The section 5.4 of SICP develops an evaluator written in an assembly, but I can't figure out why they call it explicit-control evaluator.
Up until this point we were writing interpreters that run over the underlying machine's architecture with its own registers and operations. In a way, we don't truly have control of what's happening down there.
Now, we're going to build a virtual machine in which we'll have explicit control over the registers, operations, jumps, etc. - sure, this virtual machine will also be running on top of the physical machine, but you'll be able to see how things work at a very low level ;)