A processor with k address lines can put 2ᵏ different addresses on its address bus. That whole set, 0 to 2ᵏ − 1, is its address space.
- 16 address lines → 64K addresses, 0000₁₆–FFFF₁₆
- 20 address lines → 1M addresses
- 32 address lines → 4G addresses
The address space is how much the processor could reach, not how much memory is installed. A 64K space might hold only 32K of RAM and 8K of ROM, with the rest unused. The memory map shows how the space is divided up.
Dividing it is easiest in equal blocks. The top address bits split a 2ᵏ space into blocks: the top 3 bits of a 16-bit space make eight 8K blocks (0000₁₆–1FFF₁₆, 2000₁₆–3FFF₁₆, …). That's exactly what a decoder on those bits does when it drives the chip selects.
In a byte-addressable machine each address is one byte, so a 32-bit address space covers 4 GiB.
0
32768
1
16384
1
8192
0
4096
1
2048
0
1024
1
512
1
256
0
128
0
64
1
32
0
16
0
8
0
4
0
2
0
1