VGA and integrated graphics theory

Question

I'm not really wanting to know the ins and outs of VGA but rather the basic principle of how it works (and with integrated graphics), The Intel website says -

So this stolen memory is used as the frame buffer for the VGA adapter and any reads/writes by the VGA graphics controller will be going and coming from there?

Example system with 1MB stolen VGA memory-

So if the above system was running in VGA mode and something was written to the legacy VGA address range (0xA0000 - 0xbffff), what would the process be?

Currently my understanding is that the memory controller would forward it from the CPU to the VGA adapter and then using the graphics translation table (GTT) it would translate this into a physical address at the top of DRAM between the range of 03F0_OOOOh - 03FF_FFFFh?

Would this mean that the legacy VGA memory range 0xA0000 - 0xbffff is not accessible in DRAM as the VGA adapter is using the address range for MMIO?

If anyone could help with those questions it would be greatly appreciated,

Thanks.

Answer 1

it has been quite a few years I wrote something directly for VGA so take that in mind.

The old legacy stuff (CGA/EGA,VGA) mapped all VRAM memory access to two segments only (2 x 64KByte)

1. graphic modes

   A000:0000 - A000:FFFF
   

2. text modes

   B800:0000 - B800:FFFF
   

   So booth #1 and #2 64 KByte chunks of memory are not directly accessible instead VGA forwards its own memory there. With integrated cards + shared memory they do not have own memory so the chipset takes it from the global memory (usually from the top address space). In that case yes the memory is not accessible by HW (unless some feature of the chipset is used). The memory space in global memory is usually remapped or used for shadow of ROMs

3. gfx-BIOS

   all legacy gfx cards has its own ``BIOS FLASH/EEPROM/EPROM/PROM` memory. I can't remember exactly how that works but as I remember expansion BIOS area starts around

   C000:0000
   

   where all BIOS able HW map their BIOS memory (not only gfx cards and not only entire segment in size).

   Now there are many gfx modes that need more than 64KB of VRAM so you call gfx BIOS to map appropriate memory segment to A000:0000 or set it by control registers by IO operations on gfx IO ports. Gfx card remap memory and then you can use it ...

4. VESA

   VESA VRAM can be accessed in the same way as on old legacy gfx stuff but VESA add LFB (linear frame buffer) support which can map entire VRAM to memory not just single segment and also can use extended memory (on just base it would have not much use).

As I wrote before it has been some years I deal with this stuff so if I am wrong please edit or add comment ...