The problem is that you can't hold a pointer to more than 64K of address space inside a 16-bit register.

x86 could have easily had an IP-relative addressing mode for data from the beginning (jumps and calls already had it), but to get a pointer you can pass around to use someplace else than the current instruction, it has to be either absolute, or relative to some other "base" register which stays constant. Like the segment registers.

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gpderetta 7 months ago | parent [-]

Just combining two 16 bit registers for a logical 32 bit address would have been better than the weird partially overlapping addressspace.

	▲	rep_lodsb 7 months ago \| parent \| next [-]
		How would you have redesigned the 8086 to do this? And why, other than because of some aesthetic objection to overlapping segments? The 286 and 386 in protected mode did allow segments with any base address (24 or 32 bits), so your argument about extending the address space doesn't make sense.
	▲	wvenable 7 months ago \| parent \| prev [-]
		But then you'd end up wasting memory because the address space it would be divided into 64K blocks. The first PC had only 16KB of RAM but 128KB was probably more common. With the segments setup the way you describe a 128KB machine could use only 2 segment addresses out of 65,536 -- not very efficient or useful for relocating code and data.