11. In addition to faster components, two significant design improvements were incorporated : base registers and additional hardware instructions. 12. If register 0 is used as the base register , this becomes an example of " absolute addressing ". 13. Base registers logically contain a virtual address that points to a word in a code or data bank ( segment ).14. This is a map-based register established by the Land Registration ( Scotland ) Act 1979 eventually to replace the Sasine Register. 15. Operand addresses were determined by adding the contents of the selected base register and the 10-bit displacement field ( m ). 16. The CPU had 15 index registers, a four-bit field ( x ) allowed selection of one index register as the base register . 17. Base registers B16-B23 are part of the activity state and are maintained by the Exec with no direct access by the activity.18. That can then be used as the base register in a base-plus-offset addressing mode which supplies the low-order 16 or 12 bits. 19. The base register could contain the start address of an array or vector, and the index could select the particular array element required. 20. Typical instructions contain a function code, the target ( or source ) register, an index register, a base register and a displacement field.
