Applicability of smart contracts showed significant potential to multiple industries by offering data integrity, transparency, non-repudiation, elimination of trust. An essential part of introducing programmability in a blockchain is a process virtual machine that performs the execution and computes new valid states. In this paper, a model for a programmable blockchain will be presented that can perform computations and track the state changes. Besides that, the virtual machine responsible for the execution will be defined as a deterministic state machine and there will be proposed a loosely-coupled architecture such that the virtual machine works independently from the blockchain. In addition to that, flaws and shortcomings of current execution environments of today's popular blockchain platforms will be discussed. The models provided could serve as an implementation guide for blockchains that want to add programmability to their system.