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fat12: can now write files

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This commit is contained in:
Paul Mathieu
2025-10-06 22:49:17 +02:00
parent 91cf76c251
commit 1208f1f66e
4 changed files with 200 additions and 47 deletions
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2
src/Makefile
View File

@@ -18,7 +18,7 @@ LDFLAGS = -mregparmcall -Wl,--gc-sections -Os -nostdlib -flto
fat12boot.bin:
$(OBJCOPY) -O binary $< $@
fat12boot.elf: fat12boot.o fat12.o bootsect.S
fat12boot.elf: fat12boot.o fat12.o bootsect.S stdlib.o
fat12boot.elf: LDFLAGS += -T bootsect.ld
fat12boot.bin: fat12boot.elf

190
src/fat12.c
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@@ -1,12 +1,18 @@
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include "polos.h"
#define kSectorsPerCluster 2
#define kSectorsPerTrack 9
#define kHeads 2
#define kTracks 40
#define kFatSizeSectors 2
#define kRootDirSizeSectors 7
#define kBytesPerCluster 1024
#define kDataRegionStartSector (1 + kFatSizeSectors * 2 + kRootDirSizeSectors)
#define kNumClusters ((kSectorsPerTrack * kHeads * kTracks - kDataRegionStartSector) / kSectorsPerCluster + 2)
typedef struct {
char name[8];
@@ -22,23 +28,12 @@ typedef struct {
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");
static uint8_t rwsector(uint8_t w, int c, int h, int s, void* addr) {
uint16_t ret;
uint16_t cmd = w ? 0x0301 : 0x0201;
// retry for a total of up to 3 attempts if drive not ready
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));
ret = __readwritesector__(cmd, addr, c, h, s) >> 8;
if (ret != 0x80) {
break;
}
@@ -47,6 +42,14 @@ static int readsector(int c, int h, int s, uint8_t* addr) {
return ret;
}
static uint8_t readsector(int c, int h, int s, void* addr) {
return rwsector(0, c, h, s, addr);
}
static uint8_t writesector(int c, int h, int s, void* addr) {
return rwsector(1, c, h, s, addr);
}
static int readcluster(int cluster) {
int offs = cluster * 3 / 2;
if (cluster % 2) {
@@ -57,6 +60,28 @@ static int readcluster(int cluster) {
}
}
static int writecluster(int cluster, int value) {
int offs = cluster * 3 / 2;
if (cluster % 2) {
gFat[offs] = (gFat[offs] & 0x0f) + ((value & 0x0f) << 4);
gFat[offs+1] = value >> 4;
// high nibble is lsb + whole byte
} else {
gFat[offs] = value & 0xff;
gFat[offs+1] = (gFat[offs+1] & 0xf0) + (value >> 8);
}
return value;
}
static int firstfreecluster() {
for (int i = 2; i < kNumClusters; i++) {
if (readcluster(i) == 0) {
return i;
}
}
return -1;
}
static void cluster2chs(int cluster, int* c, int* h, int* s) {
int logicalsector =
kDataRegionStartSector + (cluster - 2) * kSectorsPerCluster;
@@ -65,19 +90,9 @@ static void cluster2chs(int cluster, int* c, int* h, int* s) {
*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) {
for (int i = 0; i < entry->size; i += kBytesPerCluster) {
int c, h, s;
cluster2chs(cluster, &c, &h, &s);
if (readsector(c, h, s, addr + i)) {
@@ -88,10 +103,10 @@ static int loadfile(direntry* entry, void* addr) {
s = 1;
h++;
}
if (h > 1) {
h = 0;
c++;
}
// if (h > 1) { // this will never happen
// h = 0;
// c++;
// }
if (readsector(c, h, s, addr + i + 512)) {
return -5;
};
@@ -100,6 +115,94 @@ static int loadfile(direntry* entry, void* addr) {
return 0;
}
static direntry* findfile(const char* name) {
direntry* file = 0;
for (int i = 0; i < 16; i++) {
direntry* entry = &gRootdir[i];
if (!strncmp(entry->name, name, 11)) {
file = entry;
break;
}
}
return file;
}
static direntry* newfile(const char* name) {
direntry* file = 0;
for (int i = 0; i < 16; i++) {
direntry* entry = &gRootdir[i];
if (entry->name[0] == 0) {
file = entry;
break;
}
}
if (!file) {
return file;
}
memcpy(file->name, name, 11);
file->size = 0;
file->cluster = 0;
return file;
}
static int allocatecluster(int previous) {
int cluster = firstfreecluster();
writecluster(cluster, 0xfff);
if (previous != 0) {
writecluster(previous, cluster);
}
return cluster;
}
static void freeclusterchain(int cluster) {
if (cluster < 2 || cluster >= 0xf00) {
return;
}
freeclusterchain(readcluster(cluster));
writecluster(cluster, 0);
}
static int savefile(direntry* file, void* addr, int size) {
freeclusterchain(file->cluster);
file->cluster = 0;
int cluster = 0;
for (int i = 0; i < size; i += kBytesPerCluster) {
cluster = allocatecluster(cluster);
if (file->cluster == 0) {
file->cluster = cluster;
}
int c, h, s;
cluster2chs(cluster, &c, &h, &s);
if (writesector(c, h, s, addr + i)) {
return -7;
}
s++;
if (s > 9) {
s = 1;
h++;
}
// if (h > 1) { // no cluster will cross a cylinder boundary
// h = 0;
// c++;
// }
if (writesector(c, h, s, addr + i + 512)) {
return -7;
};
}
file->size = size;
if (writesector(0, 0, 2, gFat)) {
return -8;
}
if (writesector(0, 0, 6, gRootdir)) {
return -9;
}
return 0;
}
int fat12_init(void* fat_addr, void* rootdir_addr) {
if (readsector(0, 0, 2, fat_addr)) {
return -2;
@@ -115,21 +218,22 @@ int fat12_init(void* fat_addr, void* rootdir_addr) {
}
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;
}
}
direntry* file = findfile(name);
if (!file) {
return -4;
}
return loadfile(file, addr);
}
int fat12_writefile(const char* name, void* addr, int size) {
direntry* file = findfile(name);
if (!file) {
file = newfile(name);
}
if (!file) {
// no moar space
return -6;
}
return savefile(file, addr, size);
}

39
src/polos.h
View File

@@ -8,6 +8,9 @@
int runcomms(char until_idle);
int putchar(int);
int puts(const char*);
inline static uint8_t __get_far_u8__(uint16_t addr, uint16_t seg) {
register uint16_t ad asm("di") = addr;
register uint8_t ret asm("al");
@@ -18,3 +21,39 @@ inline static uint8_t __get_far_u8__(uint16_t addr, uint16_t seg) {
: "es");
return ret;
}
inline static void __set_far_u8__(uint16_t addr, uint16_t seg, uint8_t val) {
register uint16_t ad asm("di") = addr;
register uint8_t v asm("al") = val;
asm("mov %0, %%es \t\n"
"stosb \n\t" ::"r"(seg), "r"(ad), "r"(v) : "es", "memory");
}
inline static uint16_t __readwritesector__(uint16_t cmd, uint8_t* addr, uint8_t c, uint8_t h, uint8_t s) {
register uint8_t* dest asm("bx") = addr;
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 ret asm("ax");
asm volatile("xor %%ax, %%ax \n\t"
"mov %%ax, %%es \n\t"
"mov %1, %%ax \n\t"
"int $0x13"
: "=r"(ret)
: "ir"(cmd), "r"(dest), "r"(sect), "r"(cyl),
"r"(head), "r"(drive)
);
return ret;
}
inline static uint16_t __readsector__(uint8_t* addr, uint8_t c, uint8_t h, uint8_t s) {
return __readwritesector__(0x0201, addr, c, h, s);
}
inline static uint16_t __writesector__(uint8_t* addr, uint8_t c, uint8_t h, uint8_t s) {
return __readwritesector__(0x0301, addr, c, h, s);
}

16
src/stdlib.c
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@@ -46,7 +46,17 @@ void* memset(void* ptr, int val, size_t len) {
return ptr;
}
int _int80h_2(uint8_t fun, uint16_t arg1, uint16_t arg2) {
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 int80h_2(uint8_t fun, uint16_t arg1, uint16_t arg2) {
register uint16_t a1 asm("dx") = arg1;
register uint16_t a2 asm("cx") = arg2;
register uint8_t ah asm("ah") = fun;
@@ -79,12 +89,12 @@ int read(int fd, void* buf, size_t size) {
if (fd != kLpt1) {
return -1;
}
return _int80h_2(0x05, (uint16_t)buf, size);
return int80h_2(0x05, (uint16_t)buf, size);
}
int write(int fd, void* buf, size_t size) {
if (fd != kLpt1) {
return -1;
}
return _int80h_2(0x06, (uint16_t)buf, size);
return int80h_2(0x06, (uint16_t)buf, size);
}