Virtual memory address translation mechanism with controlled dat

Details Virtual memory address translation mechanism with controlled dat Virtual memory address translation mechanism with controlled dat

1983-09-19

1987-01-20

Shaw, Gareth D.

Boots, shoes, and leggings

G06F 1210

Patent

active

046384260

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates generally to computer memory subsystems and more particularly to such a memory subsystem organized into what is known in the art as a virtual memory. Still more particularly, the invention relates to an apparatus for converting virtual addresses into real memory addresses and for effecting certain unique control functions within the memory hierarchy.
In most modern computer system, when a program is executing, it frequently attempts to access data or code which resides somewhere in the system (that is, in some level of the cache/main store/Direct Access Storage Device (DASD) storage hierarchy or even at another node in a distributed system network). For the most primitive system, consider what the program must understand in order to make this access.
Where is the data (or code)? The location will generally determine what kind of address must be used for the access (e.g. main storage address of 24 bits, or sector address on a disk track, or node address in a network). The location will also determine what kinds of instructions must be used to accomplish the access (e.g. Load/Store/Branch for main storage accesses, channel command words for disk accesses, communication protocols for network accesses).
Is this data shared with other program executions? If it is, the access cannot proceed unless certain locks are held. If the changes which this program is about to make to data are not to be seen by others at this time, the Store instruction must be to some private address.
Is this data to be recoverable? If it is, some "journalling" strategy must be implemented so that a consistent prior state of the data can be retrieved when necessary.
Suppose, in this very primitive system, the program was in fact required to make these distinctions at each access. Then the following would result:
If the program is to be generally applicable the accesses would be very slow, even for the most frequent occurrences of "trivial, safe" requests, namely, for private, unrecoverable data in main storage.
If the program were to perform well it would be locked into one accessing mode, so that it would not run correctly against data with different characteristics.
The program would be complex, large and prone to error.
Modern systems have addressed these problems in varying degrees. For instance:
Relocate architectures generally allow private, unrecoverable, nonpersistent data and programs to be addressed uniformly, with an address size of 16 to 32 bits--(usually adequate for temporary computational requirements). When these architectures are implemented with proper "look-aside" hardware, the vast majority of such accesses are accomplished at cache or main storage speeds. Only when this look-aside hardware fails (less than one in one hundred attempts) does the system pay the cost of accessing the relocation table structure. And only when the relocation tables fail (i.e. the data is not in main storage) does the system pay the significant "page fault" overhead. Thus the penalties are paid only when they are really necessary, which is surely the goal of a good architecture and implementation.
When the data is to persist beyond this execution of this program, most modern systems require that, instead of Load/Store/Branch instructions, access be made by explicit requests to software-implemented "access methods." These access methods generally support data which are organized into certain defined aggregates, called "records" and files." The "instructions" to access are usually called "read/write" or "get/put."
This data is not shared or recoverable. It may in fact be in main storage (in some buffer area). But for every access, the program must pay the overhead of these explicit "read/write" calls. Thus access methods, when suitably defined, have resulted in programs which are less complex and more generally usable than in primitive systems, but the performance of these accesses are uniformly poorer than Load/Store, and the data accessed must have been stru

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"The 801 Minicomputer", by George Radin, ACM Sigplan Notices, vol. 17, No. 4, Apr. 1982, pp. 39-47.

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