Implementing some FAT12 code in C

2023-08-22 12:18:13 -04:00
parent 1d349f2f23
commit a22a2631b4
3 changed files with 197 additions and 2 deletions
@@ -0,0 +1,182 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ctype.h>
+
+typedef uint8_t bool;
+#define true 1
+#define false 0
+
+typedef struct
+{
+    uint8_t BootJumpInstruction[3];
+    uint8_t OemIdentifier[8];
+    uint16_t BytesPerSector;
+    uint8_t SectorsPerCluster;
+    uint16_t ReservedSectors;
+    uint8_t FatCount;
+    uint16_t DirEntryCount;
+    uint16_t TotalSectors;
+    uint8_t MediaDescriptorType;
+    uint16_t SectorsPerFat;
+    uint16_t SectorsPerTrack;
+    uint16_t Heads;
+    uint32_t HiddenSectors;
+    uint32_t LargeSectorCount;
+
+    // extended boot record
+    uint8_t DriveNumber;
+    uint8_t _Reserved;
+    uint8_t Signature;
+    uint32_t VolumeId;          // serial number, value doesn't matter
+    uint8_t VolumeLabel[11];    // 11 bytes, padded with spaces
+    uint8_t SystemId[8];
+} __attribute__((packed)) BootSector;
+
+typedef struct 
+{
+    uint8_t Name[11];
+    uint8_t Attributes;
+    uint8_t _Reserved;
+    uint8_t CreatedTimeTenths;
+    uint16_t CreatedTime;
+    uint16_t CreatedDate;
+    uint16_t AccessedDate;
+    uint16_t FirstClusterHigh;
+    uint16_t ModifiedTime;
+    uint16_t ModifiedDate;
+    uint16_t FirstClusterLow;
+    uint32_t Size;
+} __attribute__((packed)) DirectoryEntry;
+
+
+//global vars
+BootSector g_BootSector;
+uint8_t* g_Fat = NULL;
+DirectoryEntry* g_RootDirectory = NULL;
+uint32_t g_RootDirectoryEnd;
+
+
+bool readBootSector(FILE* disk) {
+    return fread(&g_BootSector, sizeof(g_BootSector), 1, disk) > 0;
+}
+
+bool readSectors(FILE* disk, uint32_t lba, uint32_t count, void* bufferOut)
+{
+    bool ok = true;
+    ok = ok && (fseek(disk, lba * g_BootSector.BytesPerSector, SEEK_SET) == 0);
+    ok = ok && (fread(bufferOut, g_BootSector.BytesPerSector, count, disk) == count);
+    return ok;
+}
+
+bool readFat(FILE* disk)
+{
+    g_Fat = (uint8_t*) malloc(g_BootSector.SectorsPerFat * g_BootSector.BytesPerSector);
+    return readSectors(disk, g_BootSector.ReservedSectors, g_BootSector.SectorsPerFat, g_Fat);
+}
+
+bool readRootDirectory(FILE* disk)
+{
+    uint32_t lba = g_BootSector.ReservedSectors + g_BootSector.SectorsPerFat * g_BootSector.FatCount;
+    uint32_t size = sizeof(DirectoryEntry) * g_BootSector.DirEntryCount;
+    uint32_t sectors = (size / g_BootSector.BytesPerSector);
+    if (size % g_BootSector.BytesPerSector > 0)
+        sectors++;
+
+    g_RootDirectoryEnd = lba + sectors;
+    g_RootDirectory = (DirectoryEntry*) malloc(sectors * g_BootSector.BytesPerSector);
+    return readSectors(disk, lba, sectors, g_RootDirectory);
+}
+
+DirectoryEntry* findFile(const char* name)
+{
+    for (uint32_t i = 0; i < g_BootSector.DirEntryCount; i++)
+    {
+        if (memcmp(name, g_RootDirectory[i].Name, 11) == 0)
+            return &g_RootDirectory[i];
+    }
+
+    return NULL;
+}
+
+bool readFile(DirectoryEntry* fileEntry, FILE* disk, uint8_t* outputBuffer)
+{
+    bool ok = true;
+    uint16_t currentCluster = fileEntry->FirstClusterLow;
+
+    do {
+        uint32_t lba = g_RootDirectoryEnd + (currentCluster - 2) * g_BootSector.SectorsPerCluster;
+        ok = ok && readSectors(disk, lba, g_BootSector.SectorsPerCluster, outputBuffer);
+        outputBuffer += g_BootSector.SectorsPerCluster * g_BootSector.BytesPerSector;
+
+        uint32_t fatIndex = currentCluster * 3 / 2;
+        if (currentCluster % 2 == 0)
+            currentCluster = (*(uint16_t*)(g_Fat + fatIndex)) & 0x0FFF;
+        else
+            currentCluster = (*(uint16_t*)(g_Fat + fatIndex)) >> 4;
+
+    } while (ok && currentCluster < 0x0FF8);
+
+    return ok;
+}
+
+int main(int argc, char** argv) {
+    if(argc <3) {
+	printf("Syntax: %s <disk image> <file name>\n", argv[0]);
+	return -1;
+    }
+
+    FILE* disk = fopen(argv[1], "rb");
+    if(!disk) {
+	fprintf(stderr, "Cannot open disk image %s!", argv[1]);
+	return -1;
+    }
+
+    if(!readBootSector(disk)) {
+	fprintf(stderr, "Could not read boot sector!");
+	return -2;
+    }
+
+    if (!readFat(disk)) {
+        fprintf(stderr, "Could not read FAT!\n");
+        free(g_Fat);
+        return -3;
+    }
+
+    if (!readRootDirectory(disk)) {
+        fprintf(stderr, "Could not read FAT!\n");
+        free(g_Fat);
+        free(g_RootDirectory);
+        return -4;
+    }
+
+    DirectoryEntry* fileEntry = findFile(argv[2]);
+    if (!fileEntry) {
+        fprintf(stderr, "Could not find file %s!\n", argv[2]);
+        free(g_Fat);
+        free(g_RootDirectory);
+        return -5;
+    }
+
+    uint8_t* buffer = (uint8_t*) malloc(fileEntry->Size + g_BootSector.BytesPerSector);
+    if (!readFile(fileEntry, disk, buffer)) {
+        fprintf(stderr, "Could not read file %s!\n", argv[2]);
+        free(g_Fat);
+        free(g_RootDirectory);
+        free(buffer);
+        return -5;
+    }
+
+    for (size_t i = 0; i < fileEntry->Size; i++)
+    {
+        if (isprint(buffer[i])) fputc(buffer[i], stdout);
+        else printf("<%02x>", buffer[i]);
+    }
+    printf("\n");
+
+    free(buffer);
+    free(g_Fat);
+    free(g_RootDirectory);
+    return 0;
+}