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Software memory management

November 23rd, 2011 No comments

I recently wrote about how computer memory capacity limits were a serious constraint for compiler writers. One technique used to increase the amount of memory available to a compiler (back in the days when pointers usually contained 16-bits) is software based paged memory management. Yes, compiler writers were generally willing to take the runtime performance hit to increase effectively accessible memory by around a factor of 10-25 (e.g., a 2 byte number used as an index into an array of 20 to 50 byte records).

The code to iterate over a data structure stored under the control of a software memory manager looks like the following (taken from a C to Pascal translator):

Var
	Flds    : Sw_Ptr;  (* in practice an integer *)
        T_Node  : Sw_Node; (* in practice a pointer to a record *)
Begin
While Flds <> Sw_Nil Do  (* Sw_Nil is the memory managers Nil value *)
  Begin
  Sw_Node_Ref(Cpswfile, Flds, T_Node, Mm_Readonly);
    If T_Node.Pn^.Node_Is<>N_Is_Field Then
      Verify_Error(Ve_Cputils, Ve_Scan_Fld);
 
  Scan(T_Node.Pn^.Field_Node.Ftype);
  Flds := T_Node.Pn^.Field_Node.Next;
  End;
End;

Where Sw_Node_Ref is a procedure in the memory management package that ensures the record denoted by Flds (whose value was obtained by a previous called to Sw_New_Node) is available in memory and returned in T_Node. Had Mm_ReadWrite rather than Mm_Readonly been specified the memory manager would assume that the record had been modified and when the page containing the record was swapped out of main memory it would write the contents of the page containing it to the swapfile (specified by the first argument, Cpswfile).

A call to Sw_Node_Ref only guarantees that the record is at the returned location until the next memory management procedure is called. This takes advantage of common usage which is: read a record, do something with its contents and then move on to the next one. The procedure Sw_Node_Ptr is available for when a record needs to be held in main memory across multiple Sw_ calls; this procedure locks a record in the limited capacity memory pool until explicitly freed (a Pascal style Mark/Release system was also available).

Multiple records were overlayed on a page (invariably 512 bytes) of storage. Some implementations used a fancy tool to create the overlay while others did it manually. The size of the pool in main memory used to hold swapped-in pages was specified when the memory manager was initialized; the maximum number of records that could be created by a call to Sw_New_Node was only limited by the maximum value of an integer and available disk space.

I learned about this implementation technique while on secondment at Intermetrics in the early 1980s, and they told me it came from the PQCC project of the mid 1970s. There is a paper in the Proceedings of the 1982 SIGPLAN symposium describing the system/library used by Intermetrics, which rambles on about nothing in particular and fails to say anything about software memory management (it is too useful an idea for a commercial company to tell anybody else); I don’t know of any other published description. Everybody I know who left Intermetrics to work on other compilers created their own implementation of a software memory management package.