[mumblemumble]

They don't have to, but they're commonly understood to refer to memory addresses, which, on most ISAs, are locations of octets.

Even if the ISA only allows word- or dword-aligned loads from memory, the addresses still typically enumerate bytes, not words or dwords.

Based on a quick summary of the MCS-51 that I googled up, it looks like its memory addressing scheme still assigns addresses to bytes, and has special operations that allow you to further specify a bit offset within that memory address.

[gnode]

> it looks like its memory addressing scheme still assigns addresses to bytes, and has special operations that allow you to further specify a bit offset within that memory address.

There are also instructions which use an addressing scheme which takes an 8-bit bit address, with the 0x00 - 0x7f corresponding to lower memory, and 0x80 - 0xff corresponding to 16 specific registers in the Special Function Register set.