OSDev.org

is is possible to write to a000:0000 in text mode and have the pixel change color or can you only write to the screen with the address b800:0000?

can you do both at the same time?

you cant write to A000:0000 in text mode (the memory is locked, you can with tricks unlock it and make it useable. my bootloader used a trick to unlock the extra 128kb memory and store itself there so all base memory was free to load a kernel anywher!)

basically if your in text mode its 0xB800, if your in a graphic mode its 0xA000

you can access that memory only if you set graphic mode, just then you will put graphic pixel there...

note that you can also do some very curious things like splitting your screen (horizontally only, though) and have a half in graphic mode and another half in text mode. This has been used in some PC demo ages ago ... not very clean, not very standard (just tricks around the VGA architecture) and sometimes doesn't work on some screens ... but possible nonetheless...