The main memory makes 511 registers available as index registers.
In main memory, 511 of the registers can be used as automatic index registers.
The storage device used by a computer to hold the currently executing program and its working data. A modern computer's main memory is built from random-access memory integrated circuits. In the old days ferrite core memory was one popular form of main memory, leading to the use of the term "core" for main memory.
Computers have several other sorts of memory, distinguished by their access time, storage capicity, cost, and the typical lifetime or rate of change of the data they hold. Registers in the CPU are fast, few, expensive and typically change every few machine instructions. Other kinds are cache, PROM, magnetic disk (which may be used for virtual memory) and magnetic tape.