A fallacy in the defendant’s argument is that it assumes that the statutory scheme prior to the revision of § 48 in 1972 entitled prisoners who satisfactorily completed their work assignments to earned good conduct deductions. It did not. It only required that they work. Their reward lay in the satisfaction to be derived from a job well done. See also G. L. c. 127, § 48A (earnings *948for prison work). It was not until St. 1973, c. 528, which was approved nearly a year after the revision of § 48, that the Legislature added § 129D, which for the first time authorized the Commissioner of Correction to grant good time credits for satisfactory participation in work and education programs. Thus, the defendant, far from being disadvantaged by statutory changes after his sentence was imposed, was in reality accorded by those changes an opportunity to earn credits that was not available to him under the statutory scheme in effect at the time of his original sentencing in 1971.

Order denying motion for postconviction relief affirmed.