Access contents of pre-specified memory address

This is a discussion on Access contents of pre-specified memory address within the C Programming forums, part of the General Programming Boards category; Hello, I am attempting to write some code to be executed in MS-DOS that will copy data from the 0xC000 ...

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    Access contents of pre-specified memory address

    Hello, I am attempting to write some code to be executed in MS-DOS that will copy data from the 0xC000 segment into the 0x8000 segment (this has to do with the video bios).

    My problem is that I can not figure out how to access the contents of a specific memory address. From what I have been reading this can be done is DOS, obviously not in windows though.

    I see that 'far pointers' will be needed to accomplish this since the data are in two different segments (the memory is represented as segment: offset), but can't seem to even be able to write code that will read the contents of a specified memory address and display them in a printf statement.

    For example, can anyone show me how to access the contents of memory address 0xC0000000 and print the contents out in a printf statement?

    Any help is greatly appreciated.

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    DOS compilers usually had some kind of MKFP macro to make a far pointer for you.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    i found the macro in the dos.h file but i don't know how to use it.

    #define _MK_FP(seg, offset) (void __far *)(((unsigned long)seg << 16) \
    + (unsigned long)(unsigned)offset)

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    void *p = _MK_FP(0xc000,0x0000)
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    ok thanks, i figured that part out. now for the second part. how do i change the INT10 vector to point to 8000h segment?

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    Quote Originally Posted by pole1080 View Post
    ok thanks, i figured that part out. now for the second part. how do i change the INT10 vector to point to 8000h segment?
    In <dos.h> there's a union struct called REGS. Create a variable of that type, initialize its member ax to to 0x8000. Now invoke the BIOS services thro' int86() with 0x10 as its first argument.
    Last edited by itCbitC; 06-08-2010 at 03:33 PM.

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