Converting from 1Mbyte to Hex

Question

I am having problem understanding the following text,

8088 supports 1 Mbyte of external memory. This memory space is organized from a software point of view as individual bytes of data stored at consecutive address over the address range 00000 to FFFFF.

Now I don't get how the author converted from 1 Mbyte to FFFFF. Can anyone please help me out?

Thanks.

Answer 1

1 Megabyte is 2^20. That means that you need 20 bits to represent it. The range 00000-FFFFF holds all possible values of 20 bits.

Answer 2

It is well explained in pages describing the 8086 (the 8088 has the same instruction set architecture, but with slightly different pins).

Basically a segment register gave a (16 bits) base address which was multiplied by 16, then an 16 bits offset was added.

And 0xfffff is 2²⁰-1, i.e. 1048575

Answer 3

Please diatinguish between CAPACITY and ADDRESSING. Capacity count start from 1, but in memory Addressing start from 0 ( address 0 is counted1).

An analogy. if we have capacity 10, and we address from 0, it would be from 0 to 9.

So the trick for addressing is Last Address = Capacity - 1.

For Capacity 1MB (dec) in Hex = 10.0000h And for addressing, last address is 10.0000 - 1 = F FFFF So addressiing for 1MB capacity is from 0 to F FFFF. (Remember address 0, is counted 1).