[Ethereum] Rationale behind 256-bit words in EVM

It will be easier to understand if we make these two definitions (note: this might not be fully correct, but is enough for the following explanation):

• the EVM is the machine executing the instructions, which has inputs and outputs
• the state manager as the one putting data in/out of the blockchain.

Many of the interactions with the blockchain (and state manager) happen via RLP encoded data. One notable exception is the ABI with the contracts (the data field in the transaction).

The EVM itself operates on 256 bit words, which means each stack and each storage space is 256 bit wide and usually you can address them that way (there are a few byte addressing methods). The EVM itself doesn't really care how the data is stored then. The arithmetic instructions will expect a certain format for the data, but that's really it.

The Contract ABI document defines how to interact with a contract and how they interact with each other. As far as I know Solidity as a language also stores data structures internally in a similar way.

The Contract ABI defines different data types each with their own layout. Numbers (int*/uint*) are stored left padded, whereas bytes/string is stored right padded.