Editing CDC 6600 (section)

===Memory organization===
User programs are restricted to use only a contiguous area of main memory. The portion of memory to which an executing program has access is controlled by the ''RA'' (Relative Address) and ''FL'' (Field Length) registers which are not accessible to the user program. When a user program tries to read or write a word in central memory at address ''a'', the processor will first verify that a is between 0 and FL-1. If it is, the processor accesses the word in central memory at address RA+a. This process is known as [[Base and bounds|base-bound]] relocation; each user program sees core memory as a contiguous block words with length FL, starting with address 0; in fact the program may be anywhere in the physical memory. Using this technique, each user program can be moved ("relocated") in main memory by the operating system, as long as the RA register reflects its position in memory. A user program which attempts to access memory outside the allowed range (that is, with an address which is not less than FL) will trigger an interrupt, and will be terminated by the operating system. When this happens, the operating system may create a [[core dump]] which records the contents of the program's memory and registers in a file, allowing the developer of the program a means to know what happened. Note the distinction with [[virtual memory]] systems; in this case, the entirety of a process's addressable space must be in core memory, must be contiguous, and its size cannot be larger than the real memory capacity.

All but the first seven [[CDC 6000 series]] machines could be configured with an optional Extended Core Storage (ECS) system. ECS was built from a different variety of core memory than was used in the central memory. This memory was slower, but cheap enough that it could be much larger. The primary reason was that ECS memory was wired with only two wires per core (contrast with five for central memory). Because it performed very wide transfers, its sequential transfer rate was the same as that of the small core memory. A 6000 CPU could directly perform block memory transfers between a user's program (or operating system) and the ECS unit. Wide data paths were used, so this was a very fast operation. Memory bounds were maintained in a similar manner as central memory, with an RA/FL mechanism maintained by the operating system. ECS could be used for a variety of purposes, including containing user data arrays that were too large for central memory, holding often-used files, swapping, and even as a communication path in a multi-mainframe complex.