop.

Of course, there are some other things with your program that could be improved. For example:

• conio.h is non-standard. You're non using anything from it, anyway, so why include it?
• void main() is also non-standard. You should use int main(). There are several FAQ articles about this, for example:
  
  • http://faq.cprogramming.com/cgi-bin/...&id=1043284376
  • cpwiki.sf.net/Void_main

Quote:

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Originally Posted by dwks

Your logic looks pretty good to me. One thing I forgot to mention is that if you're trying to fill in four consecutive blocks of memory, you'd want to use 0x4 instead of 0x8 in your loop.

Of course, there are some other things with your program that could be improved. For example:

• conio.h is non-standard. You're non using anything from it, anyway, so why include it?
• void main() is also non-standard. You should use int main(). There are several FAQ articles about this, for example:
  
  • http://faq.cprogramming.com/cgi-bin/...&id=1043284376
  • cpwiki.sf.net/Void_main

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Thanks dwks!

Also I noticed that you mentioned to modify the for loop as:

Code:

---

for(memoffset=0x0;memoffset<0x8;memoffset++)

---

instead of:

Code:

---

for(memoffset=0x0;memoffset<0x8;memoffset+=2)

---

Is that true even if my data variable contains following:

Code:

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short data=0x11;

---

instead of:

Code:

---

short data=0x1111;

---

?

For example, as far as I understand writing 0x1234 will cause the following:

e000:0000 - 34
e000:0001 - 12

What would happen if my data is just 0x12 ?
Will it just write to first location and skip to the e000:0002, like this:

e000:0000 - --
e000:0001 - 12
e000:0002 - <Next Two bytes>

Thank you once again for helping me out.

Quote:

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Originally Posted by modest

What would happen if my data is just 0x12 ?
Will it just write to first location and skip to the e000:0002, like this:

e000:0000 - --
e000:0001 - 12
e000:0002 - <Next Two bytes>

---

Then it would look like:
e000:0000 - 00
e000:0001 - 12
e000:0002 - <Next Two bytes>

I think I'll explain a bit about how pointers work. When you have a char* pointer, each element is one byte, so each time you increment the pointer it goes up by one byte. For a short* pointer, assuming 2 byte shorts, each element is 2 bytes, so each time you increment the pointer it advances two bytes.

This is actually what you want. You want to do this

Code:

---

short data[] = {1, 2, 3};
printf("%hd\n", *(data + 2));
printf("%hd\n", data[2]);

---

rather than this.

Code:

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short data[] = {1, 2, 3};
printf("%hd\n", *(data + 2 * sizeof(short)));
printf("%hd\n", data[2 * sizeof(short)]);

---

In other words, remember that a short pointer goes up in 2-byte increments.

(Incidentally, this is also why you can't dereference void pointers. Because the compiler doesn't know how big each element is, it can't calculate *(void_pointer + 1).)

Guys thank you very much for all the advice and help. It seems that my program is not writing to the desired location (e000h) I verified this with DOS "debug" utility. How can I debug my program?

Do you want to learn how to debug a program or what is wrong with your specific program?

If the latter, post your current code.

Quote:

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Originally Posted by robwhit

Do you want to learn how to debug a program or what is wrong with your specific program?

If the latter, post your current code.

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here is my program that I am using to write to address location e000h

Code:

---

//The essentials
#include <stdio.h>
#include <stdlib.h>

#define ADDR 0xe0000


    int main (void)
    {
        //Declaration of Variables     
 
        int offset; 

        //Declare a pointer to the memory
        short *memory = (short *)ADDR;

        short data=0x1234;

        for(offset=0x0;offset<0x4;offset++) 
        {
            memory[offset]=data;
        }       
    }

---

Thanks for offering your help.

I wasn't offering my help, I was just asking you so that people could help you if they wanted.

> #define ADDR 0xe0000

hmm...

But are you actually running this in DOS or some other OS (Windows, Linux, Mac)?

Quote:

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Originally Posted by Elysia

But are you actually running this in DOS or some other OS (Windows, Linux, Mac)?

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DOS.

In DOS, what memory model are you using?

You probably need to use a for most memory models (whatever compiler extension that may be), or set the compiler options to generate (pick the right flavor) LARGE or HUGE code.