Free Virtual Page Structures

VPI=0d3e pVP=ff1e8ee8 free FLink=00000000 BLink=fff13280
VPI=0d3f pVP=ff1e8ef4 free FLink=ff1e9fec BLink=ff1e8cf0
VPI=0d40 pVP=ff1e8f00 free FLink=ff1e9fec BLink=ff1e8cf0
VPI=0d41 pVP=ff1e8f0c free FLink=00001000 BLink=02450030
VPI=0d42 pVP=ff1e8f18 free FLink=00000000 BLink=ff1e8f00

Notes

• Free Page Frame Structures are grouped in bundles that are chained in a circular double link list. Each bundle comprises contiguous free VPs in the VP array. The chain pointers are only used by the head and tail of each bundle as follows:

  • For bundles of greater than one VP:

    • Blink of the head points to the tail
      
    • Flink of the head points to the head of the next bundle
      
    • Blink of the tail points to the head of the previous bundle
      
    • Flink of the tail is set to zero
    -
    For single VP bundles:

    • Blink points to the head of the previous bundle
      
    • Flink points to the head of the next bundle
    The free VP chain is headed by a pseudo-VP whose Blink points to the head of the first true free bundle and whose Flink points to the last VP in the VP array. The pseudo-VP is located at global symbol:

    _pgVPHead
    

    
    Unless a free VP is the head or tail of a bundle then its Flink and Blink will retain values from its previous use. In particular it may be possible to glean information about a previous allocation as the Flink field overlays the Flg and Block fields and the Blink field overlays the HobPg and Hob fields of an In-use VP. In the example above VPI d41 was probably allocated to page 30 of hob 245. Using the following .MOC command might reveal who the owner was and who allocated this storage as follows:

         .MOC 245
    

    For a description of the fields formatted by .MV select .MV Output Field Descriptions

    For more examples using of the .M family of commands see: Exploring Memory Management.

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