memory segmentation - Segment size in x86 real mode

Question

I have one doubt regarding the size of segments in real mode as they can't be more than 64K but can be less than that. My question is how these segment size and base address is initialized ? Like there are GDT's and LDT's in protected mode.Real mode segments can also overlapped,disjoint or adjacent.Like BIOS has some reserved area for specific things like boot code,video buffer etc does assembly programs need to do something like that ?

Answer 1

In real mode segmented addresses are hardwired into memory. To get a physical address you can use this equation:

physical address = segment * 16 + offset

Both segment and offset addreses are 16 bit. By using this equation you can make one 20 bit address and access low 640kB of RAM with no problem.

There is no table that holds where some segment is located. The problem is that you have to set both segment and offset registers to be able to access any address. So you could access maximum of 64k of RAM bytes with a simple loop that just increments the offset register, which makes memory access to larger buffers less comfortable than in flat model.