My understanding is that the original vacuum tube based IBM computers on which Lisp was implemented were accumulator based architectures (ie. had one register u

My understanding is that the original vacuum tube based IBM computers on which Lisp was implemented were accumulator based architectures (ie. had one register usually named "accumulator") with some instructions using parts of the accumulator in particular harwired ways. Also, for CISC architectures that can do indirect loads on the ISA level (nowadays this is the only remaining practical difference between RISC and CISC), the difference between value of register (for i386: eax or %eax) and value referenced by that register ([eax] or $(%eax)) leads to different syntax of the assembly with same mnemonic. i386 with intel assembly syntax is particularly good example of this as 'mov' refers to at least a two dozens of different opcodes depending on the syntax of its two arguments and current mode of the assembler.