System information

4.1 VGA Specification
Even Scans
Odd Scans
Display Screen
Pixel
Even Scan/Address
Odd Scan/Address
Reserved
Reserved
Video Memory
0x0
0x8000
Figure 4.4: The screen is divided into odd and even columns.
both mechanism: classification of the pixel into two parts and fragmentation of the pixel
itself. These modes use two video planes with two separate pixel areas per plane.
Four Plane Modes Video modes using four video planes separate a pixel into four parts.
The first three planes represent one color at a time: blue, green and red. The last plane
contains the intensity of a pixel.
4.1.3 Access to the Video Memory and Register
A video card is one of the few hardware devices which has a fixed address range inside
the computer address space. Normally, there are configuration processes at the start up of
a computer which allocates the address of every hardware device. Thus, a new installed
device can be easily integrated into the computer system, because it will get automatically a
suitable address window inside the address space. But at the time when the VGA standard
was founded, the address window of hardware devices was fixed. Furthermore, the address
space of a computer was limited to 1 MB. Finally, the VGA video memory got the address
window from 0xA0000 to 0xBFFFF and this has been valid for every Personal Computer
since 1987. But the addresses of the I/O ports of a VGA card are fixed, too. With the
aid of the I/O ports one can setup the video mode or change the color palette of the pixel
color code. Table 4.1 and 4.2 show the memory and I/O address windows of a VGA card.
A detailed description of the VGA I/O ports and the related VGA register can be found in
the Appendix H.2.
Memory Address Window Size Description
0xA0000 - 0xAFFFF 64 KB VGA graphic mode framebuffer
0xB0000 - 0xB7FFF 32 KB MDA framebuffer
0xB8000 - 0xBFFFF 32 KB VGA text/graphic mode framebuffer
Table 4.1: VGA address window to access the framebuffer.
45