Editing Aliasing (computing) (section)

== Hardware aliasing ==
The term ''aliasing'' is also used to describe the situation where, due to either a hardware design choice or a hardware failure, one or more of the available address bits is not used in the memory selection process.<ref>{{cite web | url=http://www.esacademy.com/en/library/technical-articles-and-documents/miscellaneous/software-based-memory-testing.html |author=Michael Barr |date=2012-07-27 |title=Software Based Memory Testing}}</ref> This may be a design decision if there are more address bits available than are necessary to support the installed memory device(s). In a failure, one or more address bits may be shorted together, or may be forced to [[Ground (electricity)|ground]] (logic 0) or the supply voltage (logic 1).

;Example
For this example,  assuming a memory design with 8 locations, requiring only 3 address lines (or [[bit]]s, since 2<sup>3</sup> = 8).  Address bits (named A2 through A0) are decoded to select unique memory locations as follows, in standard [[Counter (digital)|binary counter]] fashion:

{| class="wikitable" style="text-align:center"
|-
! A2 !! A1 !! A0 !! Memory location
|-
| 0 || 0 || 0 || 0
|-
| 0 || 0 || 1 || 1
|-
| 0 || 1 || 0 || 2
|-
| 0 || 1 || 1 || 3
|-
| 1 || 0 || 0 || 4
|-
| 1 || 0 || 1 || 5
|-
| 1 || 1 || 0 || 6
|-
| 1 || 1 || 1 || 7
|-
|}
In the table above, each of the 8 unique combinations of address bits selects a different memory location. However, if one address bit (say A2) were to be shorted to ground, the table would be modified as follows:

{| class="wikitable" style="text-align:center"
|-
! A2 !! A1 !! A0 !! Memory location
|-
| 0 || 0 || 0 || 0
|-
| 0 || 0 || 1 || 1
|-
| 0 || 1 || 0 || 2
|-
| 0 || 1 || 1 || 3
|-
| <span style="color:red">'''0'''</span> || 0 || 0 || <span style="color:red">'''0'''</span>
|-
| <span style="color:red">'''0'''</span> || 0 || 1 || <span style="color:red">'''1'''</span>
|-
| <span style="color:red">'''0'''</span> || 1 || 0 || <span style="color:red">'''2'''</span>
|-
| <span style="color:red">'''0'''</span> || 1 || 1 || <span style="color:red">'''3'''</span>
|-
|}

In this case, with A2 always being zero, the first four memory locations are duplicated and appear again as the second four.  Memory locations 4 through 7 have become inaccessible.

If this change occurred to a different address bit, the decoding results would be different, but in general the effect would be the same: the loss of a single address bit cuts the available memory space in half, with resulting duplication (aliasing) of the remaining space.