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Combined HASH INDEX and Primary Directory

IP.com Disclosure Number: IPCOM000034667D
Original Publication Date: 1989-Apr-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Frye, HE: AUTHOR [+3]

Abstract

The System/38 product line has evolved such that faster systems with larger main storage are required. Increasing the memory size makes efficient address translation more critical. Larger storage also increases the requirements of VMC storage management to, for example, handle more DASD devices to support the larger system. Specifically, enhancement of the virtual address translation table structure is required to: 1. Expand the maximum allowable main storage to 256 MB (2**19 entries in the primary directory). 2. Provide faster address translation. 3. Provide expanded storage for VMC storage management data (15 bytes per primary directory entry). 4. Combine relevant address translation data at one location. The virtual address translation tables in Fig.

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Combined HASH INDEX and Primary Directory

The System/38 product line has evolved such that faster systems with larger main storage are required. Increasing the memory size makes efficient address translation more critical. Larger storage also increases the requirements of VMC storage management to, for example, handle more DASD devices to support the larger system. Specifically, enhancement of the virtual address translation table structure is required to: 1. Expand the maximum allowable main storage to 256 MB

(2**19 entries in the primary directory).

2. Provide faster address translation.

3. Provide expanded storage for VMC storage management

data

(15 bytes per primary directory entry).

4. Combine relevant address translation data at one location.

The virtual address translation tables in Fig. 1 were redesigned beginning with the primary directory. To solve problems (3) and (4) above, a primary directory entry with more than 16 bytes is required.

The next acceptable size (in powers of 2) for the primary directory entry is 32 bytes. However, several bytes of storage are unused. Assuming 32 bytes for each primary directory entry and 2**19 total entries to solve problem (1) above, then a 24-bit INDEX field could be used wherein that field contains the actual 24-bit offset of the primary directory entry from the beginning of the primary directory (32 = 2**5, 5 + 19 = 24). This also solves problem (2) above because no shifting of INDEX field bits is required to compute the address of a primary directory entry. Only simple addition of the offset from the INDEX field to the primary directory origin address is required. Finally, as shown in Fig. 2, the hash index and primary directory, while logically distinct tables, can be physically interleaved and combined wherein the hash index entries are placed in otherwise unused fields of the primary directory. Thus, no additional storage is required for the hash index table. The primary directory and hash index tables are interleaved in storage. The two tables combined occupy one contiguous storage area. The primary directory begins at the address specified by the PD address field in the CAT (a system control address table elsewhere in memory), as does the hash index. The primary directory and hash index will, from this point, be referred to as the primary directory (or, abbreviated, PD). The primary directory must be aligned on a segment boundary, may cross segment boundaries, and must be contained in 1 to 256 64KB V=R (Virtual = Real) segments. That is, the primary directory for the largest sys...