/*++

Copyright (c) 2024, Quinn Stephens.
Provided under the BSD 3-Clause license.

Module Name:

   mmmd.c

Abstract:

   Provides memory descriptor routines.

--*/

#include <ntrtl.h>
#include "bootlib.h"
#include "mm.h"

#define MAX_STATIC_DESCRIPTOR_COUNT 1024

MEMORY_DESCRIPTOR MmStaticMemoryDescriptors[MAX_STATIC_DESCRIPTOR_COUNT];

PMEMORY_DESCRIPTOR MmGlobalMemoryDescriptors;
ULONG MmGlobalMemoryDescriptorCount, MmGlobalMemoryDescriptorsUsed;

PMEMORY_DESCRIPTOR MmDynamicMemoryDescriptors;
ULONG MmDynamicMemoryDescriptorCount, MmDynamicMemoryDescriptorsUsed;

#define MAX_PRECEDENCE_INDEX sizeof(MmPlatformMemoryTypePrecedence) / sizeof(MmPlatformMemoryTypePrecedence[0])

MEMORY_TYPE MmPlatformMemoryTypePrecedence[] = {
    MEMORY_TYPE_RESERVED,
    MEMORY_TYPE_UNUSABLE,
    MEMORY_TYPE_MMIO,
    MEMORY_TYPE_MMIO_PORT_SPACE,
    MEMORY_TYPE_PAL_CODE,
    MEMORY_TYPE_RUNTIME_SERVICES_CODE,
    MEMORY_TYPE_RUNTIME_SERVICES_DATA,
    MEMORY_TYPE_ACPI_NVS,
    MEMORY_TYPE_ACPI_RECLAIM,
    MEMORY_TYPE_PERSISTENT,
    MEMORY_TYPE_BOOT_APPLICATION_2,
    MEMORY_TYPE_BOOT_SERVICES,
    MEMORY_TYPE_FREE,
    MEMORY_TYPE_FREE_ZEROED
};

ULONG
GetPrecedenceIndex (
    IN MEMORY_TYPE Type
    )

/*++

Routine Description:

    Finds the index into MmPlatformMemoryTypePrecedence for Type.

Arguments:

    Type - The memory type.

Return Value:

    The precedence index if found.
    MAX_PRECEDENCE_INDEX if not found.

--*/

{
    for (ULONG Index = 0;
         Index < MAX_PRECEDENCE_INDEX;
         Index++) {
        if (MmPlatformMemoryTypePrecedence[Index] == Type) {
            return Index;
        }
    }

    return MAX_PRECEDENCE_INDEX;
}

BOOLEAN
MmMdpHasPrecedence (
    IN MEMORY_TYPE TypeA,
    IN MEMORY_TYPE TypeB
    )

/*++

Routine Description:

    Compares two memory types to determine which has precedence.

Arguments:

    TypeA - Memory type A.

    TypeB - Memory type B.

Return Value:

    TRUE if TypeA has precedence over TypeB, or if neither has precedence.
    FALSE if TypeB has precedence over TypeA.

--*/

{
    ULONG ClassA, ClassB;
    ULONG PrecedenceIndexA, PrecedenceIndexB;

    if (TypeB == MEMORY_TYPE_FREE_ZEROED) {
        return TRUE;
    } else if (TypeA == MEMORY_TYPE_FREE_ZEROED) {
        return FALSE;
    } else if (TypeB == MEMORY_TYPE_FREE) {
        return TRUE;
    } else if (TypeA == MEMORY_TYPE_FREE) {
        return FALSE;
    }

    ClassA = TypeA >> 28;
    ClassB = TypeB >> 28;
    if (ClassA != MEMORY_CLASS_APPLICATION
        && ClassA != MEMORY_CLASS_LIBRARY
        && ClassA != MEMORY_CLASS_SYSTEM) {
        return TRUE;
    } else if (ClassB != MEMORY_CLASS_APPLICATION
        && ClassB != MEMORY_CLASS_LIBRARY
        && ClassB != MEMORY_CLASS_SYSTEM) {
        return FALSE;
    }

    if (ClassA != MEMORY_CLASS_SYSTEM) {
        return (ClassB != MEMORY_CLASS_SYSTEM
                && (ClassA == MEMORY_CLASS_APPLICATION
                    || ClassB != MEMORY_CLASS_APPLICATION)) ? TRUE:FALSE;
    } else if (ClassB != MEMORY_CLASS_SYSTEM) {
        return TRUE;
    }

    PrecedenceIndexA = GetPrecedenceIndex(TypeA);
    if (PrecedenceIndexA == MAX_PRECEDENCE_INDEX) {
        return TRUE;
    }

    PrecedenceIndexB = GetPrecedenceIndex(TypeB);
    if (PrecedenceIndexB == MAX_PRECEDENCE_INDEX) {
        return FALSE;
    }

    return PrecedenceIndexA <= PrecedenceIndexB ? TRUE:FALSE;
}

BOOLEAN
MmMdpTruncateDescriptor (
    IN PMEMORY_DESCRIPTOR_LIST Mdl,
    IN PMEMORY_DESCRIPTOR Descriptor,
    IN ULONG Flags
    )

/*++

Routine Description:

    Trunactes a memory descriptor that overlaps
    with adjacent descriptors.

Arguments:

    Mdl - The MDL containing Descriptor.

    Descriptor - The descriptor to truncate.

Return Value:

    TRUE if Descriptor was deleted from Mdl.

--*/

{
    NTSTATUS Status;
    PMEMORY_DESCRIPTOR PrevDescriptor, NextDescriptor, NewDescriptor;
    ULONGLONG DescriptorEnd, PrevDescriptorEnd, NextDescriptorEnd;
    ULONGLONG MappedFirstPage;
    ULONGLONG Change;

    PrevDescriptor = (PMEMORY_DESCRIPTOR)Descriptor->ListEntry.Blink;
    NextDescriptor = (PMEMORY_DESCRIPTOR)Descriptor->ListEntry.Flink;

    DescriptorEnd = Descriptor->FirstPage + Descriptor->PageCount;
    PrevDescriptorEnd = PrevDescriptor->FirstPage + PrevDescriptor->PageCount;
    NextDescriptorEnd = NextDescriptor->FirstPage + NextDescriptor->PageCount;

    //
    // Check if overlapping with previous descriptor.
    //
    if (Descriptor->ListEntry.Blink != Mdl->Head && Descriptor->FirstPage < PrevDescriptorEnd) {
        if (MmMdpHasPrecedence(Descriptor->Type, PrevDescriptor->Type)) {
            PrevDescriptor->PageCount = Descriptor->FirstPage - PrevDescriptor->FirstPage;

            if (DescriptorEnd < PrevDescriptorEnd) {
                if (PrevDescriptor->MappedFirstPage) {
                    MappedFirstPage = PrevDescriptor->MappedFirstPage + DescriptorEnd - PrevDescriptor->FirstPage;
                } else {
                    MappedFirstPage = 0;
                }

                NewDescriptor = MmMdInitDescriptor(
                    DescriptorEnd,
                    MappedFirstPage,
                    PrevDescriptorEnd - DescriptorEnd,
                    PrevDescriptor->Attributes,
                    PrevDescriptor->Type
                );
                if (NewDescriptor != NULL) {
                    Status = MmMdAddDescriptorToList(Mdl, NewDescriptor, Flags);
                    if (!NT_SUCCESS(Status)) {
                        MmMdFreeDescriptor(NewDescriptor);
                    }
                }
            }

            //
            // Delete and free the previous
            // descriptor if it is now empty.
            //
            if (PrevDescriptor->PageCount == 0) {
                MmMdRemoveDescriptorFromList(Mdl, PrevDescriptor);
                MmMdFreeDescriptor(PrevDescriptor);
            }
        } else {
            //
            // Remove if completely overlapping.
            //
            if (DescriptorEnd <= PrevDescriptorEnd) {
                MmMdRemoveDescriptorFromList(Mdl, Descriptor);
                MmMdFreeDescriptor(Descriptor);
                return TRUE;
            }

            //
            // Otherwise, move the descriptor.
            //
            Change = PrevDescriptorEnd - Descriptor->FirstPage;
            Descriptor->FirstPage += Change;
            Descriptor->PageCount -= Change;
            if (Descriptor->MappedFirstPage) {
                Descriptor->MappedFirstPage += Change;
            }
        }
    }

    //
    // Check if overlapping with next descriptor.
    //
    if (Descriptor->ListEntry.Flink != Mdl->Head && NextDescriptor->FirstPage < DescriptorEnd) {
        if (MmMdpHasPrecedence(NextDescriptor->Type, Descriptor->Type)) {
            Descriptor->PageCount = NextDescriptor->FirstPage - Descriptor->FirstPage;

            if (NextDescriptorEnd < DescriptorEnd) {
                if (Descriptor->MappedFirstPage) {
                    MappedFirstPage = Descriptor->MappedFirstPage + NextDescriptorEnd - Descriptor->FirstPage;
                } else {
                    MappedFirstPage = 0;
                }

                NewDescriptor = MmMdInitDescriptor(
                    NextDescriptorEnd,
                    MappedFirstPage,
                    DescriptorEnd - NextDescriptorEnd,
                    Descriptor->Attributes,
                    Descriptor->Type
                );
                if (NewDescriptor != NULL) {
                    Status = MmMdAddDescriptorToList(Mdl, NewDescriptor, Flags);
                    if (!NT_SUCCESS(Status)) {
                        MmMdFreeDescriptor(NewDescriptor);
                    }
                }
            }

            //
            // Delete and free the previous
            // descriptor if it is now empty.
            //
            if (Descriptor->PageCount == 0) {
                MmMdRemoveDescriptorFromList(Mdl, Descriptor);
                MmMdFreeDescriptor(Descriptor);
            }
        } else {
            //
            // Remove if completely overlapping.
            //
            if (NextDescriptorEnd <= DescriptorEnd) {
                MmMdRemoveDescriptorFromList(Mdl, NextDescriptor);
                MmMdFreeDescriptor(NextDescriptor);
                return TRUE;
            }

            //
            // Otherwise, move the next descriptor.
            //
            Change = DescriptorEnd - NextDescriptor->FirstPage;
            NextDescriptor->FirstPage += Change;
            NextDescriptor->PageCount -= Change;
            if (NextDescriptor->MappedFirstPage) {
                NextDescriptor->MappedFirstPage += Change;
            }
        }
    }

    return FALSE;
}

NTSTATUS
MmMdAddDescriptorToList (
    IN PMEMORY_DESCRIPTOR_LIST Mdl,
    IN PMEMORY_DESCRIPTOR Descriptor,
    IN ULONG Flags
    )

/*++

Routine Description:

    Adds a descriptor to a MDL.

Arguments:

    Mdl - the MDL to add Descriptor to.

    Descriptor - the descriptor to add to Mdl.

Return Value:

    STATUS_SUCCESS if successful,
    STATUS_INVALID_PARAMETER if Mdl or Descriptor are invalid.

--*/

{
    PLIST_ENTRY Entry;
    PMEMORY_DESCRIPTOR CurrentDescriptor;

    if (Mdl == NULL || Descriptor == NULL) {
        DebugPrint(L"MmMdAddDescriptorToList(): Mdl and/or Descriptor are NULL\r\n");
        return STATUS_INVALID_PARAMETER;
    }

    if (Mdl->Current) {
        if (Descriptor->FirstPage < ((PMEMORY_DESCRIPTOR)Mdl->Current)->FirstPage) {
            Entry = Mdl->Head->Flink;
        } else {
            Entry = Mdl->Current;
        }
    } else {
        Entry = Mdl->Head->Flink;
    }

    //
    // Search for an existing descriptor for this region.
    //
    while (Entry != Mdl->Head) {
        CurrentDescriptor = (PMEMORY_DESCRIPTOR)Entry;

        if (
            Descriptor->FirstPage >= CurrentDescriptor->FirstPage && (Descriptor->FirstPage != CurrentDescriptor->FirstPage
            || !MmMdpHasPrecedence(Descriptor->Type, CurrentDescriptor->Type))
        ) {
            Entry = Entry->Flink;
            continue;
        }

        //
        // Insert descriptor into the list
        // right before the current entry.
        //
        Descriptor->ListEntry.Blink = Entry->Blink;
        Descriptor->ListEntry.Flink = Entry;
        Entry->Blink->Flink = &Descriptor->ListEntry;
        Entry->Blink = &Descriptor->ListEntry;

        //
        // Truncate overlapping descriptors
        // into one larger descriptor.
        //
        if (Flags & MDL_OPERATION_FLAGS_TRUNCATE) {
            MmMdpTruncateDescriptor(Mdl, Descriptor, Flags);
        }

        return STATUS_SUCCESS;
    }

    //
    // If there are no existing descriptors for this region
    // (or the list is empty), insert and truncate the descriptor.
    //
    InsertTailList(Mdl->Head, &Descriptor->ListEntry);
    if (Flags & MDL_OPERATION_FLAGS_TRUNCATE) {
        MmMdpTruncateDescriptor(Mdl, Descriptor, Flags);
    }

    return STATUS_SUCCESS;
}

VOID
MmMdRemoveDescriptorFromList (
    IN PMEMORY_DESCRIPTOR_LIST Mdl,
    IN PMEMORY_DESCRIPTOR Descriptor
    )

/*++

Routine Description:

    Removes a descriptor from a MDL.

Arguments:

    Mdl - MDL to remove Descriptor from.

    Descriptor - Descriptor to remove from Mdl.

Return Value:

    None.

--*/

{
    PLIST_ENTRY Blink;

    Blink = Descriptor->ListEntry.Blink;
    RemoveEntryList(&Descriptor->ListEntry);

    //
    // Check if the removed descriptor was cached.
    //
    if (Mdl->Current != &Descriptor->ListEntry) {
        return;
    }

    //
    // Cache the previous descriptor if possible.
    //
    if (
        (
            (ULONG_PTR)Blink < (ULONG_PTR)MmGlobalMemoryDescriptors
            || (ULONG_PTR)Blink >= (ULONG_PTR)&MmGlobalMemoryDescriptors[MmGlobalMemoryDescriptorCount]
        )
        && Blink != Mdl->Head
    ) {
        Mdl->Current = Blink;
    } else {
        Mdl->Current = NULL;
    }
}

PMEMORY_DESCRIPTOR
MmMdFindDescriptorFromMdl (
    IN PMEMORY_DESCRIPTOR_LIST Mdl,
    IN ULONGLONG Page,
    IN ULONG Flags
    )

/*++

Routine Description:

    Searches an MDL for the descriptor containing a page.

Arguments:

    Mdl - The MDL to search.

    Page - The page to search for.

    Flags - MDL_OPERATION_FLAGS_*
        MDL_OPERATION_FLAGS_PHYSICAL if Page is physical.
        MDL_OPERATION_FLAGS_VIRTUAL if Page is virtual.

Return Value:

    Pointer to the descriptor if successful.
    NULL if an error occurs.

--*/


{
    BOOLEAN Mapped;
    PMEMORY_DESCRIPTOR Descriptor;
    PLIST_ENTRY ListEntry;
    ULONGLONG FirstPage;

    Mapped = FALSE;
    if (Flags & MDL_OPERATION_FLAGS_VIRTUAL) {
        if (Mdl->Type == MDL_TYPE_PHYSICAL) {
            Mapped = TRUE;
        }
    } else {
        //
        // If the MDL is virtual, the
        // virtual flag must be set.
        //
        if (Mdl->Type == MDL_TYPE_VIRTUAL) {
            DebugPrint(L"MmMdFindDescriptorFromMdl(): Flags is invalid\r\n");
            return NULL;
        }
    }

    //
    // Check if the cached descriptor is in range.
    //
    if (!Mapped && Mdl->Current != NULL) {
        Descriptor = (PMEMORY_DESCRIPTOR)Mdl->Current;
        if (Page < Descriptor->FirstPage) {
            ListEntry = Mdl->Head->Flink;
        } else {
            ListEntry = Mdl->Current;
        }
    } else {
        ListEntry = Mdl->Head->Flink;
    }

    while (ListEntry != Mdl->Head) {
        Descriptor = (PMEMORY_DESCRIPTOR)ListEntry;

        if (Mapped) {
            FirstPage = Descriptor->MappedFirstPage;
        } else {
            FirstPage = Descriptor->FirstPage;
        }

        //
        // Check if this descriptor contains Page.
        //
        if ((!Mapped || FirstPage)
            && Page >= FirstPage
            && Page < FirstPage + Descriptor->PageCount) {
            return Descriptor;
        }

        ListEntry = ListEntry->Flink;
    }

    return NULL;
}

NTSTATUS
MmMdRemoveRegionFromMdlEx (
    IN PMEMORY_DESCRIPTOR_LIST Mdl,
    IN ULONGLONG RemoveStart,
    IN ULONGLONG PageCount,
    IN ULONG Flags,
    OUT PMEMORY_DESCRIPTOR_LIST Unused
    )

/*++

Routine Description:

    Removes a region from a MDL.

Arguments:

    Mdl - MDL to remove the region from.

    RemoveStart - The first page in the region.

    PageCount - The number of pages in the region.

    Flags - MDL_OPERATION_FLAGS_*.
        MDL_OPERATION_FLAGS_PHYSICAL if the region is physical.
        MDL_OPERATION_FLAGS_VIRTUAL if the region is virtual.

    Unused - Unused.

Return Value:

    STATUS_SUCCESS if successful,
    STATUS_INVALID_PARAMETER if Flags value is invalid.

--*/

{
    NTSTATUS Status;
    ULONG Offset;
    BOOLEAN Mapped;
    ULONGLONG RemoveEnd;
    PLIST_ENTRY ListEntry;
    ULONGLONG DescriptorStart, DescriptorEnd;
    PMEMORY_DESCRIPTOR Descriptor, NewDescriptor;

    (VOID)Unused;

    Mapped = FALSE;
    if (Flags & MDL_OPERATION_FLAGS_VIRTUAL) {
        if (Mdl->Type == MDL_TYPE_PHYSICAL) {
            Mapped = TRUE;
        }
    } else {
        //
        // If the MDL is virtual, the
        // virtual flag must be set.
        //
        if (Mdl->Type == MDL_TYPE_VIRTUAL) {
            DebugPrint(L"MmMdRemoveRegionFromMdlEx(): Flags is invalid\r\n");
            return STATUS_INVALID_PARAMETER;
        }
    }

    RemoveEnd = RemoveStart + PageCount;
    ListEntry = Mdl->Head->Flink;
    while (ListEntry != Mdl->Head) {
        Descriptor = (PMEMORY_DESCRIPTOR)ListEntry;

        if (Mapped) {
            DescriptorStart = Descriptor->MappedFirstPage;
        } else {
            DescriptorStart = Descriptor->FirstPage;
        }
        DescriptorEnd = DescriptorStart + Descriptor->PageCount;

        //
        // Check if the region to be removed
        // is inside the current descriptor.
        //
        if (RemoveStart <= DescriptorStart && RemoveEnd > DescriptorStart) {
            //
            // The region is around the start of the descriptor, or
            // they have identical locations and sizes.
            // | RemoveStart | DescriptorStart | RemoveEnd | DescriptorEnd |
            // | Lower address ............................ Higher address |
            //

            if (RemoveEnd < DescriptorEnd) {
                Offset = RemoveEnd - DescriptorStart;
            } else {
                Offset = DescriptorEnd - DescriptorStart;
            }

            //
            // Shrink the descriptor.
            //
            Descriptor->FirstPage += Offset;
            Descriptor->PageCount -= Offset;
            if (Descriptor->MappedFirstPage) {
                Descriptor->MappedFirstPage += Offset;
            }

            //
            // Remove descriptor if now empty.
            //
            if (Descriptor->PageCount == 0) {
                MmMdRemoveDescriptorFromList(Mdl, Descriptor);
                MmMdFreeDescriptor(Descriptor);
            }
        } else if (RemoveStart < DescriptorEnd && RemoveEnd >= DescriptorEnd) {
            //
            // The region is around the end of the descriptor.
            // | DescriptorStart | RemoveStart | DescriptorEnd | RemoveEnd |
            // | Lower address ............................ Higher address |
            //

            //
            // Simply shrink the descriptor.
            //
            Descriptor->PageCount -= DescriptorEnd - RemoveStart;
        } else if (RemoveStart > DescriptorStart && RemoveEnd < DescriptorEnd) {
            //
            // The region is completely inside the descriptor.
            // In this case, the descriptor must be split in two.
            // | DescriptorStart | RemoveStart | RemoveEnd | DescriptorEnd |
            // | Lower address ............................ Higher address |
            //

            //
            // Create a new descriptor before the removed region.
            //
            NewDescriptor = MmMdInitDescriptor(
                Descriptor->FirstPage,
                Descriptor->MappedFirstPage,
                RemoveStart - DescriptorStart,
                Descriptor->Attributes,
                Descriptor->Type
            );

            //
            // Shrink and move the current descriptor.
            //
            Offset = NewDescriptor->PageCount + PageCount;
            Descriptor->FirstPage += Offset;
            Descriptor->PageCount -= Offset;
            if (Descriptor->MappedFirstPage) {
                Descriptor->MappedFirstPage += Offset;
            }

            //
            // Now check if MmMdInitDescriptor() actually worked.
            //
            if (NewDescriptor == NULL) {
                return STATUS_NO_MEMORY;
            }

            Status = MmMdAddDescriptorToList(Mdl, NewDescriptor, Flags);
            if (!NT_SUCCESS(Status)) {
                MmMdFreeDescriptor(NewDescriptor);
                return Status;
            }
        }

        ListEntry = ListEntry->Flink;
    }

    return Status;
}

NTSTATUS
MmMdRemoveRegionFromMdl (
    IN PMEMORY_DESCRIPTOR_LIST Mdl,
    IN ULONGLONG RemoveStart,
    IN ULONGLONG PageCount,
    IN ULONG Flags
    )

/*++

Routine Description:

    Removes a region from a MDL.
    Wrapper around MmMdRemoveRegionFromMdlEx().

Arguments:

    Same as MmMdRemoveRegionFromMdlEx(), except for Unused.

Return Value:

    Any status code returned by MmMdRemoveRegionFromMdlEx().

--*/

{
    return MmMdRemoveRegionFromMdlEx(
        Mdl,
        RemoveStart,
        PageCount,
        Flags,
        NULL
    );
}

NTSTATUS
MmMdFreeDescriptor (
    IN PMEMORY_DESCRIPTOR Descriptor
    )

/*++

Routine Description:

    Frees a memory descriptor.

Arguments:

    Descriptor - the descriptor to free.

Return Value:

    STATUS_SUCCESS if successful,
    other NTSTATUS value if an error occurs.

--*/

{
    if (
        (
            MmDynamicMemoryDescriptors != NULL
            && (ULONG_PTR)Descriptor >= (ULONG_PTR)MmDynamicMemoryDescriptors
            && (ULONG_PTR)Descriptor <= (ULONG_PTR)&MmDynamicMemoryDescriptors[MmDynamicMemoryDescriptorCount]
        )
        || (
            (ULONG_PTR)Descriptor >= (ULONG_PTR)MmStaticMemoryDescriptors
            && (ULONG_PTR)Descriptor <= (ULONG_PTR)&MmStaticMemoryDescriptors[MAX_STATIC_DESCRIPTOR_COUNT]
        )
    ) {
        //
        // Clear the descriptor from static/dynamic MDL.
        //
        RtlZeroMemory(Descriptor, sizeof(MEMORY_DESCRIPTOR));
        return STATUS_SUCCESS;
    }

    //
    // TODO: Free the descriptor from the heap.
    // Need BlMmFreeHeap().
    //
    DebugPrint(L"MmMdFreeDescriptor(): Heap not available\r\n");
    return STATUS_NOT_IMPLEMENTED;
    // return BlMmFreeHeap(Descriptor);
}

VOID
MmMdFreeList (
    IN PMEMORY_DESCRIPTOR_LIST Mdl
    )

/*++

Routine Description:

    Frees a memory descriptor list (MDL).

Arguments:

    Mdl - the MDL.

Return Value:

    None.

--*/

{
    PLIST_ENTRY Entry;

    Entry = Mdl->Head->Flink;
    while (Entry != Mdl->Head) {
        MmMdRemoveDescriptorFromList(Mdl, (PMEMORY_DESCRIPTOR)Entry);
        MmMdFreeDescriptor((PMEMORY_DESCRIPTOR)Entry);
        Entry = Entry->Flink;
    }
}

PMEMORY_DESCRIPTOR
MmMdInitDescriptor (
    IN ULONGLONG FirstPage,
    IN ULONGLONG MappedFirstPage,
    IN ULONGLONG PageCount,
    IN ULONG Attributes,
    IN MEMORY_TYPE Type
    )

/*++

Routine Description:

    Initializes a memory descriptor.

Arguments:

    FirstPage - The first page in the region.

    MappedFirstPage - The first page in the mapping.

    Attributes - Memory attributes of the region.

    Type - Memory type of the region.

Return Value:

    Pointer to the memory descriptor,
    NULL if an error occurs.

--*/

{
    PMEMORY_DESCRIPTOR Descriptor;

    if (MmGlobalMemoryDescriptorsUsed >= MmGlobalMemoryDescriptorCount) {
        DebugPrint(L"MmMdInitDescriptor(): No free descriptors available\r\n");
        return NULL;
    }

    Descriptor = &MmGlobalMemoryDescriptors[MmGlobalMemoryDescriptorsUsed++];
    Descriptor->FirstPage = FirstPage;
    Descriptor->MappedFirstPage = MappedFirstPage;
    Descriptor->PageCount = PageCount;
    Descriptor->Attributes = Attributes;
    Descriptor->Type = Type;
    InitializeListHead(&Descriptor->ListEntry);

    return Descriptor;
}

NTSTATUS
MmMdDestroy (
    )

/*++

Routine Description:

    Cleans up after any actions performed by the memory descriptor manager.
    After calling this, the memory descriptor manager can no longer be used.

Arguments:

    None.

Return Value:

    STATUS_SUCCESS.

--*/

{
    //
    // TODO: Implement this routine.
    //

    return STATUS_SUCCESS;
}

VOID
MmMdInitialize (
    IN ULONG Stage,
    IN PBOOT_LIBRARY_PARAMETERS LibraryParameters
    )

/*++

Routine Description:

    Initializes the memory descriptor manager.

Arguments:

    Stage - Which stage of initialization to perform.

        Stage 0: Initializes the static memory descriptor pool.

    LibraryParameters - Pointer to the library parameters structure.

Return Value:

    None.

--*/

{
    (VOID)LibraryParameters;

    if (Stage == 0) {
        //
        // Initialize static memory descriptor pool.
        //
        MmGlobalMemoryDescriptors = &MmStaticMemoryDescriptors[0];
        MmGlobalMemoryDescriptorCount = MAX_STATIC_DESCRIPTOR_COUNT;
        MmGlobalMemoryDescriptorsUsed = 0;
        RtlZeroMemory(MmGlobalMemoryDescriptors, MAX_STATIC_DESCRIPTOR_COUNT * sizeof(MEMORY_DESCRIPTOR));
    }

    //
    // TODO: Implement stage 1 initialization.
    //
}