从正确的 GCC 基本内联汇编和扩展内联汇编的角度来看，该代码存在一些严重的问题，但从根本上来说，访问逻辑块地址 19 (LBA) 的问题在于计算。 LBA 19 是 CHS (Cylinder, Head, Sector) = (0, 1, 2) 其中 OP 建议它是 (Head: 0, Track: 1 , 扇区 2) 这是不正确的。
Int 13h/ah=2 采用 CHS 值。您可以使用以下公式（或等效公式）将 LBA 转换为 CHS 值：
C = (LBA ÷ SPT) ÷ HPC
H = (LBA ÷ SPT) mod HPC
S = (LBA mod SPT) + 1
HPC = Heads per cylinder (aka Number of Heads)
SPT = Sectors per Track,
LBA = logical block address
"mod" is the modulo operator (to get the remainder of a division)
我在 LBA 到 CHS 的转换部分的另一个 Stackoverflow answer 中写了更多关于 LBA 到 CHS 计算的内容。
一个观察结果是，最大扇区数根本没有考虑到等式中。这里真正的问题是 OP 的公式不正确：
int sector_count = 2880;
int heads = 2; /* Head per cylinder */
int tracks = 18; /* Sectors per Track */
int h = sector/(sector_count/heads);
int c = (sector-h*(sector_count/heads))/tracks;
int s = sector-c*tracks-h*(sector_count/heads)+1;
等式中唯一产生正确结果的部分（以循环方式）是s（扇区）。 c（圆柱）和h（头部）计算不正确。因此，它导致了在问题和 OP 的后续答案中观察到的问题。为了了解 OP 方程产生的值，我编写了一个程序，使用适当的公式将它们的值与正确的值进行比较：
LBA = 0: CHS = ( 0, 0, 1) | CHS = ( 0, 0, 1)
LBA = 1: CHS = ( 0, 0, 2) | CHS = ( 0, 0, 2)
LBA = 2: CHS = ( 0, 0, 3) | CHS = ( 0, 0, 3)
LBA = 3: CHS = ( 0, 0, 4) | CHS = ( 0, 0, 4)
LBA = 4: CHS = ( 0, 0, 5) | CHS = ( 0, 0, 5)
LBA = 5: CHS = ( 0, 0, 6) | CHS = ( 0, 0, 6)
LBA = 6: CHS = ( 0, 0, 7) | CHS = ( 0, 0, 7)
LBA = 7: CHS = ( 0, 0, 8) | CHS = ( 0, 0, 8)
LBA = 8: CHS = ( 0, 0, 9) | CHS = ( 0, 0, 9)
LBA = 9: CHS = ( 0, 0, 10) | CHS = ( 0, 0, 10)
LBA = 10: CHS = ( 0, 0, 11) | CHS = ( 0, 0, 11)
LBA = 11: CHS = ( 0, 0, 12) | CHS = ( 0, 0, 12)
LBA = 12: CHS = ( 0, 0, 13) | CHS = ( 0, 0, 13)
LBA = 13: CHS = ( 0, 0, 14) | CHS = ( 0, 0, 14)
LBA = 14: CHS = ( 0, 0, 15) | CHS = ( 0, 0, 15)
LBA = 15: CHS = ( 0, 0, 16) | CHS = ( 0, 0, 16)
LBA = 16: CHS = ( 0, 0, 17) | CHS = ( 0, 0, 17)
LBA = 17: CHS = ( 0, 0, 18) | CHS = ( 0, 0, 18)
LBA = 18: CHS = ( 1, 0, 1) | CHS = ( 0, 1, 1)
LBA = 19: CHS = ( 1, 0, 2) | CHS = ( 0, 1, 2)
LBA = 20: CHS = ( 1, 0, 3) | CHS = ( 0, 1, 3)
LBA = 21: CHS = ( 1, 0, 4) | CHS = ( 0, 1, 4)
LBA = 22: CHS = ( 1, 0, 5) | CHS = ( 0, 1, 5)
LBA = 23: CHS = ( 1, 0, 6) | CHS = ( 0, 1, 6)
LBA = 24: CHS = ( 1, 0, 7) | CHS = ( 0, 1, 7)
LBA = 25: CHS = ( 1, 0, 8) | CHS = ( 0, 1, 8)
LBA = 26: CHS = ( 1, 0, 9) | CHS = ( 0, 1, 9)
LBA = 27: CHS = ( 1, 0, 10) | CHS = ( 0, 1, 10)
LBA = 28: CHS = ( 1, 0, 11) | CHS = ( 0, 1, 11)
LBA = 29: CHS = ( 1, 0, 12) | CHS = ( 0, 1, 12)
LBA = 30: CHS = ( 1, 0, 13) | CHS = ( 0, 1, 13)
LBA = 31: CHS = ( 1, 0, 14) | CHS = ( 0, 1, 14)
LBA = 32: CHS = ( 1, 0, 15) | CHS = ( 0, 1, 15)
LBA = 33: CHS = ( 1, 0, 16) | CHS = ( 0, 1, 16)
LBA = 34: CHS = ( 1, 0, 17) | CHS = ( 0, 1, 17)
LBA = 35: CHS = ( 1, 0, 18) | CHS = ( 0, 1, 18)
LBA = 36: CHS = ( 2, 0, 1) | CHS = ( 1, 0, 1)
LBA = 37: CHS = ( 2, 0, 2) | CHS = ( 1, 0, 2)
LBA = 38: CHS = ( 2, 0, 3) | CHS = ( 1, 0, 3)
LBA = 39: CHS = ( 2, 0, 4) | CHS = ( 1, 0, 4)
LBA = 40: CHS = ( 2, 0, 5) | CHS = ( 1, 0, 5)
LBA = 41: CHS = ( 2, 0, 6) | CHS = ( 1, 0, 6)
LBA = 42: CHS = ( 2, 0, 7) | CHS = ( 1, 0, 7)
LBA = 43: CHS = ( 2, 0, 8) | CHS = ( 1, 0, 8)
LBA = 44: CHS = ( 2, 0, 9) | CHS = ( 1, 0, 9)
LBA = 45: CHS = ( 2, 0, 10) | CHS = ( 1, 0, 10)
LBA = 46: CHS = ( 2, 0, 11) | CHS = ( 1, 0, 11)
LBA = 47: CHS = ( 2, 0, 12) | CHS = ( 1, 0, 12)
LBA = 48: CHS = ( 2, 0, 13) | CHS = ( 1, 0, 13)
LBA = 49: CHS = ( 2, 0, 14) | CHS = ( 1, 0, 14)
LBA = 50: CHS = ( 2, 0, 15) | CHS = ( 1, 0, 15)
LBA = 51: CHS = ( 2, 0, 16) | CHS = ( 1, 0, 16)
LBA = 52: CHS = ( 2, 0, 17) | CHS = ( 1, 0, 17)
LBA = 53: CHS = ( 2, 0, 18) | CHS = ( 1, 0, 18)
LBA = 54: CHS = ( 3, 0, 1) | CHS = ( 1, 1, 1)
LBA = 55: CHS = ( 3, 0, 2) | CHS = ( 1, 1, 2)
LBA = 56: CHS = ( 3, 0, 3) | CHS = ( 1, 1, 3)
LBA = 57: CHS = ( 3, 0, 4) | CHS = ( 1, 1, 4)
LBA = 58: CHS = ( 3, 0, 5) | CHS = ( 1, 1, 5)
LBA = 59: CHS = ( 3, 0, 6) | CHS = ( 1, 1, 6)
LBA = 60: CHS = ( 3, 0, 7) | CHS = ( 1, 1, 7)
LBA = 61: CHS = ( 3, 0, 8) | CHS = ( 1, 1, 8)
LBA = 62: CHS = ( 3, 0, 9) | CHS = ( 1, 1, 9)
LBA = 63: CHS = ( 3, 0, 10) | CHS = ( 1, 1, 10)
LBA = 64: CHS = ( 3, 0, 11) | CHS = ( 1, 1, 11)
LBA = 65: CHS = ( 3, 0, 12) | CHS = ( 1, 1, 12)
LBA = 66: CHS = ( 3, 0, 13) | CHS = ( 1, 1, 13)
LBA = 67: CHS = ( 3, 0, 14) | CHS = ( 1, 1, 14)
LBA = 68: CHS = ( 3, 0, 15) | CHS = ( 1, 1, 15)
LBA = 69: CHS = ( 3, 0, 16) | CHS = ( 1, 1, 16)
LBA = 70: CHS = ( 3, 0, 17) | CHS = ( 1, 1, 17)
LBA = 71: CHS = ( 3, 0, 18) | CHS = ( 1, 1, 18)
LBA = 72: CHS = ( 4, 0, 1) | CHS = ( 2, 0, 1)
LBA = 73: CHS = ( 4, 0, 2) | CHS = ( 2, 0, 2)
LBA = 74: CHS = ( 4, 0, 3) | CHS = ( 2, 0, 3)
LBA = 75: CHS = ( 4, 0, 4) | CHS = ( 2, 0, 4)
LBA = 76: CHS = ( 4, 0, 5) | CHS = ( 2, 0, 5)
LBA = 77: CHS = ( 4, 0, 6) | CHS = ( 2, 0, 6)
LBA = 78: CHS = ( 4, 0, 7) | CHS = ( 2, 0, 7)
LBA = 79: CHS = ( 4, 0, 8) | CHS = ( 2, 0, 8)
...
OP 的结果在左边，正确的在右边。 LBA 0 到 LBA 17 是正确的。如果您开始读取 LBA 小于 18 的一个或多个扇区，那将是正确的。如果您使用为 LBA 19 计算的 CHS 值，它们是不正确的。
OP 在他们的回答中建议柱面和磁头值的文档不正确，并且寄存器被颠倒了。文档是正确的：
AL = number of sectors to read (must be nonzero)
CH = low eight bits of cylinder number
CL = sector number 1-63 (bits 0-5)
high two bits of cylinder (bits 6-7, hard disk only)
DH = head number
DL = drive number (bit 7 set for hard disk)
ES:BX -> data buffer
OP 的回答建议解决方法是交换磁头和气缸。事实上，这恰好使他的代码意外地为 LBA 0 到 LBA 35 工作。LBA >= 36 不正确。
解决方法是在 OP 的代码中使用正确的计算：
c = (sector / tracks) / heads;
h = (sector / tracks) % heads;
s = (sector % tracks) + 1;
测试 LBA 到 CHS 方程的代码
#include <stdio.h>
int main()
{
const int sector_count = 2880;
const int heads = 2;
const int tracks = 18; /* tracks per sector */
unsigned char h, h2;
unsigned char c, c2;
unsigned char s, s2;
int sector; /* LBA */
for (sector=0; sector < sector_count; sector++) {
/* Improper calculation */
h = sector/(sector_count/heads);
c = (sector-h*(sector_count/heads))/tracks;
s = sector-c*tracks-h*(sector_count/heads)+1;
/* Proper calculation */
c2 = (sector / tracks) / heads;
h2 = (sector / tracks) % heads;
s2 = (sector % tracks) + 1;
printf ("LBA = %4d: CHS = (%2d, %2d, %2d) | CHS = (%2d, %2d, %2d)\n",
sector, c, h, s, c2, h2, s2);
}
return 0;
}
使用内联汇编执行磁盘读取的示例 GCC 代码
biosdisk.h
#ifndef BIOSDISK_H
#define BIOSDISK_H
#include <stdint.h>
/* BIOS Parameter Block (BPB) on floppy media */
typedef struct __attribute__((packed)) {
char OEMname[8];
uint16_t bytesPerSector;
uint8_t sectPerCluster;
uint16_t reservedSectors;
uint8_t numFAT;
uint16_t numRootDirEntries;
uint16_t numSectors;
uint8_t mediaType;
uint16_t numFATsectors;
uint16_t sectorsPerTrack;
uint16_t numHeads;
uint32_t numHiddenSectors;
uint32_t numSectorsHuge;
uint8_t driveNum;
uint8_t reserved;
uint8_t signature;
uint32_t volumeID;
char volumeLabel[11];
char fileSysType[8];
} disk_bpb_s;
/* State information for CHS disk accesses */
typedef struct __attribute__((packed)) {
uint16_t segment;
uint16_t offset;
uint16_t status;
/* Drive geometry needed to compute CHS from LBA */
uint16_t sectorsPerTrack;
uint16_t numHeads;
/* Disk parameters */
uint16_t cylinder;
uint8_t head;
uint8_t sector;
uint8_t driveNum;
uint8_t numSectors; /* # of sectors to read */
/* Number of retries for disk operations */
uint8_t retries;
} disk_info_s;
extern fastcall uint8_t
reset_disk (disk_info_s *const disk_info);
extern fastcall uint8_t
read_sector_chs (disk_info_s *const disk_info);
/* Forced inline version of reset_sector */
static inline fastcall always_inline uint8_t
reset_disk_i (disk_info_s *const disk_info)
{
uint16_t temp_ax = 0x0000;
uint8_t carryf;
__asm__ __volatile__ (
"int $0x13\n\t"
#ifdef __GCC_ASM_FLAG_OUTPUTS__
: [cf]"=@ccc"(carryf),
#else
"setc %[cf]\n\t"
: [cf]"=qm"(carryf),
#endif
"+a"(temp_ax)
: "d"(disk_info->driveNum)
: "cc");
disk_info->status = temp_ax;
return (carryf);
}
/* Forced inline version of read_sector */
static inline fastcall always_inline uint8_t
read_sector_chs_i (disk_info_s *const disk_info)
{
uint16_t temp_ax;
uint16_t temp_dx;
uint8_t carryf = 0;
uint8_t retry_count = 0;
#ifndef BUGGY_BIOS_SUPPORT
temp_dx = (disk_info->head << 8) | disk_info->driveNum;
#endif
do {
/* Only reset disk if error detected previously */
if (carryf)
reset_disk_i (disk_info);
/* Need to reload AX during each iteration since a previous
* int 0x13 call will destroy its contents. There was a bug on
* earlier BIOSes where DX may have been clobbered.
*/
temp_ax = (0x02 << 8) | disk_info->numSectors;
#ifdef BUGGY_BIOS_SUPPORT
temp_dx = (disk_info->head << 8) | disk_info->driveNum;
#endif
__asm__ __volatile__ (
"push %%es\n\t"
"mov %w[seg], %%es\n\t"
#ifdef BUGGY_BIOS_SUPPORT
"stc\n\t" /* Some early bioses have CF bug */
"int $0x13\n\t"
"sti\n\t" /* Some early bioses don't re-enable interrupts */
#else
"int $0x13\n\t"
#endif
"pop %%es\n\t"
#ifdef __GCC_ASM_FLAG_OUTPUTS__
: [cf]"=@ccc"(carryf),
#else
"setc %[cf]\n\t"
: [cf]"=qm"(carryf),
#endif
#ifdef BUGGY_BIOS_SUPPORT
"+a"(temp_ax),
"+d"(temp_dx)
:
#else
"+a"(temp_ax)
:
"d"(temp_dx),
#endif
"c"(((disk_info->cylinder & 0xff) << 8) |
((disk_info->cylinder >> 2) & 0xC0) |
(disk_info->sector & 0x3f)),
"b"(disk_info->offset),
[seg]"r"(disk_info->segment)
: "memory", "cc");
} while (carryf && (++retry_count < disk_info->retries));
disk_info->status = temp_ax;
return (carryf);
}
/* Forced inline version of read_sector_lba */
static inline fastcall always_inline uint8_t
read_sector_lba_i (disk_info_s *const disk_info, const uint32_t lba)
{
disk_info->cylinder = lba / disk_info->sectorsPerTrack / disk_info->numHeads;
disk_info->head = (lba / disk_info->sectorsPerTrack) % disk_info->numHeads;
disk_info->sector = (lba % disk_info->sectorsPerTrack) + 1;
return read_sector_chs_i (disk_info);
}
#endif
biosdisk.c：
#include <stdint.h>
#include "biosdisk.h"
fastcall uint8_t
reset_disk (disk_info_s *const disk_info)
{
return reset_disk_i (disk_info);
}
fastcall uint8_t
read_sector_chs (disk_info_s *const disk_info)
{
return read_sector_chs_i (disk_info);
}
fastcall uint8_t
read_sector_lba (disk_info_s *const disk_info, const uint32_t lba)
{
return read_sector_lba_i (disk_info, lba);
}
x86helper.h：
#ifndef X86HELPER_H
#define X86HELPER_H
#define fastcall __attribute__((regparm(3)))
/* noreturn lets GCC know that a function that it may detect
won't exit is intentional */
#define noreturn __attribute__((noreturn))
#define always_inline __attribute__((always_inline))
#define used __attribute__((used))
#endif
可以在on my website找到一个在 GCC 中创建 2 阶段引导加载程序的小型概念验证项目。
注意事项
-
Int 13h/AH=0h 重置磁盘系统。此操作在软盘等真实硬件上可能需要相当长的时间，因为它还会重新校准驱动器磁头。您只应在检测到错误后重试磁盘操作之前重置磁盘。
-
使用 GCC 创建将在实模式下运行的代码充其量是有问题的。使用-m16生成的代码一般只能在80386或更高版本的处理器上运行。
-
编译器不保证多个asm 语句按照它们在代码中出现的顺序发出。您应该将多个asm 语句合并为一个。 GCC documentation 是这样说的：
不要期望 asm 语句序列在编译后保持完全连续，即使您使用 volatile 限定符也是如此。如果某些指令需要在输出中保持连续，请将它们放在单个多指令 asm 语句中。