From:	XXguille AT XXiies DOT XXes (Guillermo Rodriguez Garcia)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Allocate dos memory
Date:	Sun, 25 Apr 1999 23:58:01 GMT
Organization:	Telefonica Transmision de Datos
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El día Fri, 23 Apr 1999 01:48:10 GMT, tdu AT enter DOT net (Tim Updegrove)
escribió:

>I'm still trying to allocate memory and get a physical address for a
>DMA controller.  As a temporary approach to my physical address
>problem, I've tried to use __dpmi_allocate_dos_memory.  The below
>piece of code works but I have several questions:

Here you have a function which allocates DOS memory suitable for DMA
transfers, ensuring that the block doesn't cross a page boundary; it
is based in Allegro, but for 16 bit DMA it supports blocks up to 128
KB (I think Allegro's was limited to 64 KB). On success, it fills
*paddr with the physical address of the block to use, and *psel with
the selector (needed to free the block) and returns 0. On error, it
returns -1.

You don't need to manually lock this memory.

Hope this helps.

=== Begin ===
int dma_alloc(unsigned size, int *paddr, int *psel)
{
    int seg;

    if (size > 131072) return -1;

    seg = __dpmi_allocate_dos_memory((2 * size + 15) >> 4, psel);

    if (seg == -1)
    {
        *paddr = 0;
        *psel  = 0;
        return -1;
    }

    *paddr = seg << 4;

    if (size <= 65536)
    {
        if ((*paddr >> 16) != ((*paddr + size) >> 16))
            *paddr = (*paddr + size) & 0xFFFF0000;
    }
    else
    {
        if ((*paddr >> 17) != ((*paddr + size) >> 17))
            *paddr = (*paddr + size) & 0xFFFE0000;
    }

    return 0;
}

=== End ===

Regards,
GUILLE
----
Guillermo Rodriguez Garcia
XXguille AT XXiies DOT XXes (ya sabes :-)

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