Minimum Alternate Tax requires companies to compute tax from book profits when normal tax is lower, with advance tax obligations. Section 115JB constitutes a self-contained MAT code requiring that where tax on total income is lower than the prescribed benchmark based on book profits, tax shall be computed with reference to book profits; specified exempt incomes are excluded from MAT computation. The provision operates within the wider Income-tax Act for charge, assessment and recovery except as modified, and companies liable under MAT must compute and pay advance tax with the general provisions relating to interest on defaults applying where applicable.