Quick answer:

What is the difference between S7-PLCSIM and a "real" automation system?

S7-PLCSIM supports four accumulators like a S7-400 CPU. In some cases a program running on S7- PLCSIM with four accumulators might behave differently than on an S7-300 CPU that only has two accumulators.

Longer answers:

SIMATIC S7-300 Instruction list S7-300 CPUs and ET 200 CPUs

ACCU1 and ACCU2 (32 bit)

The accumulators are registers for processing bytes, words or double words. The addresses are loaded into the accumulators, where they are logically gated.

The result of the logic operation (RLO) is in ACCU1.

The accumulators are 32 bit long.

Instruction List S7-400 CPU 412-2 PN/DP, 414-3 PN/DP, 414F-3 PN/DP, 416-3 PN/DP, 416F-3 PN/DP

ACCU1 to ACCU4 (32 Bit)

The accumulators are registers for processing bytes, words or double words. The address identifiers are loaded into the accumulators, where they are logically gated.

The result of the logic Instr. (RLO) is in ACCU1 and can be transferred from there to a memory cell.

The accumulators are 32 bits long.

For registers, maybe you should clarify your request, i do not understand exactly what you are looking for.

Hope this helps,
Daniel Chartier