Add fat12 lib

fat12.c

@@ -0,0 +1,138 @@
+#include <stdint.h>
+#include <stdlib.h>
+
+#define kSectorsPerCluster 2
+#define kSectorsPerTrack 9
+#define kHeads 2
+#define kFatSizeSectors 2
+#define kRootDirSizeSectors 7
+#define kDataRegionStartSector (1 + kFatSizeSectors*2 + kRootDirSizeSectors)
+
+
+typedef struct {
+    char name[8];
+    char ext[3];
+    uint8_t attr;
+
+    uint8_t dontcare[14];
+
+    uint16_t cluster;
+    uint32_t size;
+} direntry;
+
+static uint8_t* gFat;
+static direntry* gRootdir;
+
+
+static int readsector(int c, int h, int s, uint8_t* addr) {
+    register uint8_t* dest asm ("bx") = addr;
+    register uint8_t nsects asm ("al") = 1;
+    register uint8_t func asm ("ah") = 0x02;
+    register uint8_t sect asm ("cl") = s;
+    register uint8_t cyl asm ("ch") = c;
+    register uint8_t head asm ("dh") = h;
+    register uint8_t drive asm ("dl") = 0;
+    register uint16_t seg asm ("es") = 0;
+
+    register uint8_t ret asm("ah");
+
+    for (int i = 0; i < 3; i++) {
+        asm volatile ("int $0x13"
+                : "=r" (ret)
+                : "r" (dest), "r" (nsects), "r" (func), "r" (sect),
+                  "r" (cyl), "r" (head), "r" (drive), "r" (seg));
+        if (ret == 0x80) {
+            continue;
+        } else {
+            return ret;
+        }
+    }
+
+    return ret;
+}
+
+static int readcluster(int cluster) {
+    int offs = cluster * 3 / 2;
+    if (cluster % 2) {
+        // high nibble is lsb + whole byte
+        return ((gFat[offs] & 0xf0) >> 4) + (gFat[offs+1] << 4);
+    } else {
+        return gFat[offs] + ((gFat[offs+1] & 0x0f) << 8);
+    }
+}
+
+static void cluster2chs(int cluster, int* c, int* h, int* s) {
+    int logicalsector = kDataRegionStartSector + (cluster - 2) * kSectorsPerCluster;
+    *s = (logicalsector % kSectorsPerTrack) + 1;
+    *h = (logicalsector / kSectorsPerTrack) % kHeads;
+    *c = logicalsector / (kHeads * kSectorsPerTrack);
+}
+
+static int strncmp(const char* s1, const char *s2, size_t len) {
+    for (int i = 0; i < len && s1[i]; i++) {
+        if (s1[i] != s2[i]) {
+            return 1;
+        }
+    }
+    // XXX: really we should return 0 only if *s1 == *s2 here too
+    return 0;
+}
+
+static int loadfile(direntry* entry, void* addr) {
+    int cluster = entry->cluster;
+    for (int i = 0; i < entry->size; i+=1024) {
+        int c, h, s;
+        cluster2chs(cluster, &c, &h, &s);
+        if (readsector(c, h, s, addr + i)) {
+            return -5;
+        }
+        s++;
+        if (s > 9) {
+            s = 1;
+            h++;
+        }
+        if (h > 1) {
+            h = 0;
+            c++;
+        }
+        if (readsector(c, h, s, addr + i + 512)) {
+            return -5;
+        };
+        cluster = readcluster(cluster);
+    }
+    return 0;
+}
+
+int fat12_init(void* fat_addr, void* rootdir_addr) {
+    if (readsector(0, 0, 2, fat_addr)) {
+        return -2;
+    }
+    if (readsector(0, 0, 6, rootdir_addr)) {
+        return -3;
+    }
+
+    gFat = fat_addr;
+    gRootdir = rootdir_addr;
+
+    return 0;
+}
+
+int fat12_readfile(const char* name, void* addr) {
+    direntry* file = 0;
+
+    for (int i = 0; i < 16; i++) {
+        direntry* entry = &gRootdir[i];
+        if (entry->name[0] == 0) {
+            break;
+        }
+        if (!strncmp(entry->name, name, 11)) {
+            file = entry;
+            break;
+        }
+    }
+    if (!file) {
+        return -4;
+    }
+
+    return loadfile(file, addr);
+}

fat12.h

@@ -0,0 +1,15 @@
+#pragma once
+
+/*
+ * Arguments:
+ * - fat_addr: 1 KiB
+ * - rootdir_addr: 3.5 KiB
+ */
+int fat12_init(void* fat_addr, void* rootdir_addr);
+
+/*
+ * Returns:
+ *  -4 if file is not found
+ *  -5 if there is a disk error
+ */ 
+int fat12_readfile(const char* name, void* addr);