1. A data processor comprising: registers including first, second, and third registers, said first and second registers being general-purpose registers and sharing read and write ports, said third register being a non-general-purpose register having dedicated read and write ports; and an execution unit for executing a first instruction so as to transfer more data than can fit into said first register collectively from external memory to said first and third registers, and a second instruction so as to transfer more data than can fit into said second register collectively from said second and third registers to external memory.