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I've written a very basic kernel that displays text to the screen, sets up protected mode, and listens to keyboard input. However I don't know where to go from here. I'm told by some resources to make a memory manager but I'm not sure that I need it at the moment. As aren't memory managers only used for multi tasking? At the moment I am unsure whether to make a memory manager or a hard disk driver. Could you give me some advice please.

http://www.osdev.org/wiki/What_order_sh ... _things_in

As aren't memory managers only used for multi tasking?

No. You need a memory manager for nearly all things, even for a hard disk driver. It has not to be a very complex one in the beginning, but I think you don't want to plan statically which parts of memory you want to use for whatever.