[JonathonW]

Is it even an emulator? There's an SM83 (the Game Boy Color's weird not-quite-Z80 CPU) hiding inside the Game Boy Advance's SoC.

[edit] Reading a bit more, it is an emulator-- and apparently an inefficient, poorly-written one at that.

[lxgr]

The original GBA models (including the SP) did actually feature a Sharp SM83 (the Z80-compatible CPU used in the GB and GBC) for GB/GBC backwards compatibility.

It was never accessible by GBA games (or at least intended to be accessed, so to my knowledge no official titles did), and so the Game Boy Micro and the DS and DS Lite, which both feature GBA but not GB/GBC backwards compatibility, could drop it without causing any problems for GBA games.

[mmaniac]

The Game Boy Micro still contained a useable SM83 and GBC PPU on the SoC, but the cartridge is inaccessible. In theory you might be able to run a tiny payload from WRAM (32KB?) but it's pretty useless otherwise.

https://github.com/AntonioND/gba-switch-to-gbc

[Dwedit]

GBC mode (with the z80-like processor) cannot be accessed in GBA mode at all.