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From:	Eric <NoOne AT invalid DOT com>
Subject:	Re: dos buffer
Newsgroups:	comp.os.msdos.djgpp
Date:	Thu, 20 Apr 2006 15:39:40 -0700
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Rod Pemberton wrote:

> 
> "Eric" <NoOne AT invalid DOT com> wrote in message
> news:A8adnbpWz9FuPdjZnZ2dnUVZ_sednZ2d AT comcast DOT com...
>> Can anyone point me to some sample code that shows how to allocate a dos
>> buffer (below 1 meg). I want to understand how to do things this way and
>> not rely on tb.
>> For instance:
>>   allocate buffer below 1 meg - size -32k
>>   fill buffer with data and read it back
>>   store actual buffer address in say regs.r.ax and regs.r.bx
>>   free buffer
>>
>> I looked at __dpmi_allocate_dos_memory but it gets you a selector and I
> dont
>> know what to do to convert that to a pointer
>>
> 
> Now, the memory below 1Mb is physically mapped per the DPMI specification,
> but a buffer in that region still needs to be allocated.  Unfortunately,
> DOS memory must be allocated through the DPMI host and not directly
> through DOS
> itself.  It handles some other issues and allocates from DOS.  Therefore,
> to allocate DOS memory, you must use either one of these:
> 
> 1) __dpmi_allocate_dos_memory
> 2) _go32_dpmi_allocate_dos_memory
> 
> Now, _go32_dpmi_allocate_dos_memory() uses segments and offsets through a
> structure, like you want.  However, I'm not fond of the _go32_xxx
> functions
> because the set of functions is incomplete.  So, you end up spending all
> your time reformating segments, offsets, etc. to switch between the _go32
> functions and _dpmi functions...
> 
> Anyway, I've written two examples.  They both compile cleanly, but haven't
> been thoroughly tested (i.e., possible bugs...).  If there are any bugs,
> you'll need to sort them out yourself.
> 
> Method 1)
> ----
> #include <dpmi.h>
> #include <go32.h>
> #include <sys/farptr.h>
> 
> #define BUFF 32768
> int main(void)
> {
>    unsigned long base;
> /*   unsigned short segment,offset; */
>    int selector;
>    char buf[BUFF];
> 
> /* base is the physical address of the low mem buffer */
> /* selector is used for virtual addressing, i.e., within application space
> */
>    __dpmi_allocate_dos_memory((BUFF+15)>>4,&selector);
>    __dpmi_get_segment_base_address(selector,&base);
> 
> #if 0
>    /* this is how to calculate segment & offset, if needed */
>    segment=(unsigned long)base>>4;
>    offset=(unsigned long)base&0x0F;
> #endif
> 
>    dosmemput(buf, BUFF, base); /* copy to low mem */
>    /* something done to low mem buffer: DOS call, farpoke */
>    _farpokeb(selector,12,0x20); /* set low mem buf[12] to 0x20 */
>    dosmemget(base, BUFF, buf); /* copy from low mem */
>    __dpmi_free_dos_memory(selector);
> 
> return(0);
> }
> 
> Method 2)
> ----
> #include <string.h>
> #include <dpmi.h>
> #include <go32.h>
> #include <sys/farptr.h>
> #include <sys/nearptr.h>
> 
> #define BUFF 32768
> int main(void)
> {
>    unsigned long base,CS_base;
> /*   unsigned short segment,offset; */
>    int selector;
>    unsigned char buf[BUFF];
>    unsigned char *bufl;
> 
> /* base is the physical address of the low mem buffer */
> /* selector is used for virtual addressing, i.e., within application space
> */
>    __dpmi_allocate_dos_memory((BUFF+15)>>4,&selector);
>    __dpmi_get_segment_base_address(selector,&base);
> 
> /* get the physical starting address of the code segment */
> /* and enable full address range */
>    __dpmi_get_segment_base_address(_my_cs(),&CS_base);
>    __djgpp_nearptr_enable();
> 
> #if 0
>    for physical-to-virtual addressing use: (e.g., screen=0xB8000)
>             #ifdef __DJGPP__
>                 "variable"  -=CS_base;
>             #endif
>    for virtual-to-physical addressing use: (e.g., lgdt,lidt,etc.)
>             #ifdef __DJGPP__
>                 "variable"  +=CS_base;
>             #endif
> #endif
> 
>    bufl=(unsigned char *)base; /* create a pointer to low mem */
>    bufl+=CS_base; /* convert virtual to physical address */
> 
> #if 0
>    /* this is how to calculate segment & offset, if needed */
>    segment=(unsigned long)bufl>>4;
>    offset=(unsigned long)bufl&0x0F;
> #endif
> 
>    memcpy(bufl,buf,BUFF); /* copy to low mem */
>    /* something done to low mem buffer: DOS call, farpoke */
>    _farpokeb(selector,12,0x20); /* set low mem buf[12] to 0x20 */
>    memcpy(buf,bufl,BUFF); /* copy from low mem */
>    __dpmi_free_dos_memory(selector);
> 
> return(0);
> }
> 
> 
> HTH,
> 
> Rod Pemberton
Hey! Thanks a lot, that helped immensely!
Now i have something i can work with.
Thanks again,
Eric

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