In these cases, retro/rev blogs refer to special memory areas or APIs (interrupts, etc.).
In this case, he's working on a Commodore 64, and $A000 is the beginning of the BASIC ROM/RAM (http://sta.c64.org/cbm64mem.html).
In this case, he's working on a Commodore 64, and $A000 is the beginning of the BASIC ROM/RAM (http://sta.c64.org/cbm64mem.html).