Re: Mode 13h

["Don Williamson" <DonWilliamson@aqua-pacific.com>]

This is just a polite request, but could you PLEASE check your sources
before you go giving example information to people? The memory location you
specified in the mail is wrong.

The location number is dependant upon whether you're programming in real
mode or protected mode. In real mode, memory is wildly segmented and the
location of video memory is at 0xA000:0000, that's segment 0xA000, offset 0,
which is completely different. In this case your pointer would be...

unsigned char far *vbuf = (unsigned char far *)0xA0000000L;

It looks like most people use DJGPP though, which means addressing is in
protected mode. Usually this means you can have access to memory linearly,
all your segments and offsets are disolved and you can calculate an
equivalent protected mode address like so...

linear address = segment * 10h + offset

This is because each segment in real mode is seperated by a 16-byte
boundary. So for DJGPP (or, protected mode), your address would become...

A000h * 10h + 0 = A0000h
unsigned char *vbuf = (unsigned char *)0xA0000L;

It can be a little more complicated than that I believe with DJGPP since in
protected mode we still have segments, albeit much more bigger and flexible
ones (up to 4 gig in size, depending on the granularity bit). The thing is,
DOS extenders like DOS4G/W will allow you to do the above but the extender
that comes with DJGPP (CWSDPMI?) needs you to do the following...

unsigned char *vbuf = (unsigned char *)0xA0000 + __djgpp_conventional_base;

(DOS4G/W actually does this calculation for you dynamically, reducing your
worries - great extender that :0)

Seeya,
- Don

-----Original Message-----
From: Nick Hill <nikhilwiz@yahoo.com>
To: gameprogrammer@gameprogrammer.com <gameprogrammer@gameprogrammer.com>
Date: 08 September 1999 08:37
Subject: Re: Mode 13h


>make a pointer to 0xA000
>unsigned char *VidMem=0xA000. This makes a pointer to tha vid. mem.
>Mode 13h is a linear buffer where 1 byte represents one pixel. u can
>plot a pixel by writing the color of tha pixel to tha particular mem.
>loc. Since the data in the vid. mem. start from (0,0) and proceeds
>along tha row, the first 320 bytes represents tha first row., u get tha
>idea. so tha formula to calc. tha byte at (x,y) on screen will be :
>
>pos=y * 320 + x;
>VidMem[pos]=color;
>
>where color is tha color of tha pixel u wanna plot. Keep it under 255,
>as Mode 13h s'portz only that much. thazz why tha data type on VidMem
>also is defined as unsigned char... so that u wont end up writing
>NEthin' oth' than 0-255. Hope this helped.
>
>Nikhil.
>
>--- Peter Peterssen <getoutnow@Hotmail.com> wrote:
>> Hello..
>>
>> Mode 13H. Wow. Then, perhaps you can tell me how the
>> heck I draw a dot on
>> the screen like X and Y ?
>>
>> I do call 0xA0000 and the use the..
>> outportb(0x3c9, red);
>> outportb(0x3c9, green);
>> outportb(0x3c9, blue);
>>
>> .. for the background layout and then ..
>>
>> b= 0;
>> pokeb(0xa0000, B,30)
>>
>> .. for the actual dot.
>>
>> This doensnt work unless i add 1 to red OR green OR
>> blue. ( I dont see the
>> dot on the screen).
>>
>> So, as you see Im totaly lost and really dont get
>> the whole thing. Do you
>> have an example how to structure something?
>>
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