1. A language processing system for a computer system having a data memory divided into a plurality of banks designated by bank numbers, a bank in use at any given time being known as a current bank, the language processing system converting a source program described by a program language into a computer executable machine language, the source program including a bank switching instruction for switching the bank number used to designate the current bank, the computer system handling data in the data memory, each location in the data memory being designated by a bank number and an address, said system comprising: means for reading a statement of said source program; bank information storage means for storing the bank number designating the current bank; bank information setting means for determining when the statement is said bank switching instruction and for storing a bank number designated by the bank switching instruction; bank information discriminating means operative when the statement is an instruction statement for handling the data of a data memory location, for reading out the content of said bank information storage means and comparing the current bank number and the bank number designated by the instruction statement; bank switching instruction generating means for generating the bank switching instruction to switch the current bank number in the bank information storage means to the bank number designated by said instruction statement when the bank information discriminating means determines that the current bank number and the bank number designated by the instruction statement are different; and converting means for converting the statement of said source program and said generated bank switching instruction into the computer executable machine language.