6 External Pager Protocol

6.1 Overview

The following chapters describe the general pager protocol.

In L3 there is a standard pager which is part of the kernel and handles all data spaces ([here]). But if there is a need for the handling of external filesystems like a UNIX file system, rather use an external pager.

The task of an external pager is to provide an external memory or a part of an external memory as one or more linear addressed data spaces, which can be mapped into the address space of other users.

It is possible to use different external pagers. One of the pagers can handle a hard disk and provide it as data space, one can handle a floppy disk, and the next can provide files as data spaces, which can be mapped into the address spaces of som e UNIX users.

But all pagers use the same protocol, called the general pager protocol, which is described in the following chapters.

The pager distinguishes between services, used

by the users of the pager

by the kernel.

To distinguish the origin of the message there are two protocols:

the PageFaultProtocol

the General Pager Protocol

The messages sent to the pager are of the following form:

typedef struct { 
     TMsgDope size;
     TMsgDope msg;
     DWord codeword1;
     DWord codeword2;
   }

In some cases there can be one or more additional parameters.

The results are given back by the pager with the two code words.

6.2 Page Fault Protocol

The service used by the kernel is the handling of some page faults, which the kernel ascertains. The kernel recognizes which pager is the owner of a page or a data space when a page fault occurs. In this case the kernel sends a message to the pager fourcing it to provide the page where the page fault occurs. The pager answers with the address of this page inside its own address space.

The page fault protocol has a special protocol identifier, like every L3 protocol. It is defined like follows:

#define pfp     0x30000000

The ix86 distinguishes between two kinds of access, read access and write access. Therefor the page fault procol knows about two access modes:

ReadAccess: forces the pager to provide the page, where the page fault occured, the requested page is given back for read access. The pager can decide, whether the page is given back for write access, if the page was modified earlier.
WriteAccess: the same with write access

To supply the necessary information to resolve the page fault the kernel sends two dwords describing the page fault.

dword 1: PFP + data space id
dword 2: pfaddress + access (pfaddress & ~31)

The pager tries to find the necessary pages and sends a reply consisting of two dwords and one or more flex pages. The dwords are the same like in the page fault message. The pager simply replace the access with the result of its own operation.

result code word 1

pfp + data space id

result code word 2

pfaddress + result (pfaddress & ~31)

0x00000 - ACK

0x00001 - illegal data space id

0x00010 - illegal address

0x100rw - illegal access (rw proposed)

There is a header file named pagefault.h which defines some structures and constants necessary to implement the external pager protocol.

6.3 General Pager Protocol

All other services of the pager indicated by the 2 in the code word are used by applications of the pager. These are in general the same services as the standard pager provides (see section [here]).

The use of the functions is a bit different, that's why they will be explained in the following chapter.

The external pagers provide named and unnamed data spaces. Data spaces are created by copying an existing data space. By this the user will get an identifier of the data space. This is a 32 bit long identifier, which holds the task identifier of the called pager, the task number of the calling thread and an index, which is processed by the pager.

An existing data space can be named by the user by an operation of the pager. An already named data space can be renamed in the same way. The name of an existing data space can be determined by another operation.

Other operations are similiar to the operations of the standard pager like

#define GPP          0x20000000

Functions

1 CopyDataSpace

2 DataSpace

3 SetDSName

4 GetDSName

5 Pages

6 Type

7 Storage

8 Delete

9 NextPage

A DeletePages

B CopyPages

The code words are formed as follows:

code word 1: GPP + data space identifier
code word 2: function + additional info

The additional information and parameters depend on the function. It is described below.

Description of Functionsions

CopyDataSpace

copies a data space from source to destination.

code word 1

GPP + data space identifier of the source data space. If this is 0, a new data space is created.

code word 2

CopyDataSpace

result code word 1

0 - ok + data space id

1 - illegal data space id

DataSpace

provides the data space identifier of the data space associated with the given name

code word 1

GPP

code word 2

DataSpaces

additional

StrDopeT name of the data space

result code word 1

0 (ok) + data space identifier

1 - illegal name

SetDSName

sets the specified name to the given data space.

code word 1

GPP + data space identifier

code word 2

SetDSName

additional

StrDopeT name of the data space

result code word 1

0 (ok)

1 - illegal name

2 - illegal data space id

GetDSName

determines the specified name to the given data space.

code word 1

GPP + data space identifier

code word 2

GetDSName

result code word 1

0 (ok) - Name is stored at the given address

1 - illegal data space id

additional

StrDopeT name of the data space

DeleteDataSpace

deletes the data space with the given identifier

code word 1

GPP + data space identifier

code word 2

DeleteDataSpace

result code word 1

0 - ok

1 - illegal data space id

Pages

determines the total number of the used pages of the data space with the given identifier

code word 1

GPP + data space identifier

code word 2

Pages

result code word 1

0 - ok

1 - illegal data space id

result code word 2

total number of pages

Index

provides the number of the data space specified by the given identifier inside the data spaces sequence provided by the external pager

code word 1

GPP + data space identifier

code word 2

Index

result code word 1

0 - ok

1 - illegal data space identifier

result code word 2

index of the data space

DeletePages

deletes the specified pages of the data space

code word 1

GPP + data space identifier

code word 2

Delete Pages + number of the pages to be deleted

additional parameter> from page , to page

result code word 1

0 - ok

1 - illegal data space id

2 - illegal pages

3 - illegal access

CopyPages

copies the specified pages of the given data space to the given pages of the destination data space. The pages are copied in it's indata space sequence. If the number of pages of both intervalls isn't identical, the number of pages of the source data space is copied.

code word 1

GPP + source data space identifier

code word 2

CopyPages + destination data space identifier

additional parameter

DWord from_page, to page inside the source

additional parameter

DWord from_page, to_page inside the destination

result code word 1

0 - ok

1 - illegal data space id

2 - illegal pages

3 - illegal access

NextPage

provides the number of the page following the page with the given number inside the data space

code word 1

GPP + data space identifier

code word 2

NextPage + number of the page whose follower should be provided, if this is -1, the first used page should be provided

result code word 1

0 - ok

1 - illegal data space id

2 - illegal page

result code word 2

number of the next used page, it is -1,if there are no more pages

6.4 C - Library

The functions of the gpp are provided in a library.

CopyDataSpace

  void PCopyDataSpace(DataSpaceId source, DataSpaceIdT *dest)

Parameter:

source: This parameter specifies the data space to copy. If this is 0, a new data space is created.
dest: This parameter specifies the identifier of the new created data space.

DataSpace

  DtaSpaceIdT PDataSpace(char * name)

Parameter:

name: This parameter is the name of the data space.
result: The result of this function is either the data space identifier, if the the data space associated with name exists, or 0, if it doesn't exist.

SetDSName

  void PSetDSName(DataSpaceId ds, char *name)

Parameter:

ds: This parameter specifies the data space to be named.
name: This parameter specifies the name, which is given to the data space.

GetDSName

  void PSetDSName(DataSpaceId ds, char *name)

Parameter:

ds: This parameter specifies the data space, whose name should be traced.
name: This parameter holds the name after the operation. If the data space was unnamed, it is NIL.

Pages

  int PPages(DataSpaceId ds)

Parameter:

ds: This parameter specifies the data space, whose number of pages should be provided.
result: The result of this function is either the number of the used pages of the given data space or -1, if the data space doesn't exist.

Type

  int PType(DataSpaceId ds)

Parameter:

ds: This parameter specifies the data space, whose type is traced.
result: The result of the function is either the type of the data space or -1 if the data space doesn't exist.

Storage

  int PStorage(DataSpaceId ds)

Parameter:

ds: This parameter specifies the data space, whose storage size has to be provided.
result: The result of this function is either the size of the used storage of the data space or -1 if the dat space doesn't exist.

Delete

  void PDelete(DataSpaceId ds)

Parameter:

ds: This parameter specifies the data space to be deleted.

NextPage

  int PNextPage(DataSpaceId ds, int start_page)

Parameter:

ds: This parameter specifies the data space.
start_page: This parameter specifies the number of the page, whose follower should be provided. Is this -1, the first used page of the data space is provided.
result: The result of the function is either the number of the next used page or -1 if there are no more used pages.

DeletePages

  void PDeletePages(DataSpaceId ds, int from_page, int to_page, int pages)

Parameter:

ds: This parameter specifies the data space, whose pages have to be deleted.
from_page: This parameter specifies the first page to be deleted.
to_page: This parameter specifies the last page to be deleted.
pages: This parameter specifies the total number of pages to be deleted.

CopyPages

  void PCopyPages(DataSpaceId source, DataSpaceIdT dest, int from_page,
                  int to_page, int pages)

Parameter:

source: This parameter specifies the data space, whose pages have to be copied.
dest: This parameter specifies the identifier of the data space, where the pages are copied to.
from_page: This parameter specifies the first page to be copied.
to_page: This parameter specifies the last page to be copied.
pages: This parameter specifies the total number of pages to be copied.

Marion Schalm, Jean Wolter, Michael Hohmuth
26.12.1995 (unfinished)