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//===- MSFCommon.h - Common types and functions for MSF files ---*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/MathExtras.h"
#include <cstdint>
#include <vector>
namespace llvm {
namespace msf {
static const char Magic[] = {'M', 'i', 'c', 'r', 'o', 's', 'o', 'f',
't', ' ', 'C', '/', 'C', '+', '+', ' ',
'M', 'S', 'F', ' ', '7', '.', '0', '0',
'\r', '\n', '\x1a', 'D', 'S', '\0', '\0', '\0'};
// The superblock is overlaid at the beginning of the file (offset 0).
// It starts with a magic header and is followed by information which
// describes the layout of the file system.
struct SuperBlock {
char MagicBytes[sizeof(Magic)];
// The file system is split into a variable number of fixed size elements.
// These elements are referred to as blocks. The size of a block may vary
// from system to system.
support::ulittle32_t BlockSize;
// The index of the free block map.
support::ulittle32_t FreeBlockMapBlock;
// This contains the number of blocks resident in the file system. In
// practice, NumBlocks * BlockSize is equivalent to the size of the MSF
// file.
support::ulittle32_t NumBlocks;
// This contains the number of bytes which make up the directory.
support::ulittle32_t NumDirectoryBytes;
// This field's purpose is not yet known.
support::ulittle32_t Unknown1;
// This contains the block # of the block map.
support::ulittle32_t BlockMapAddr;
struct MSFLayout {
MSFLayout() = default;
uint32_t mainFpmBlock() const {
assert(SB->FreeBlockMapBlock == 1 || SB->FreeBlockMapBlock == 2);
return SB->FreeBlockMapBlock;
uint32_t alternateFpmBlock() const {
// If mainFpmBlock is 1, this is 2. If mainFpmBlock is 2, this is 1.
return 3U - mainFpmBlock();
const SuperBlock *SB = nullptr;
BitVector FreePageMap;
ArrayRef<support::ulittle32_t> DirectoryBlocks;
ArrayRef<support::ulittle32_t> StreamSizes;
std::vector<ArrayRef<support::ulittle32_t>> StreamMap;
/// Describes the layout of a stream in an MSF layout. A "stream" here
/// is defined as any logical unit of data which may be arranged inside the MSF
/// file as a sequence of (possibly discontiguous) blocks. When we want to read
/// from a particular MSF Stream, we fill out a stream layout structure and the
/// reader uses it to determine which blocks in the underlying MSF file contain
/// the data, so that it can be pieced together in the right order.
class MSFStreamLayout {
uint32_t Length;
std::vector<support::ulittle32_t> Blocks;
/// Determine the layout of the FPM stream, given the MSF layout. An FPM
/// stream spans 1 or more blocks, each at equally spaced intervals throughout
/// the file.
MSFStreamLayout getFpmStreamLayout(const MSFLayout &Msf,
bool IncludeUnusedFpmData = false,
bool AltFpm = false);
inline bool isValidBlockSize(uint32_t Size) {
switch (Size) {
case 512:
case 1024:
case 2048:
case 4096:
return true;
return false;
// Super Block, Fpm0, Fpm1, and Block Map
inline uint32_t getMinimumBlockCount() { return 4; }
// Super Block, Fpm0, and Fpm1 are reserved. The Block Map, although required
// need not be at block 3.
inline uint32_t getFirstUnreservedBlock() { return 3; }
inline uint64_t bytesToBlocks(uint64_t NumBytes, uint64_t BlockSize) {
return divideCeil(NumBytes, BlockSize);
inline uint64_t blockToOffset(uint64_t BlockNumber, uint64_t BlockSize) {
return BlockNumber * BlockSize;
inline uint32_t getFpmIntervalLength(const MSFLayout &L) {
return L.SB->BlockSize;
/// Given an MSF with the specified block size and number of blocks, determine
/// how many pieces the specified Fpm is split into.
/// \p BlockSize - the block size of the MSF
/// \p NumBlocks - the total number of blocks in the MSF
/// \p IncludeUnusedFpmData - When true, this will count every block that is
/// both in the file and matches the form of an FPM block, even if some of
/// those FPM blocks are unused (a single FPM block can describe the
/// allocation status of up to 32,767 blocks, although one appears only
/// every 4,096 blocks). So there are 8x as many blocks that match the
/// form as there are blocks that are necessary to describe the allocation
/// status of the file. When this parameter is false, these extraneous
/// trailing blocks are not counted.
inline uint32_t getNumFpmIntervals(uint32_t BlockSize, uint32_t NumBlocks,
bool IncludeUnusedFpmData, int FpmNumber) {
assert(FpmNumber == 1 || FpmNumber == 2);
if (IncludeUnusedFpmData) {
// This calculation determines how many times a number of the form
// BlockSize * k + N appears in the range [0, NumBlocks). We only need to
// do this when unused data is included, since the number of blocks dwarfs
// the number of fpm blocks.
return divideCeil(NumBlocks - FpmNumber, BlockSize);
// We want the minimum number of intervals required, where each interval can
// represent BlockSize * 8 blocks.
return divideCeil(NumBlocks, 8 * BlockSize);
inline uint32_t getNumFpmIntervals(const MSFLayout &L,
bool IncludeUnusedFpmData = false,
bool AltFpm = false) {
return getNumFpmIntervals(L.SB->BlockSize, L.SB->NumBlocks,
AltFpm ? L.alternateFpmBlock() : L.mainFpmBlock());
Error validateSuperBlock(const SuperBlock &SB);
} // end namespace msf
} // end namespace llvm