Why does the 8086 use an extra register to address 1MB of memory?

I heard that the 8086 has 16-bit registers which allow it to only address 64K of memory. Yet it is still able to address 1MB of memory which would require 20-bit registers. It does this by using another regi开发者_如何转开发ster to to hold another 16 bits, and then adds the value in the 16-bit registers to the value in this other register to be able to generate numbers which can address up to 1MB of memory. Is that right?

Why is it done this way? It seems that there are 32-bit registers, which is more than sufficient to address 1MB of memory.

Actually this has nothing to do with number of registers. Its the size of the register which matters. A 16 bit register can hold up to 2^16 values so it can address 64K bytes of memory.

To address 1M, you need 20 bits (2^20 = 1M), so you need to use another register for the the additional 4 bits.

The segment registers in an 8086 are also sixteen bits wide. However, the segment number is shifted left by four bits before being added to the base address. This gives you the 20 bits.

the 8088 (and by extension, 8086) is instruction compatible with its ancestor, the 8008, including the way it uses its registers and handles memory addressing. the 8008 was a purely 16 bit architecture, which really couldn't address more than 64K of ram. At the time the 8008 was created, that was adequate for most of its intended uses, but by the time the 8088 was being designed, it was clear that more was needed.

Instead of making a new way for addressing more ram, intel chose to keep the 8088 as similar as possible to the 8008, and that included using 16 bit addressing. To allow newer programs to take advantage of more ram, intel devised a scheme using some additional registers that were not present on the 8008 that would be combined with the normal registers. these "segment" registers would not affect programs that were targeted at the 8008; they just wouldn't use those extra registers, and would only 'see' 16 addres bits, the 64k of ram. Applications targeting the newer 8088 could instead 'see' 20 address bits, which gave them access to 1MB of ram

I heard that the 8086 has 16 registers which allow it to only address 64K of memory. Yet it is still able to address 1MB of memory which would require 20 registers.

You're misunderstanding the number of registers and the registers' width. 8086 has eight 16-bit "general purpose" registers (that can be used for addressing) along with four segment registers. 16-bit addressing means that it can only support 2¹⁶ B = 64 KB of memory. By getting 4 more bits from the segment registers we'll have 20 bits that can be used to address a total of 2⁴*64KB = 1MB of memory

Why is it done this way? It seems that there are 32 registers, which is more than sufficient to address 1MB of memory.

As said, the 8086 doesn't have 32 registers. Even x86-64 nowadays don't have 32 general purpose registers. And the number of registers isn't relevant to how much memory a machine can address. Only the address bus width determines the amount of addressable memory

At the time of 8086, memory is extremely expensive and 640 KB is an enormous amount that people didn't think that would be reached in the near future. Even with a lot of money one may not be able to get that large amount of RAM. So there's no need to use the full 32-bit address

Besides, it's not easy to produce a 32-bit CPU with the contemporary technology. Even 64-bit CPUs today aren't designed to use all 64-bit address lines

• Why can't OS use entire 64-bits for addressing? Why only the 48-bits?
• Why do x86-64 systems have only a 48 bit virtual address space?

It'll takes more wires, registers, silicons... and much more human effort to design, debug... a CPU with wider address space. With the limited transistor size of the technology in the 70s-80s that may not even come into reality.

8086 doesn't have any 32-bit integer registers; that came years later in 386 which had a much higher transistor budget.

8086's segmentation design made sense for a 16-bit-only CPU that wanted to be able to use 20-bit linear addresses.

Segment registers could have been only 8-bit or something with a larger shift, but apparently there are some advantages to fine-grained segmentation where a segment start address can be any 16-byte aligned linear address. (A linear address is computed from (seg << 4) + off.)