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Is it nessecary to enable the A20 to enter the protected mode? Why enable so much OS the A20?

thx!

No, you don't HAVE TO, but you probably WANT TO. First, let me tell you what A20 is. It's line 20 on the address bus (which corresponds with bit 20 of a 32-bit address). If the A20 line is disabled, that means that bit 20 of the address is treated as a 0, even if it's really a 1. So the addresses between 1MB and 2MB are treated as though they were between 0 and 1MB; the addresses between 3MB and 4MB are treated as though they were between 2MB and 3MB; and so on with the rest of the odd megabytes. So basically, if you're going to use any memory above 1MB, then you want to enable the A20 line.