What are address lines?

[Isaac]

Hi,

I understand that the ancient processors only had 20 address lines and later Intel added a 21st address line. But how much memory do I gain by enabling the A20 line? And what are address lines anyway?

[dmatlack]

When a computer needs to access memory, it needs to communicate what address it wants to access. This communication is done via the address lines. There is one address line for each bit of an address. If you want use 32-bit addresses, you need 32 address lines.

If you have 20-bits for an address, that's 2^20 possible address = 2^20 bytes = 1MB
If you enable the A20 line on a 32-bit computer, you will have 2^32 possible addresses = 2^32 bytes = 4GB.

Of course, this is all about *addressable* memory. Your physical memory limits (how much RAM you have) may obviously be different.

[VolTeK]

Just in case.. you really are blind. Save your self the trouble and thousand question filled posts, and get reading.

http://wiki.osdev.org/Main_Page

[Love4Boobies]

If you enable the A20 line on a 32-bit computer, you will have 2^32 possible addresses = 2^32 bytes = 4GB.

And if you don't, you will have 2 GiB since that's how much 2^31 is (you're only disabling one address line).