CODEWITHSUNDEEP
x86memory-addressreal-mode
Oxdeadbeef
Oxdeadbeef

Reputation: 1063

how does cpu calculate 20-bit address in real mode

i know it uses physical address = segment register << 4 + offset register. Although these two registers are 16-bits, how can 8086 handle a 20-bit plus operation?

Upvotes: 5

Views: 2695

Answers (2)

jonsca
jonsca

Reputation: 10381

When you shift a 16 bit number 4 places to the left, you're essentially creating a 20 bit number. The offset then indicates how far into that range you will go. See the Wikipedia article on Real mode addressing.

Example:

0x0001  << 4  ->  0x00010  (5 sets of 4 bits in each hex digit -> 20 bits)

Upvotes: 1

DarkDust
DarkDust

Reputation: 92335

The Bus Interface Unit consists of segment registers, adder to generate 20 bit address and instruction prefetch queue. Once this address is sent out of BIU, the instruction and data bytes are fetched from memory and they fill a First In First Out 6 byte queue.

See a document called "8086_Internal_Block_diagram_enotes.pdf", easily to be found via Google. Also see this document, the section about the Bus Interface Unit.

So the processor generates these 20-bit addresses "on demand" with a dedicated internal 20-bit register, if you will.

Upvotes: 5

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